A common question among enthusiasts is whether a given engine is capable of being turbocharged. The answer to this question varies depending upon several considerations. First of all, is the engine sound and in good shape mechanically? What is the expected use of the engine and what is your horsepower goal?
If your objective is to create a street/strip machine that also serves as a daily driver, then you should consider the engine’s general condition. A turbocharger is not a cure for a poor performing engine that has lost power due to age and internal wear. If your engine is strong with relatively low miles, and your overall vehicle is worth the time and investment, then a turbocharger may be just what the doctor ordered! There are limits for stock components though, and we’ll discuss those, too.
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For the most part, today’s fuel injected engines are very capable of being turbocharged for a 50-percent increase in power without making any real internal engine modifications. That correlates to about 7–8 lbs of boost. This is assuming that the engine’s duty cycle is relatively low meaning that the high-horsepower potential is used only occasionally and for relatively short bursts.
A twin-turbo small-block Chevrolet by Gale Banks Engineering is being assembled in the Banks engine room. This room would probably pass a hospital cleanliness inspection! When Banks builds the engine for one of his twin kits, he starts from the bottom up, to make sure everything is just right. Weak links kill engines. (Courtesy Gale Banks Engineering)
This competition Buick Grand National set-up runs mid-8s. The 3.8-liter powerhouse belongs to Dan Strezo, owner of DLS Engine Development in Wheatfield, Indiana.
Note that this book does not specifically discuss the merits of individual engines or their capability of being turbocharged. Most engines of the last 10 years have been designed with more robust features for higher quality and longer life. Turbocharging your engine to raise the horsepower level can shorten its useful life, but how much all depends upon how frequently the power is used, and how well your system is tuned. Remember that engine manufacturers use higher horsepower levels than an engine is designed for in order to perform accelerated durability testing.
So before you begin your project you may want to check out your particular engine platform to research where its weak links are, if any, and account for those things. This step may save you a lot of headaches and heartaches down the road.
The above is all true for mild street/strip projects and hot street cars, but if your project engine is a bit tired, and/or you want to make even more power, then you may need to consider some internal modifications to handle the stress. Beyond a 50 to 100 percent power increase, you’ll need to plan on it. Power increases above that range are typically found in competition vehicles and there is much to consider.
For the most part, you can think of your boost level as a basis of need for engine modifications. The danger in saying this is that it dwells upon boost and horsepower being tied together, but they aren’t necessarily. Remember that boost and turbine backpressure are undesirable values, but are inherently necessary in increasing air mass flow into the engine. Therefore assuming you’ve done your homework correctly, and the boost level you intend to run is efficient and accurate for the intended level of horsepower desired, we can say that up to the 7–8 lbs of boost, you can probably get away with little to no engine modifications. In this situation, you’ll likely be able to run on premium pump gas rated between 91 and 94 octane with an air-fuel ratio of about 12-12.5:1 for max power. Above this boost level, it’s hard to find fuel with enough octane to eliminate detonation, which will quickly destroy your engine and your wallet.
Detonation will occur at different boost levels on different engines. The detonation threshold will depend upon several factors such as static compression ratio, vehicle weight, combustion chamber design, camshaft design, valve lash and timing, intake air temperature, and others. It just isn’t practical to expect to run more than 15 lbs of boost on a street engine. Regardless of the variables, you’re most likely to have a detonation problem that will defeat the purpose of running a turbo system. Despite this statement, I know there will still be some people who will insist upon race-level equipment on the street, there always are. I know this because I grew up with several of them! But, high-end street rods, for example, that are lightweight and built well, and operated by those who understand those variables, can somewhat get away with it. This is possible due the fact that turbos are load-sensitive machines and boost is developed on demand. A wise old street rodder who wants a high boost engine for that occasional trip to the track for low ET bragging rights, will know that his engine in a 2,300-lb vehicle will not detonate nearly as soon as the very same engine in a 3,500-lb slug of a car. Further, if you don’t have your racing gas handy, just stay out of the throttle. That’s one of the beauties of turbocharged engines versus a high-compression, naturally aspirated mill that will see detonation much more quickly by just slightly juicing the throttle. Once you’re over 20–25 lbs of boost, you’re in need of some real octane, or just switch to alcohol! The current trend toward higher amounts of ethanol in pump gasoline, such as E85, where up to 85 percent of the fuel is ethanol, actually raises the effective octane rating and can provide greater detonation resistance to the turbo motor.
It would be impossible to cover preparation specifics for all engine designs in a single book, but this chapter will cover modifications that pertain to turbocharged engines in general. Most of the following recommendations are necessary with a radical horsepower increase of 100 percent or more. In this case, you should also find a book that dwells upon the specifics of that particular engine. Most of the naturally aspirated high-horsepower engine build recommendations include bottomend strengthening. Even many of the top-end engine recommendations will apply, such as installing oversized valves, porting, best head designs, etc. Remember to think like an air molecule. Many of the optimized airflow tricks that work best in a naturally aspirated engine will also benefit a boosted motor. While the specific increases may not be as dramatic since the air fills the combustion chamber with a slightly different characteristic (since it’s filling from a static pressure charge), you still have a very short time to fill the chamber at high RPM, so there are improvements to be made to minimize the resistances to the cylinder charge. Think of air as being lazy; you need all the tricks in the book to get it to move.
Quality, Quantity & Balance: A Philosophical Approach
I’ve stated that most of the same engine building recommendations for a high-horsepower, naturally aspirated engine will also apply to a turbocharged engine. But did you ever wonder why some engines produce more power and live longer than others? If you’re planning to push the horsepower envelope, it’s time to use a philosophical approach.
Building an extremely high-horsepower engine is about quality, quantity, and balance. These simple words can formulate the framework for success. Quality refers to the proper selection of key components with a high-quality design for your intended use, and the materials they are made from. The stresses imparted onto a boosted motor will be way beyond what the original engine was designed to handle, so you have to question the strength and durability of every single part. Further, the quality of your engine build includes whether you’ve taken the time to measure all the critical clearances to make sure they are what they’re supposed to be.
