The valve angles on stock big-block cylinder heads are as follows: The intake is at 26 degrees incline with 4 degrees of cant while the exhaust is 17 degrees incline with 4 degrees of cant. Throughout the book I refer to cylinder heads that directly replace stock heads at 24 degrees. The reason is that 26 degrees is too much and the flatter angle means that valve cutouts would be less severe and the chamber could be reduced in volume more easily. I am not sure who started the 24-degree trend (it may have been Dart) but most of the industry followed suit.
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Port Shape and Volume
This section quantifies a few major aspects of big-block heads, so you have better command of cylinder head decision making and can determine the best modifications to make. A number of prime elements concerning heads need to be dealt with. But first I want to reiterate the fact that the Chevy big-block has pitifully small valves for the displacement available. As a consequence, you have to make well-educated cylinder head choices or an already difficult situation just gets worse. What you are after is the best possible combination of airflow and port velocity. The list of relevant subjects includes port shapes (oval or rectangle) and port volume. Determining port volume is critical, so you need to know when big is just too big, matching flow curves and port volume to cam lift, and finally the compression ratio. Knowing where you are going and making good choices in these areas will, on a typical 496, add as much as 60 ft-lbs and 80 hp. For the most part, that extra output often involves no extra cost.
Fig. 4.1. This 100-cc chamber (as used in a rolled Edelbrock E-Street head) might not look like much, but if you are on a tight budget, it’s an excellent choice and could mean a 15- to 20-hp increase for your dollar-constrained build.
Many novice Chevy big-block engine builders see the big rectangular port as the big-horsepower maker and the oval port as its poor cousin. If that is what you were thinking, it’s a concept you need to drop right now. When only factory-built heads were available, rectangular ports were a quick fix for more top-end power for NASCAR racers. A factory-pattern rectangular port is not the best shape to have when power is the prime requirement. It’s the correct choice only if the needed port flow cannot be found by other means.
However, I should not paint all rectangular-port heads with the same brush. The basic problem with the big rectangular-port factory heads is that it flows very poorly on the port floor, so it is a sluggish port that does not ram very well until high RPM is reached. Some of those early rectangular-port heads actually made as much as 25 hp more with the bottom 1/4 to 3/8 inch of the port filled in. Doing only this marginally reduces flow but it does considerably aid what seems to be a vital Chevy big-block power production requirement, namely, port velocity.
To sum up the oval- versus rectangular-port debate, it’s my opinion that the big factory rectangular port is a poor fix for not putting enough flow bench time into the design in the first place. If enough time had been spent, you would have seen something closer to the current oval-port designs. Unless the cross-sectional area required of the port for the build spec exceeds the area that can be had with an oval-port head there is no justification to use a factory-style big rectangular port.
If you start looking at port shapes only, you find a progression from a true oval to a hybrid oval/rectangular port and, finally, a rectangular port. Many heads have what is basically a rectangular port, but may also have a relatively small-volume port. Plenty of aftermarket heads fall into this category. The heads up to about 300 cc from Air Flow Research (AFR), Brodix, Edelbrock, Pro-Filer, and Trick Flow Specialties (TFS) are prime examples. Such heads can deliver impressive results, not because they are a rectangular design, but because the volume is appropriate for the application.
You should spend a day at the tech desk of any cylinder head manufacturer. If the customer on the phone is considering the purchase of a set of Chevy big-block heads, the first question is often, “How big a port volume do I need?” The tech guys usually give them good advice, but so many feel that they do not want to recommend a head that’s too small and doesn’t deliver top-end horsepower. So they often recommend heads with big ports.
However, large-port heads do not perform well for low- and mid-RPM street driving. Although it seems a logical way to view the situation, overly large is in fact the wrong way. If you over-estimate the port volume and, consequently, the port’s cross section, the price you pay is at least a reduction in output until the very top end of the RPM range. But even that is a generous viewpoint.
I have seen heads swapped out for ones with as much as 20 cc less port volume, and the whole power curve from low to top improved. I’ve swapped out 345-cc port heads for 325s that flow even slightly less air on the bench but have seen a 25-hp increase and a gain of 30 ft-lbs in the 2,800- to 3,000-rpm range.
To help you make a choice here, check out Figure 4.2 and be sure to read and absorb the caption. From the examples, you can see that a big-port volume does not show up as big-horsepower results. It really is a question of being conservative to achieve the best results.