The quantity is key to potential horsepower production. Horsepower is determined by pounds of fuel burned. Boost pressure doesn’t make power, burning more fuel does. But having the correct quantities of air and fuel is key to producing power. Quantity can also be about an orifice size, such as the intake valve or the lift and duration of the camshaft. Maximizing all the variables relative to quantity will produce the maximum horsepower, but it can only be supported by the quality of the components.
Balance is about proportions, as well as the literal balance of the engine’s rotating and reciprocating parts. A balanced engine will not only survive longer, it will also produce more horsepower. Balance is just as important when it refers to balanced proportions such as combustion chamber sizes, air-fuel ratios, and the pressure balance across the engine. Raising the boost pressure from 20 lbs to 30 lbs by a wastegate adjustment won’t likely help you much if you’ve gone from a turbine backpressure of 19 lbs to 38 lbs. You’ve lost the pressure balance across the engine that made efficient power. Now you’ve got an engine that’s working hard to overcome tremendous pumping losses. It’s time for a different turbo with a different flow capacity. The following discussions will philosophically pertain to those modifications that are essential for boosted motors. They’re intended to direct your thinking toward the blend of quality, quantity, and balance in order to achieve success in engines where the specific horsepower output is going to approach or exceed 200 hp per liter.
The best place to start is the engine block. The engine block is your foundation. If it isn’t up to snuff, then nothing else matters. Engine blocks come in both aluminum and cast iron. Researching your particular engine will tell you where to find the strongest factory block, or if an aftermarket block is the only way to go. For example, even the best OEM 5.0L Ford blocks don’t live long near or above 500 hp. If your turbo project is being done on an older cast-iron block, make sure you’ve got a good one. One way to tell is whether your block has experienced any core shift during its original casting. One way to check this is by simply looking at the casting boss where the cam bearings are installed. If there’s an unequal amount of casting machined around this area it can indicate a block that has experienced a casting core shift. The point here is to look for evidence as to whether all the cylinder bores have been cut straight in the casting, which gives a more uniform cylinder wall thickness and therefore a stronger cylinder wall.
Strength aside, this is an old street racer’s trick to tell the best block candidates for large overbores to make mountain motors when raw cubic inches were king. Times have changed. In a boosted engine, forget about boring for displacement benefits. If you need to freshen up the block, take the lightest cut (smallest overbore) for which you can find pistons. Keep your cylinder walls thick for strength and thermal retention. Your turbocharger will more than make-up the small amount of difference that a 0.030–0.060-inch overbore will add. There are trade-offs in many choices, and giving up a few cubic inches for strength is a wise one.
After the cylinders have checked out the next step for block prep is to deck the block to make sure it is not only smooth for gasket sealing, but also exactly perpendicular to the cylinders. You can’t assume that the factory production machining is good enough.
Later-model aluminum and iron blocks are probably more precision cast, but there is a good way to ensure you’ve got a viable candidate for a high-performance turbo motor. Dan Strezo, president of DLS Engine Development, is perhaps one of the very best 3.8-liter Buick Grand National engine builders today. While his specialty is the 3.8-liter engine, he also commonly builds small-block Chevrolets and Hondas. He has actually made 700 hp using stock 3.8-liter components with 26 to 27 lbs of boost. That’s about a 200 percent increase over its stock rating.
Dan’s philosophy is: make the motor sound, strong, and powerful and then add the turbo. This is right in line with our quality, quantity, and balance principal. Dan uses a 3-step process to prep all his used engine blocks.
- The block is first heated in an oven at 500 degrees F for 30 minutes. This will dry out all the oils and carbon.
- The block is bead blasted to break loose any remaining debris and remove rust.
- Next, a shaker setup removes the blast media and other debris.
Immediately after this process, the block is placed in a high-pressure wash tank to thoroughly clean the block. The next step is to magnaflux the casting to find any cracks. Dan also believes in performing a sonic check of the block’s cylinder walls for thickness on both the major and minor thrust surfaces. These are the sides of the cylinder walls perpendicular to the crankshaft. As a general rule, the sonic check typically reveals a cylinder wall thickness of 0.180 to 0.210 inch. When preparing a high-boost competition engine he’ll look for a 0.210- inch thick wall cylinder block. If you’ve got a thin one, get rid of it! Your measurements and standards may vary depending on what engine you’re working with. Research on your specific engine should guide you.
One of the common misconceptions Dan encounters is the racer who believes that a main bearing cap that breaks means that the main caps need girdles or other aftermarket add-ons for superior strength. While the wisdom of strengthening the saddles and main caps in the block is based upon conventional wisdom, and sound knowledge, Dan sees many repeated engine failures due to improper failure diagnosis. Failure diagnosis is as key to engine failure analysis as it is to turbo failure analysis. The 3.8-liter engine, like many others, has a cast crankshaft that bends under pressure. Strengthening the mains won’t correct for a crankshaft that is whipping around breaking main bearing caps. In such a case main cap reinforcing is addressing the symptom, not the cause. Lastly, be sure to use honing plates for the block’s final finish hone. Honing plates are flat plates that torque onto the engine block’s deck to simulate the cylinder distortion that can occur when the head bolts are torqued. This helps to ensure that the bore is perfectly round once the engine is completely assembled. Even though an engine block is the heaviest part of any engine, the bores will easily flex. You can witness this yourself by simply placing a 10ths-reading dial indicator inside the bore exactly perpendicular to the crankshaft where your reading is on the piston’s major and minor thrust surfaces. Then, using your own muscle power, squeeze the outside of the block and witness the changes on the dial. Now think of the much greater force inside the bore caused by the torque of head bolts. If this exercise won’t convince you of the value of using honing plates, nothing will. A turbo motor is expected to seal very high pressures when making 700, 800, or more than 1,000 hp. The rings can’t do their job keeping the combustion pressure in the cylinder if the bore isn’t kept round. Some racers even fill the water jackets with different compounds to further reinforce the bores from distortion during operation.
Some research into your particular engine will tell you what kind of luck people have with various available cranks, both factory and aftermarket.
In many cases, a stock cast crankshaft will not meet durability requirements in a high-horsepower turbo motor. The crankshaft is no place to skimp on your budget. Spend the money on a quality forging! Strezo will use forged crankshafts, made from 4340 steel, on engines that produce up to about 1,000 hp. Above 1,000 hp, he uses a billet crank also machined from 4340 material.