Most engine builders focus on the high-lift flow numbers to estimate the head’s power potential. Although helpful, it is more meaningful to consider high-lift flow in conjunction with the 0.200-inch-lift flow figures. The flow numbers and dyno results of numerous head tests indicate that the best peak and average outputs are seen when the 0.000 to 0.200 flow numbers are high, and the reason seems to be the short rod/stroke ratio. When this is the case the piston hangs around BDC longer.
When this condition is combined with an under-valved big-block Chevy, good low-lift flow helps fill a cylinder better toward the end of the induction stroke. In other words, when the rod is short, good low-lift flow helps make up for a deficit during the main part of the induction stroke.
Many successful head porters claim low-lift flow is not important even to the extent that too much low-lift flow actually costs power. Here is the other side of the story. On four occasions, my engines have won every single race for a championship win using heads that have outstanding low-lift flow. For the record, a straight A-versus-B dyno test of good low-lift flow versus average low-lift flow is invalid. (For more information see Chapter 9, Camshafts and Valvetrain Events.)
Fig. 4.2. To determine required intake port volume, first decide on your target horsepower; be realistic or it will be a step backward. Next, reference the output number on the left-hand scale. When you have located it, go across the graph to the right until you intersect the green zone, then drop down to the lower scale. That is the port cubic centimeters required for a 24-degree head.
Just where in the green zone you need to drop to the lower scale depends on the compression ratio and cam spec. High-compression engines, especially those with a well-spec’d cam, tend to be toward the upper edge of the bright green zone. A typical street engine falls toward the bottom of this zone.
Although it is possible to build engines that fall toward the extremes of the blue lines, the majority of well-built engines have a port volume-to-horsepower ratio that falls somewhere within the bright green zone. The best engines are those that have port volumes toward the top of the bright green zone. This indicates that selecting too much port volume is not a good move by any standard.
To give an idea of how this works in practice, let’s look at some engines that have produced some creditably strong results: a 305-cc port producing 825 hp, a 355-cc producing 1,080 hp, a 280-cc producing 685 hp, and a 320-cc producing 880 hp.
Flow versus Power
The most common question concerning cylinder head airflow is, “How much horsepower does a certain flow capability support?” My typical and somewhat cynical response is: “That depends on how badly spec’d the rest of the engine is.” For the moment, let’s assume the rest of the engine is spec’d correctly, including the induction system, valve events, piston selection, compression ratio, and the header and exhaust system. On a 10.5:1–CR street engine, you can expect that for every 3.8 to 4.2 cfm available to the cylinder at full valve lift, it generates 1 hp. So, assuming a middle-of-the-road figure of 4 cfm per hp, and a typical intake flow of 390 cfm at 0.700 peak valve lift, the power potential would be 97.5 hp per cylinder (390 ÷ 4).
From all eight cylinders, you could reasonably expect to see 780 hp (97.5 x 8). But there is a caveat: These flow-to-horsepower figures only work as long as available flow is not needlessly squandered; that flow is needlessly squandered if you use heads with inappropriate-spec parts elsewhere in the build.
Lobe Centerline Angle
Almost certainly, the selection of an inaccurate cam is the worst mistake made when building a big-block for maximum output for the application. And I don’t mean choosing a cam with too little or too much duration but choosing one with a lobe centerline angle (LCA) that isn’t even close to correct.
Another important point is: You cannot necessarily avoid the incorrect cam spec by calling a cam company for advice. Doing so still risks a 75-percent chance of falling into a cam-selection black hole. An LCA selection mistake costs a substantial amount of horsepower and torque. Your low-buck 468-ci street build may only make 525 hp and 530 ft-lbs while a cam costing exactly the same money with exactly the same intake and exhaust profiles (but having the correctly timed valve events) for the head/short-block combo can deliver 600 hp and 600 ft-lbs.
Fig. 4.3. For this test, the stock 2.06-intake valve was replaced with a 2.190 valve, and the new seat was blended into the existing bowl. A limited amount of pocket porting was also done on both the intake and exhaust. The dotted lines show the stock heads’ flow (average of good/bad intake) and the solid lines indicate the reworked heads. The green curve is the percentage of improvement of the intake flow. Notice that with the stock port the flow improvements were mostly confined to the lower lift range. This means that any flow increase was achieved along with a port velocity increase.
Fig. 4.4. Here are the results of the dyno tests of the head mods that produced the airfl ow increases in Figure 4.3. The cam for this test was a 268 flat-tappet hydraulic grind on a 106 LCA. Peak torque climbed by 20 ft-lbs and peak power rose by 37 hp. At 5,600 rpm the power increased from 440 to 512 hp.
For the record, I have built 10.5:1–big-blocks that have produced 1 hp for every 3.59 cfm available at peak valve lift. Doing this means paying very close attention to every aspect of the build and understanding that the number-one point of failure is the selection of an incorrect camshaft. This means fully absorbing the contents of Chapter 9, Camshafts and Valvetrain Events, and Chapter 10, Valvetrain Optimization.
In terms of bang-for-the-buck porting, stock heads are the cheapest way to go. However, reconditioning a set of Chevy big-block heads is not cheap, and almost anything that is more than about five years old may need a guide and seat job. In addition, milling to increase the compression is usually about $150 or so and that needs to be taken into consideration when deciding whether or not to go aftermarket. In previous writings, I covered the fact that stock heads can be made to deliver some respectable results. The downside is the time involved doing porting and other work to prepare them for high-performance use.
In the past, I have also given peanut-port heads a performance rating that was probably much less than they deserve. I don’t want to dwell on these heads, but the fact they have ports of only about 250 cc does allow for an easy demonstration of the effect bigger valves rather than bigger ports has on output.
Figure 4.3 shows the change in fl ow that improved mostly in the 0- to 0.200-inch lift range. Figure 4.4 shows the resultant improvement in output. This test demonstrates the effect an increase in low-lift flow has as long as it is not compromised by incorrect valve event timing.
There are times when race rules call for OEM iron casting to be used. When this is the case, to be competitive it is necessary to have the heads ported by an expert in this field. In 2013, a racer approached Terry Walters of TWPE about building an engine for him. He needed this new engine for the class in which he was already successfully competing. Terry wanted to know if I knew anybody who could port a set of factory iron heads really well. The power figure he had to beat was 800 hp from a 468. I knew of several head porters who could do well in this situation, so I put him in contact with Chad Speier of Speier Racing Heads. Chad has run the gamut from stock to Pro Stock, and definitely knows how to make heads perform. A set of 088 heads was chosen for the work. Figure 4.8 shows the results. Undoubtedly these heads contributed greatly to the 468’s 850 race-winning hp.
Outside of following the basic porting steps shown in my previous Chevy big-block book, getting the best airflow possible requires a flow bench. But before getting into porting and flow bench testing, all of the heads I cover here typically respond very well to the basic no-flow-bench-needed porting procedures I talked about in the previous book. If you want to extract the best from any heads you need a flow bench, but if you think obtaining a flow bench is too costly, refer to my book How to Port and Airflow Test Cylinder Heads. It shows how to build a really effective bench for about $150 in addition to the cost of a pair of stout vacuum cleaners. And it can be up and running within a weekend.
Once a particular as-cast head in out-of-the-box form is tested, I almost always do a basic porting job, which involves narrowing the guide bosses and blending out the rest of the port. You can port for street-cruising applications. This means a couple of weekends’ work, which is within the capability of anyone with average dexterity.
Fig. 4.5. The intake ports started out at 318 cc on the Speier modified 088 factory casting, and the porting process increased the volume by only 10 to 12 cc. The flow increase was far greater, thus showing that the port’s velocity and efficiency had improved.
Fig. 4.6. As expected, seatwork is a measurable contributor toward making big horsepower numbers. Also note the lack of a polished finish on the ports.
Fig. 4.7. The stock intake valve measures 2.19 inches but a bigger 2.3-inch intake valve was installed. As a result, the now-closer chamber wall increased intake valve shrouding. This meant cutting the chamber wall while minimizing the amount taken out.
Fig. 4.8. To avoid a clutter of curves, the intake flow figures shown are the average of the big-block’s two styles of intake port. These are creditable figures as far as flow goes. On the dyno, the Chad Speier–modified heads proved their worth by delivering 850 hp from a 468-ci engine. The dotted lines are stock and the solid lines are modified.