Castings are weaker because the metal has been poured from a molten state. Castings contain porosity, which are very small voids in the metal like air pockets and they cause structural weaknesses because there isn’t a homogenous condition of the material. Further there isn’t a consistant grain structure; it’s ramdom in nature. In a forging there will be almost no voids from porosity, a grain structure will be present and the material will therefore be stronger. A billet crankshaft, or billet anything is where the total piece was shaped and finished from a solid block and where the material is most homogenous. A billet formed part is typically the strongest method of constructing a part within that material spec.
When machining your crankshaft, use the typical oil hole chamfering for improved oil distribution, but do not cross drill the journals of the rods or main bearings because the oiling benefit does not add enough significant value, but it does weaken the crankshaft that’s expected to handle extremely high combustion pressures. When it comes to oil pumps, Dan will use a high-pressure, high-volume internal pump from sources like Melling. On critical high-horsepower, high-RPM engines, you may even consider external oiling systems.
This is a Dan Strezo race-prepped crankshaft. Oil holes are chamfered, but the crank is not cross drilled. Dan uses stock oil clearances and feeds the oil galleries with a high-pressure, high-flow oil pump by Melling.
Research your engine model and make for oil clearances that work best for your intended application. When I moved from big-block Chryslers to big-block Chevrolets during my drag racing era (a sponsor’s request) I sought out one of the best big-block Chevrolet engine builders in the country, John Mattingly of Mattingly Automotive in Greenfield, Indiana. His competition engines always made gobs of horsepower and they never failed. He told me, to grind the crank to 0.001 inch under low-standard, 0.010 under. That produced oil clearances of 0.0035 inch on the rods and 0.0045 on the mains, and slightly lowered the surface speed. This sounded scary to me, but it worked and we never failed an engine in five years of competition. The point here is to remember that part of being smart is “knowing what you’re dumb at.” I was completely ignorant relative to how to clearance the new engine, so I did my research. Factory specs are for factory engine builds. If you’re going racing with your own car, do your homework! Spend a season in the pits, get out of the stands, and make some contacts that can teach you what to avoid.
Be sure to verify your crankshaft’s straightness prior to final assembly. This is very easy to do. Simply place only the two outer main bearing inserts into the main bearing saddles, and lightly coat them with oil to protect the bearing. Position a long reaching dial indicator, or very small face indicator (the crankshaft counter weights can get in the way), on the center main bearing journal, which does not have a bearing insert underneath it. Slowly turn the crankshaft by hand and note the reading variation on the indicator. Other engine builders may have their own opinions, but I’ve never assembled an engine that had any measurable reading when performing this check for crankshaft straightness. I look for the indicator’s needle to stay completely still.
Pistons & Rings
The subject of piston and ring selection may have the most unique set of characteristics and considerations when building a high-boost, high-horsepower engine. When selecting pistons for a boosted engine, you need to consider compression ratio, material, type of construction, ring type and material, top ring land location relative to the crown, clearances to bore relative to skirt size, and coatings. These issues are mostly taken into account when you select pistons designed for your particular use. Stock pistons don’t typically come with these design features, and hence they are one of the first elements that will limit the power you can make with a boosted stock engine. Let’s review some piston basics to help us understand what piston features are necessary in a boosted motor. The basic piston engine has been around for nearly 140 years. While the concept of using a piston as the bottom half of the combustion chamber hasn’t changed, the design and construction has. Over the years, piston skirts have shortened for less internal drag and frictional losses. This means that piston-to-wall clearances have gotten tighter to stabilize the piston in the cylinder, which minimizes piston rocking in the bore.
Factory engines built over the past several decades have had to be emissions certified and are also designed for maximum fuel economy. A critical consideration for heat management and emissions is the location of the top ring relative to the piston crown or top. If the top ring is located closer to the top of the piston, there is less area to become a combustion dead spot that resides between the piston crown, the cylinder wall, and the top ring. That little crevice creates a hiding place for airfuel mixtures to escape complete burning during combustion and therefore cause higher emissions. While the area is relatively small, the amount to consider is multiplied by the number of cylinders times the engine speed and then it becomes a significant amount. Unfortunately for turbo enthusiasts, this causes a thinner piston crown to withstand boosted engine pressures, plus it exposes the top ring to higher heat from combustion. Pistons have been cast, forged, and made from powdered metal. Piston materials in the past were alloyed using aluminum, like SAE 332, which is called hypoeutectic. It contains between 8.5 and 10.5 percent silicon. Many current production designs will use more eutectic alloy pistons, which have between 11 and 12 percent silicon, while hypereutectic pistons will have 12 to 16 percent silicon. The silicon content improves strength in high-heat environments. It also reduces the coefficient of thermal expansion, allowing tighter tolerances without excessive pistonto- wall scuffing. The hypereutectic alloys are also about 2 percent lighter than standard alloys. The stronger alloy also allows thinner castings for further weight reduction, as well. Hypereutectic pistons are more difficult to cast because the silicon does not always disperse itself evenly throughout the aluminum as it cools. Some of these pistons are heat treated for additional strength. The T-6 heat treatment often used on performance pistons will increase strength by as much as 30 percent.
However, nothing replaces the forged piston for ultimate strength in a high-boost engine. When in doubt, go for the forged pistons, but recognize that your cylinder wall clearances may be different than what the manufacturer recommends if your piston selection includes an alloy shift. While heat-treated hypereutectic pistons should work well for the high-horsepower street machine, forgings should be used on all drag motors. The main thing to remember is that the alloy used is a big determining factor when sizing your cylinder bore for proper pistonto- wall clearance. Be sure to ask the piston manufacturer for their piston- to-wall clearance recommendations, and ask a few successful racers running the same engine too.
Strezo of DLS Engine Development prefers JE pistons for his competition engines. He also uses Lo-Ko coatings to coat the piston skirts for reduced friction and a ceramicmetallic coating for the crown to create a thermal barrier. The thermal barrier helps keep energy in the cylinder where it can do more work, plus it keeps the piston and rings running cooler. There is a downside for this kind of piston coating. The tendency for detonation increases due to the higher heat in the combustion chamber. For this reason, it may not be wise to use coatings on street-driven engines because fuel octane will become a bigger problem. One way of combating this condition is to pull timing out by retarding it a bit, but then you may have just moved away from your optimum tuning point, as well. The best plan is to keep the piston coatings on the strip, not the street.
Lo-Ko Performance Coatings, Incorporated of Oak Lawn, Illinois, uses a poly-ceramic coating for the piston crown, and a special blend of four compounds including Teflon for the skirts that both reduces friction and improves heat transfer from the piston to the cylinder wall. This formulation is superior over Teflon alone. Testing performed by Indy car race teams quantified about a 2 percent horsepower gain, partially from the heat kept in the combustion chamber, but also from the reduction of friction between the piston skirt and cylinder wall.
The JE piston used by DLS Engine Development on the left is finished and ready for build, as compared to a non-prepped piston on the right. Note the ceramic coating on the crown and the black low-friction coating on the skirt.
This JE piston is specially designed for competition turbo engines and has the top ring located approximately 1/4 inch down from the piston crown.
Shown is a stock piston from a Buick 3.8-liter naturally aspirated engine. Note the thin piston crown thickness (1). This piston would not stand-up well to high boost.
This piston section is from a 1987 3.8-liter factory turbocharged Buick Grand National engine. Note the more robust design, where the piston has a thicker crown (1) and heavier top (2). Also note the dished top that lowers static compression ratio for the turbo boost (3).
This is a sectioned JE competition piston used by Dan Strezo. Note the thick crown (1) like in the factory turbo piston, the similar dished top (2), and also note what’s called a reverse dome (3) for added strength where valve relief is machined for high-lift cams used in competition.
According to John Vander- Meulen, president of Lo-Ko Coatings, when you use this coating, he recommends increasing the pistonto- wall clearance by an additional 0.0005 inch. The coating bonded to the skirt is thicker than some factory coatings. Lo-Ko adds about 0.0007 inch per side, or 0.0014 inch on the diameter. John says that about 0.0001 to 0.0002 inch will abrade off during engine break-in. The remaining increase in diameter tightens up the fit, but this is offset by the fact the piston is now running cooler and will not expand as much.
Compression ratio is a major concern when building a boosted motor. Again, your intended use and fuel selection will play an important role in this choice. There simply isn’t any magic formula for proper compression ratio because there are too many variables to consider. If you’re going for a higher boost on the street, you’ll want to stay lower on the compression ratio to avoid detonation. However, if you’re contemplating a mild boost of say 7 to 10 lbs, a static compression ratio in the range of 9 to 9.5:1 may prove to give you better all-around drivability and off-idle acceleration. Basically, the higher the boost level, the lower you’ll want to be on your compression ratio.
Other variables will include cam grind, cam timing, combustion chamber design, ignition advance, fuel octane, vehicle weight, and others. Since each engine design varies as to how sensitive it is to detonation, it would be wise to research your particular engine and find out what others have learned about optimum compression ratio for your particular boost pressure level.
The necessary compromise is that a higher compression ratio will launch the vehicle harder and make your throttle more responsive, but if it gets too high, you’ll enter detonation at max boost levels, which will destroy power development and potentially the engine along with it. For the average street motor you should stay in the 8 to 8.5:1 range. If you have a fairly lightweight car you can probably get away with 9:1 or a little more, but watch your boost and detonation threshold. If you’re running on the strip, iron heads can go above 9:1, while aluminum heads, used on most sport compacts, can typically run around 9.5:1.
The guys at DLS have an interesting approach where they feel that, in a drag motor, you still have to launch the car and you’ll need the static compression to build horsepower before the turbo comes on to boost. There are some racers running upwards of 10:1, which is pretty high compression for a boosted engine, but they are typically running alcohol or very high-octane gasoline.
Never modify any type of domed piston by machining it down to lower the compression ratio. If you do, you’re asking for trouble. That will weaken the entire structure by making the crown thinner and narrowing the top ring to piston crown strength. Short cuts of this type intended to save money can end up costing you more before it’s all over. Be cautious about building your engine based upon the compression ratio rating for your pistons when you purchased them. This is an approximate rating. Variations in production tolerances will allow combustion chambers to vary in size. You also have to consider the head gasket thickness, which may be a special consideration in a boosted engine due to the popularity of using thicker copper gaskets, along with the deck height of the block. These things all go into determining the actual compression ratio for your engine. If you’re building a serious boosted engine, then you should also consider a complete blueprinting, which will allow you to more accurately calculate your compression ratio.
Blueprinting an engine is a commonly overused term and means much more than just making the combustion chambers equal volume and balancing the rotating and reciprocating parts. It’s all about making sure that every single dimension throughout the entire engine is precisely what the blueprint for that engine design says it should be, size, weight, clearances, etc. It’s beyond the scope of this book to go into blueprinting details, but consider it an area worth chasing if you’re out to build a serious motor. Even if your plans don’t include blueprinting, you should accurately measure your compression ratio to make sure you know where you are when building a boosted engine. This also includes making sure that all combustion chambers are exactly the same size for balanced power production and to ensure that you know the compression ratio in all cylinders. Balancing the engine’s rotating parts is important, too. This isn’t just about life at high speed; a balanced engine will yield more horsepower.
It would be very easy to think you’ve got a 9.5:1 motor, but in reality the final build is actually only 8.9:1 and you’re just not running with the pack. If you haven’t spent the time to learn your engine, you’ll be chasing tuning features endlessly trying to find the right combination to compete, when the answer may be in your original engine build. The process of actually calculating your compression ratio appears later in this chapter.
Piston rings are an extremely important consideration as well. Consider the fact that all the trouble you go through to build an engine and pack more air in to the cylinders to match with your proper fuel flow rate comes down to whether you can hold it altogether and keep it inside the combustion chamber. All of the work and trouble to design and tune your engine are lost if these small parts don’t do their job. Your piston rings will make or break your motor’s success.
Pistons typically use three rings: a compression ring on top, a second compression/scraper ring, followed by an oil control ring on the bottom. Piston rings have three major design considerations. They must be able to seal the piston in the bore, they need to dissipate heat from the piston to the watercooled cylinder wall, and they must have the proper tensile strength to withstand the loads the engine will see including some percent of detonation. In a boosted engine, those traits are all the same, except more severe. For this reason there are several types of materials used for success in engines that have extreme demands in these areas. The correct choice of piston ring alloy and ring dimension needs to be matched with the engine’s intended application.