Basic porting power gains are very satisfying. The heads used may have some variances. However, if stock heads on a 600-hp engine receive a basic porting exercise as detailed in my previous Chevy big-block book, the output often increases to 630 to 640 hp. As good as that looks, consider that most of the big gains from porting show up at the higher lifts. The pursuit of higher valve lifts via valvetrain parts selection becomes even more critical. This makes your porting efforts, combined with a valvetrain accessing the extra airflow, add up to yet more power.
Aftermarket Iron Heads
In my previous Chevy big block book, I extensively covered Dart’s Iron Eagle heads and I noted various design and airflow points. But since that was published, I have had the opportunity to be involved with builds that utilized World Products’ latest iron heads and the new (as of 2014) Engine Quest (EQ) heads. As I write this, I have had more experience with the EQ heads than with the World Products’ heads. As you may expect, that experience has allowed me to plumb the potential of the EQ heads more than the World Products’ heads. Although both worked well out of the box, a few days of experimentation showed the EQ heads to have great potential.
Fig. 4.9. The Dart Iron Eagle heads with 345-cc ports work well on engines from about 556 ci and more. However, if I am using Iron Eagles, I prefer the 325-cc port head for most smaller displacement budget builds.
Fig. 4.10. With a basic porting job, these 325-cc port 2.25/1.88 valve split Iron Eagles produced 370 and 282 intake/exhaust CFM at 0.700 lift. Also note the use of the small bee-hive springs. Learn how these heads allowed a 482-ci engine to generate more than 675 streetable horsepower on page 135.
Fig. 4.11. If the exhaust heat crossover passage is set up in the stock configuration, an oval-port World head can be used as a straight replacement for an old worn stock head. Even as a stock replacement there is a significant performance advantage.
Fig. 4.12. The EQ heads flowed well out of the box, but as these figures show they acquitted themselves very well when a basic porting job was done.These heads should be very good for a marine application.
If you are building a street engine and have a big enough budget, aluminum heads are the number-one choice. But rest assured, iron heads will deliver the power but carry a 75-pound weight penalty. If you are building for marine use, where lake or ocean water cools the engine, you need iron heads to combat corrosion. For this scenario Dart has heads with an anti-corrosion coating. Apart from the fact that they are made of iron instead of aluminum, you can say that in terms of power or porting techniques, what goes for aluminum is much the same for iron.
Aluminum Head Sources
Let’s cover heads having 305 cc or less for an intake port. Such heads are most likely to be used on true street engines up to about 700 hp. These are the heads that most readers should consider buying.
Air Flow Research
In the aluminum category, AFR has 265-, 290-, 300-, and 305-cc port heads. Apart from being extremely functional, AFR produces a high-quality, 100-percent American-made product at a very good price. The heads come with some of the best valve- springs installed on aftermarket cylinder-head packages. This can be a big bonus for good valvetrain control.
At one time or another, I have used heads with various levels of preparation: as-cast chamber; partially CNC-machined chamber, bowls, and port entry; and fully prepped. At every stage, AFR heads produced top-quality results, and as a result, these have become firm favorites with professional shops that pride themselves on delivering real performance. If you are buying an as-cast or partially CNC-finished set of heads, and you just have to do something with the die grinder, you will be very pleased these heads are extremely porter friendly.
AFR also cuts some of the most effective valveseats in the industry. The result is that these heads perform better than most pro engine builders would even anticipate. When making a port-volume decision, do not go too large because these ports are among the most efficient in the industry, and as such, they make strong low-speed torque as well as superior top-end horsepower.
To again illustrate the dangers of having too big a port, I have seen 265-cc port AFR heads make more top-end output than 315-cc port heads, and thus showing that they can make big numbers when used with the appropriate displacement and an appropriate cam. Still in the oval category are the 290- and 300-cc heads from AFR. Matched with the right displacement and cam, my dyno tests show you can expect equally good results from these heads.
Fig. 4.13. The solid lines are for the AFR head; the dotted lines are for the peanut-port head. Blue is intake, red is exhaust. Even though the AFR head has an intake port only 4 percent larger it flowed up to 40 percent more air. The biggest-percentage increases were at 0.150 lift, due to a superior valveseat form.