Some production engines have gone to thinner and lower tension rings for improved fuel economy. Piston ring to cylinder wall contact makes up nearly 40 percent of an engine’s internal friction loss. That’s a major design consideration when your design objective is emissions and fuel economy. Further, estimates range that between 60 and 70 percent of the piston’s heat is rejected through the piston ring to cylinder contact. If a piston ring fails to do one of its jobs, like transfer heat from the piston, failure can occur. Older ring designs were cast iron and they worked well because the cast iron was soft and seated in the bore rather quickly. Cast-iron rings aren’t really suitable for high-performance engines. They are brittle and have a melt temperature of approximately 2,000 degrees F. Chrome-plated iron rings are stronger and their melt point is about 3,200 degrees F. Nodular-iron rings coated with Molybdenum, or Moly rings, are commonly used in competition engines. The Molybdenum has a melt temperature of over 4,700 degrees F. The double Moly ring sets use a Moly top and second compression ring combination. These are good choices for high-horsepower street/strip engines. High-boost, high-horsepower strip engines will also use a stainless top ring and a Moly second ring combination. Extremely high-boost applications will typically need the top ring end gap set a little wider to allow for greater expansion at peak horsepower. Most high-performance ring sets will come larger than what you want so that you can adjust the end gap for your particular build. This is another important area where it pays to ask the right people what works best for your engine and application. Don’t ask the counter person at the local speed shop what works best for the rings you’re buying unless they have successfully built a turbocharged engine.
The best policy when determining what variable, like ring end-gap or piston clearances, to use in an engine build is to get three sources of data. For something like a piston ring you could logically ask a techrep from the company whose rings you’ve purchased, a representative from your piston supplier, and a successful racer who runs your same engine. The reason for three sources is really quite simple. If you asked one you have no basis for comparison, unless you really know your source well. If you ask two and their answers differ, who’s correct? Asking a third source is seeking consistency. If all three sources provide the same answer, you’ve probably got good information. If two of the three agree, you know which one to toss out. If you’ve not built an engine like this before, simple logical research can quickly make you smart enough to be successful, rather than privately funding your very own rolling research platform complete with headaches and frustrations. Sometimes just knowing what questions to ask, and to whom they should be asked, is half the battle.
An engine is the epitome of the saying that something “is only as strong as its weakest link.” Many engines fail because the connecting rods cannot handle the forces that develop in a high-boost, high-horsepower engine. The connecting rod has a tough job. It must have high compression strength, high tensile or pulling strength, and be able to carry high shear forces by its mechanical design. All these features are also packed into a component that needs to be as light as possible. That’s a tall order.
There are many good name brand rods on the market. It is a good idea to Zyglo or Magnaflux your rods, even new aftermarket rods, right out of the box. If you’re intent on using a stock iron rod, be sure to polish the beams to remove the forging lines, which tend to be stress risers where the various forces can concentrate themselves and cause rod breakage. Then shot peen them to remove surface stress that can migrate internally to cause total failure. As with every part of the bottom end of a turbocharged engine, think strength.
Thanks to DLS Engine Development for this 3.8-liter rod comparison. Regardless of your engine type, your rods will look surprisingly similar (though the dimensions will vary greatly!). From left to right: stock, the Carillo H-Beam, the Oliver Parabolic I-beam, and the Crower I-beam. Dan Strezo’s favorite at this time is the Crower I-Beam because he’s never broken one.
This is a close-up of the Crower I-beam connecting rod. This rod has proven to work very well in high-boost, high-horsepower engines. You could certainly do a lot worse.
The super strong Giannone connecting rods are made by Giannone Performance Products in Glendale, California. They are used by DLS Engine in extremely high horsepower engines and are a favorite among many racers in the NHRA Comp Eliminator ranks.
The Giannone rod end and cap uses a series of interlocking labyrinths to precisely align the rod and cap together on both the X and Y axis, which helps speed up engine build while insuring a perfect fit. It also allows an even thrust surface between rods sharing the same main bearing journal.
With that said, the vast majority of turbo engine builds require forged aftermarket rods. For mild to moderately boosted engines, a good set of forged rods is a must. As with pistons, they are the ultimate in strength. The first step up is the forged H-beam design, available from many manufacturers. Some manufacturers also offer forged I-beam rods for even more strength. Rods get their “H” or “I” designation from their shape (see the nearby photos for examples). A little research into your particular engine will probably reveal which rods can handle which power levels.
Rod bolts are commonly the weak link in connecting rods. If you’re using stock rods it’s a good idea to upgrade to larger size rod bolts. Companies like ARP offer oversize rod bolts. Many engine builders always replace rod bolts at every engine build, while still others will magnaflux all rod bolts new or used as a form of insurance and quality in their build.
Cylinder Heads & Valves
I’ve heard it said that in a turbocharged engine it really isn’t necessary to do all of the porting and polishing to heads that are typically done in a naturally aspirated race engine. Not true! What is true is that a turbocharged engine can achieve higher volumetric efficiency with stock ports and combustion chambers than a naturally aspirated engine with gazillion-dollar heads. A turbocharged engine is boosted to achieve over 100 percent volumetric efficiency, and head work will take that further. While the improvements won’t usually add as much in percentage of gain as they will in a naturally aspirated engine, they do help, especially if you’re talking about competition.
First, let’s review volumetric efficiency again. Simply stated, VE is the measure of how close the actual volumetric airflow rate is to the theoretical flow rate. Few naturally aspirated engines get past about 90 percent, and most fall below that. The turbocharged engine will exceed 100 percent VE since the turbo is cheating the formula by forcing the air into the cylinder under pressure.
Dan Strezo’s turbocharged 3.8-liter race heads are completely ported and the combustion chambers have been polished. These heads have been professionally prepared and are extremely expensive. This level of investment is not for everyone. But there are several modifications that you can do.
This is a close up of Strezo’s 3.8-liter head’s combustion chamber. The valve guide castings have been completely ground away then new valve guides pressed into place. The intake and exhaust valve openings are Siamesed and are as large as the head design will allow.
The turbocharged engine actually has static pressure sitting on the manifold side of the intake valve such that as soon as the valve opens it fills the cylinder. It’s much like the garden hose already filled with water pressure and the spray nozzle closed. When you squeeze open the spray nozzle all the way then let it close again you get a displacement of water much like the cylinder filling process. If you repeat the same process, but do it twice as fast, you’ll get about half as much water because the orifice wasn’t opened as long. That’s no real revelation, but it’s the same principal in an engine in terms of the challenges to fill the cylinder. It’s pretty obvious that if we used a spray nozzle that was twice the size of the first, we would get more water displaced for the same valve opening duration. What it comes down to in part, is time. The faster an engine revs in RPM the less time there is to charge the cylinders with air and fuel, and that’s the whole objective to begin with. An engine’s ability to develop horsepower at higher RPMs is limited by its ability to breath the air-fuel mixture when there’s so little time to get the cylinder filled. But we can’t help the fact that time is a function of engine RPM, or can we? That aspect is an important component of basic cam design to be discussed later in this chapter.
The other restrictions to flow are the obstacles in the path of that flow, as well as the orifice size. The air sees obstacles and they slow down the flow. In the cylinder head, everything is an obstacle, the valve, the valve guide casting, even the port wall itself. Opening up those paths and smoothing their way for less drag on the boundary layer will create an easier flow path for the air, turbocharged or not.
Of course, I’m not condoning a complete port-hogging session. Head modifications are time consuming but important to overall airflow. The head, port size and shape, and valve orifice are major controlling factors to cylinder filling. All of the subtle head modifications you do to a high-horsepower, naturally aspirated engine are worthwhile doing to your turbocharged engine if you’re chasing ultimate power.
If you want to improve your flow, but you don’t want to spend the money on high-dollar heads, there is much you can do yourself. You may say your valves are as large as they can be, what else can you do? You can still open your port orifice through some manipulation of the valve edge and seat. Many valve seats are about 3/32- inch wide, but an adequate seal can be achieved with a seat width as little as 1/16 inch, or even less. The valve’s seat is where and how the valve cools itself. Narrowing the seat width can be a dangerous proposition, and you’ll need to use high-temp valves. It also dramatically increases the psi on the valve and weak valves may not stand up to the pressure. This can be a risky modification, but if you’re racing that’s part of the game. Check with the valve’s maker whether such modifications will be compatible with your engine and valve selection; it may save you down the road, literally. This modification is not extremely difficult, but it must be done carefully.
For higher airflow, open up the intake port such that “B” is wider than “A,” and reduce the valve diameter such that “D” is smaller than “C.” The upper sketch shows the valve and seat in a standard grind. The dotted lines illustrate how the seat is to be narrowed. This illustrates how the same size valves can still allow for a larger orifice through modifications that will allow more airflow. This is not an uncommon trick, but one many don’t understand. The point is that it’s not just the valve size but the way the valve grind is done to maximize port opening.
You can reduce the valve diameter by mounting the valve in a grinder and removing the desired amount of material. The outer edge should be rounded off using a fine emery cloth to create a nice smooth radius, but the valve should not be reduced in thickness since the radius is used to merge the narrowed seat into the valve body.
The combustion chambers need to be measured and matched for calculating final compression ratio. The process of matching combustion chambers is not too terribly difficult, but it is very time consuming and will give you an appreciation of why big-flow heads cost big dollars— they’re very labor intensive. To match the combustion chambers, all you really need is a small air compressor, a pneumatic hand grinder with emery rolls and arbor, a calibrated burette that has about a 100-cc capacity, and a round piece of Plexiglas about 6 inches in diameter with a small hole in the center. Install the modified valves with springs that have enough tension to make the valves seal. Secure the cylinder head on a bench, gasket side up so, that its mounting surface is perfectly level. Modify a spark plug by filling it with Epoxy so that it contains no volume and plug the first combustion chamber.
Use Vaseline to seal the gasket surface around the combustion chamber and place the Plexiglas onto the head’s surface so that the combustion chamber is sealed except for the hole. Fill the calibrated burette to its maximum value with a mixture of kerosene with enough ATF for easier visibility. Position the burette over the hole and fill the combustion chamber completely to the bottom of the hole leaving no air pockets. Subtract the reading left on the burette from the starting point and that’s your combustion chamber volume for that chamber. Repeat this process for each combustion chamber and record their readings, marking the size right on the gasket surface with a permanent marker.
Starting with the largest chamber, polish all the sharp edges and remove just enough material to smooth the casting. Measure that cylinder again and that becomes your target size. Move up to the next largest combustion chamber for the best feeling of how much material to remove. Polish and measure back and forth until you’ve finished the polish and the chambers are all the same size. The maximum variation you’re looking to achieve varies by builder, but a good reference would be one-half of the finest graduation on your calibrated burette. Now you can calculate your actual static compression ratio.
The method of sealing your heads to the deck of the block can vary. People have used thick copper gaskets for many years. These are often used in conjunction with a groove cut into the cylinder head where a small wire ring is placed to allow compression into the copper material as the head is torqued into place. This is often referred to as O-ringing. Any engine producing 200 hp or more per liter of displacement should definitely use a positive type head gasket seal such as Oringing. Head studs are also recommended for true torque and even clamping force.
The deck-sealing surface has been prepared for an O-ring seal. Most any competent machine shop can perform this modification.
The Cometic gasket company also makes a triple-shim gasket that is slightly larger in the bore area. This area accommodates a nitrogen-filled O-ring. The O-ring is coated with silver for better sealing. Not all engines can use this design. If the cylinders are especially Siamesed, then there is insufficient room for the design to work. However, if your engine will accept it this process works well and will save time and machine shop money relative to traditional O-ringing.
Measuring Compression Ratio
Measuring your actual compression ratio isn’t really that difficult, it’s just time consuming. Compression ratio is simply the total volume of the cylinder and combustion chamber when the piston is at bottom dead center (BDC) divided by the volume of the combustion chamber when the piston is at top dead center (TDC). The time consuming part of this exercise is the measurement of a the elements required.
The formula looks like this:
CCV + HGV + DH + PDV + CV
------------------------------------------ = Compression Ratio
CV + HGV + DH + PDV
CCV = Combustion Chamber Volume
HGV = Head Gasket Volume (compressed)
DH = Deck Height of the Piston Crown
PDV = Piston (Dome or Dish) Volume
CV = Cylinder Volume
You should have the CCV value from when you adjusted the combustion chambers to equal volume. If not, you’ll have to find an accurate figure for your specific engine. Your HGV is an easy issue; simply measure the thickness of a used head gasket of the type you intend to use. The compressed thickness may also be available from the manufacturer. Then apply the calculation to find the volume of a cylinder using your basic geometry because the head gasket is just a very short cylinder.
Volume = πr² x Height
a simpler version:
Bore² x stroke x 0.7854 = Cylinder Volume (The .7854 is a constant, equal to one-quarter the value of π)
The DH (deck height) is simply bringing the piston up to TDC using a dial indicator and then measuring another very short cylinder, which is the distance from the deck down to the piston crown. Your dial indicator can do this job easily. Now that you have the height, you use your cylinder bore and calculate the volume using the same formula shown above.
The PDV is perhaps the trickiest to obtain (assuming you now know your head cc value). If you have domed pistons, which you shouldn’t have, the dome is a takeaway from your volumes. In this case, enter it into the formula as expressed, but as a negative value. If you have a dish, or reverse dome as they are sometimes called, it’s a positive value. You can obtain the dome/dish value a couple of different ways that are relatively easy. The easiest way is to call the manufacturer and ask them. If this isn’t possible, you can make an impression of the piston dome in a large mass of modeling clay and use your calibrated burette and measure the volume of the indentation yourself. If it’s a dish, use your combustion chamber measuring equipment again and level one piston.
Your cylinder volume is easy, all you need is your bore and stroke. Run the numbers through the formula used for HGV and DH. Now you’re ready to run the math!
Fortunately, when adding a turbocharger to a street-driven engine, the stock camshaft will normally do nicely. This aspect, along with the stock compression ratios ranging 8.5 to 9.5:1, is what makes a mild turbo setup mostly an external installation and supplemental tuning issue. But if you’re building an extremely highpowered engine, the cam, like most internal engine parts, should be optimized. Certainly all of the various components you plan to use contribute to developing higher torque and horsepower. However, to develop the torque and horsepower at a certain RPM range—the cam becomes critical. This is true regardless what type of camshaft you’re using, conventional push rod type engine, SOHC, or DOHC. We’re talking about the control of the valve events relative to piston position and the quality of this critical time metered event.
As we discuss cams, a certain level of basic camshaft knowledge will be assumed since this isn’t a chapter dedicated to all the design theory relative to camshafts. I’ve seen it written many times how cams are one of the least understood components in the engine, but it really doesn’t have to be that way. Understanding cams can become easier if you take the concept one step at a time. The cam is a very old mechanical concept that simply transforms rotary motion into linear motion. In other words, the cam turns around and creates linear motion to operate the valves. The motion is transferred to the valve either through a pushrod- to-rocker valve actuation or directly onto the valve in the case of an overhead cam engine. Since the four-stroke engine must complete two full revolutions for every cylinder to complete all four cycles, the camshaft is geared at a 2:1 ratio, meaning it travels at 1/2 the rotational speed of the crankshaft. Since the valve actuation is critically timed relative to piston position, cam specs that discuss duration, or the time the valve is actuated, are always expressed in terms of degrees of crankshaft rotation.
The valves are open for just fractions of a second at high speed. How far the valve opens and the characteristic of how long it’s open to its maximum lift is critical to VE whether the engine is turbocharged or not. Maximizing volumetric efficiency is the primary design objective of any camshaft. Regardless of cam design, there is only a maximum amount of time the intake and/or exhaust valve can be opened. Of course, the time varies relative to engine speed, because the cam doesn’t open the valve for a certain time, it opens it for a certain number of degrees. Longer duration means the cam is opening the valve longer for a given RPM. Maximum lift, the linear distance the valve moves, is also dictated by the cam.
Remember that the piston in an engine is almost always accelerating, positively or negatively (acceleration = rate of change of velocity). When the piston is halfway down the bore, the crankpin is almost moving linearly, which creates the maximum speed of the piston. If the intake valve is wide open at that point, the valve opening opportunity will be maximized because the valve will be at its max opening when the piston is traveling the fastest. This rapidly moving air develops a considerable amount of force (force = mass x acceleration). However, once the piston is at or near BDC, it has slowed down considerably. Due to the inertia of the intake air, there are several degrees of crankshaft rotation, even after the crankshaft begins forcing the piston back up the bore, where having the valve open will aid in cylinder charging.
It’s been said that the most critical of all valve events (as it relates to making horsepower) is the point of intake valve closure. At slower engine speeds, the cylinder has more time to charge, and the intake air carries very little inertia. Consequently, the intake valve likes to close earlier before the pressure of the piston coming back up the bore forces intake air back out the intake valve. This creates a smoother idle quality. In a high-RPM engine, closing the intake valve later maximizes the intake air inertia and additional cylinder charging is accomplished because this high-speed air will have a force that is greater than the force exerted by the piston on the air charge in the cylinder, to a point. This is why performance cams will typically have an ideal RPM range where their duration is more compatible with higher engine speeds, yet cause an unstable and erratic idle quality.
The dynamics are a bit different in a turbocharged engine. Since the intake air charge is now forced in under pressure, the air charge pressure in the cylinder rises at a faster rate, so people usually want to close the valve a bit sooner. This is also why a stock camshaft works very well with a forced air induction system. But the basic principle of delaying the intake valve closing for higher RPM operation still holds true. Remember that an intake valve is a time metering device. In an extreme engine, such as a Bonneville racer, the primary concerns are power development at sustained high speed obtained from maximizing VE. Even though you have boost pressure present, the amount of time you have to charge the cylinder is a constant challenge to horsepower development. In road racing, torque development at lower speeds is critical for acceleration, and you want the engine to move up and down the RPM band freely.
Valve overlap is another critical factor for turbo cams. Valve overlap is a situation where both the intake and exhaust valves are open at the same time. Usually, when the exhaust valve is almost closed, the intake valve begins to open. The duration of crankshaft degrees rotation when both valves are off their seats is referred to as overlap. In a naturally aspirated engine operating at high speed, there is a bit of cylinder scavenging done during overlap where the force of the incoming air helps to fully exhaust the cylinder. In a turbocharged engine, this happens very quickly since the intake charge is boosted. If the exhaust valve remains open too long, it will allow the cylinder to loose too much cylinder charge. For this reason, the lobe centerline angle is usually 110 degrees or more to minimize the amount of valve overlap.
The shape of the cam lobe is critical as it relates to the quality of the time-metered event of valve opening. Two cam lobes with the same lift and duration could have completely different mechanical profiles. A given cam could open and close the valves much faster, thus lengthening the time the valve is at max lift. This maximizes total average opening size of the timemetered event. This also means the valve will be traveling at a much faster rate. Reading advertised lift and duration on a cam card wouldn’t tell you the shape of the lobe. Only plotting it onto a graph by taking dial indicator readings about every 10 degrees and plotting them would give you a real picture of what’s going on. This is partly why different cams that have nearly the same specs can perform differently in your engine.
Cams have to have entrance ramps and closing ramps. These are the gradual inclines at each end of the cam lobe that allow the valvetrain to load and unload without too much shock to the entire system. The ramps allow the smooth engagement of the valvetrain and the exit ramp keeps from slamming the valve closed. Without these features, the cam would hammer the valvetrain apart. Advertised duration used to be misleading years ago because there wasn’t a standard for what point the lift began. There is minimal flow below a certain level of lift, so the industry now uses 0.050 inch as a standard from which to reference duration. Therefore if a cam’s duration “@ .050” is 275 degrees, that means from 0.050 inch of lift to 0.050 inch before closure takes 275 degrees of crankshaft rotation.
Aggressive cams will have very steep flanks, or ramps, where the valve is accelerated to the full open position to maximize the amount of duration at peek valve lift, thereby increasing the average valve opening as a function of time. Older flat-tappet cams are somewhat limited in this area because a very aggressive cam lobe with fast flanks can virtually dig into the side of the lifter. This is the reason most modern engines use roller-tappet cams. It’s a common notion that roller cams reduce friction. While that’s theoretically true, it’s not the real reason for their use, they help us use a better cam profile. Cams are typically thought of in terms of lift and duration, as these elements focus on how long the valves are open. That’s logical, but how long they’re closed is critical, too. The average street enthusiast typically doesn’t think about it, but the second most critical valve event is the opening of the exhaust valve.
The exhaust valve will typically begin to open before the piston reaches BDC. Holding the valve closed longer allows more total combustion pressure to be applied to the piston, raising the mean effective pressure, which translates directly into horsepower. However, opening the exhaust valve too late causes the piston to do more of the work to push the expanded gases out of the cylinder and thereby gives up more energy to pumping losses. There’s a happy medium. In the case of a turbocharged engine, the burn lasts a bit longer due to the increase in massflow. If the exhaust valve opens too early it will blow-down the cylinder and allow the engine to lose BMEP (brake mean effective pressure).
Two cams can have the same lift and duration as shown, but they will perform very differently. The blue lobe obviously has the same total duration as the red, but you can see more duration in the blue lobe at full lift.
This is a graphical representation of the two cam lobes illustrated in the picture to the left. Using a dial indicator and a degree wheel and placing the cam between centers will allow you to obtain the data necessary to make this type of graph for your camshaft. By taking dial indicator lift readings every 5 to 10 degrees of rotation, you will be able to plot your cam on a graph. Analyzing your cam in this manner versus other cam grinds you’ve run, and comparing them relative to your track results may help tell you what your engine wants in the way of a cam grind for best performance results.
Most cams are ground in a symmetrical manner, meaning that the profile of each lobe looks the same either side of the cam centerline. While this is typically the case, it isn’t always true. The conventional wisdom for turbocharged engines, such as less overlap, less duration, and closing the intake earlier generally apply to the masses of street and street/strip engines for good allaround drivability and power. That’s logical, but there is another side. If you’re building a race engine, and you have both access to a dyno and a decent budget, you can do better. The conventional wisdom doesn’t have to apply to you. In a conversation I was having with Gale Banks, I taunted him with a question intentionally filled with conventional wisdom. I asked, “In a turbocharged engine you don’t want to hold the intake valve open as long as you do in a naturally aspirated engine, do you?” Gale never concerns himself with conventional wisdom; he likes to push the envelope. His response was, “Why not?”
In extreme classes of professional motorsports, like drag racing or Bonneville type cars, the conventional wisdom goes right out the window. These cars don’t have to operate on the street and have good idle quality and pull vacuum for accessories. They’re purebreds, built for speed and power.
In these situations, testing cams with progressively longer and longer duration would likely pay dividends. One way of predicting justifiable increases in duration would be to locate a pressure tap either inside the port of the cylinder head as close to the valve as possible, or in the intake manifold as close to the head as possible, where it could not “see” adjacent ports. If your engine was calculated to run at say 9,000 rpm and you were testing a long intake valve duration that closed the intake very late, you could possibly see this using a pressure transducer. You would be looking for a 75Hz signal that would tell you the valve is closing too late. If at the speed range you intend to run, the signal does not reach this level, you can change cams and test progressively later and later valve closure to maximize the time the valve is open. Now that’s tuning! Figuring the frequency you are looking for would be:
Engine RPM / 2 = number of intake pulses per minute
Intake pulses per minute / 60 = Hz
The hardest part may be to find a cam grinder who will make your custom grinds. It’s probably not likely you’ll find anyone who will set up to make special one-off asymmetrical lobes. But Dave Crower of Crower Cams told me that if someone was doing this sort of tuning they can typically alter cam centerline angles, adjust duration, and recommend the installation advance or retard to basically achieve what kind of valve timing events you’re looking for.
One interesting item to note is that the Banks Sidewinder Duramax diesel will produce a pressure wave that you can feel with your hand 12 inches in front of the turbo air inlet when the engine is at idle. But as Banks says, “Who cares what a competition vehicle runs like below its intended operating range?”
Written by Jay K. Miller and Posted with Permission of CarTechBooks
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