Fig. 4.14. The main point of these tests is to show how well the AFR 265 heads work (red curves). However, it is worth noting that the overall output produced by the peanut-port heads is very good, when compared to what is normally produced from typical performance magazine tests. The reason is that a typical cam of 112 to 114 degree LCA was not used. Such a wide LCA compounds the problem of small valve size, especially with the peanut-port head’s 2.06/1.72 valves. Here, a flat-tappet 274-duration single-pattern cam on a 106 LCA was used instead of the normally recommended (and wrongly so) wider LCA. Crane 1.8/1.7 rockers were used. Intake lift was 0.600.
As far as the functionality of the AFR heads are concerned note that they delivered more torque everywhere in this RPM range. Peak torque was up from 572 to 605 ft-lbs and peak horsepower was up from 525 to 610. At 6,000 rpm, the AFR heads posted a staggering 130-hp gain!
Fig. 4.15. AFR’s CNC chamber and seatwork is among the best in the industry. The seat-cutting equipment they use is the same as commonly found in the shops of top NASCAR teams and Formula 1 engine manufacturers
Fig. 4.16. This is an AFR 300-cc “oval” port. In reality, it is a hybrid “oval-rectangle” port. As such, it functions very well..
Fig. 4.17. AFR as-cast heads feature CNC pocket porting. This allows the benefits of a good seat form to be better realized.
Fig. 4.18. In an effort to keep the exhaust flow from going into reversion, I cut the chamber wall flatter in the area indicated to move the quench pad farther from the exhaust valve. Back-to-back tests show a worst-case scenario of a 10-hp increase with gains starting at about the 5,000-rpm mark.
Fig. 4.19. This is a ported pair of “as-cast” 325-cc port AFR heads. With little other than a tidying of the port finish and some work in the chamber and around the pushrod on the intake, these heads delivered an average of 387 cfm on the intakes and 298 on the exhaust at 0.700 lift. This amount of air produced good port velocity, which was evidenced by the output. It allowed a 572-ci engine that runs on 87-octane pump fuel to deliver a solid 812 hp and 799 ft-lbs of torque.
AFR’s smallest rectangular heads are commonly referred to as “as-cast” heads, but they’re actually semi-CNC finished because the bowls are machined. In addition, these heads are available with CNC-finished chambers, and for the minimal extra cost, I recommend them. Going this route gives outstanding results. If you decide to port the as-cast part of the head, you can get a fully finished set of higher flowing heads with minimal work on your part. The extra flow starts to show at about 0.400-inch lift and reaches an increase typically of 18 to 20 cfm at 0.700 lift.
I have to say that AFR’s rectangular-port heads are among my favorites to use either “as cast” or on which to conduct a simple porting exercise. I really don’t have space to cover all the relevant aspects. Suffice it to say, I have had hot street 496 builds making very close to 800 hp on the 325-cc port heads and 540 street builds using the 335 all-CNC-finished head at 825 hp. All on pump gas! If you are building a 10.5:1 632, AFR’s all-CNC-finished 385-cc port with its 2.35 intake valve is a good bet. For a well-spec’d build with these heads, you can expect to see about 950 hp.
Bill Mitchell Products
I’ve used a number of BMP 24-degree heads from late 2010 on but I have never actually done any definitive dyno tests. So I have not generated conclusive findings on these heads but I look forward to comparative testing of them.
BMP offers two 24-degree heads. The smaller-port heads have 310-cc runners and the larger-port headshave 350-cc runners. My first encounter was to rebuild and port a set of well-used (abused might be a better term) 310 heads for an acquaintance. These stock heads were hardly in any shape to flow test, but after putting in new seats and porting them, they performed very well even though there was nothing trick about the porting job done. The flow figures are shown in Figure 4.20.
These heads replaced a borrowed set of unported heads from a reputable manufacturer. Although the engine never went on the dyno, the car owner reported that his first pass with the new heads was the fastest to date. After timing and carb calibration was completed, the car went even faster, and for a low-buck effort, it was said to be “very competitive.”
Fig. 4.20. The BMP heads showed some good flow figures without any real effort going into the porting procedure. The “on track” performance reflected the high-flow figures seen for the relatively small port volume.
Fig. 4.21. This is one of the chambers of a set of 335-cc oval-port Brodix heads reconditioned by Mark Dalquist of Throttle’s Performance. I had a peripheral hand in helping out with this high-effort street engine build. I got to dyno test the final 565-ci build that used these heads. These Brodix oval-port heads are very strong on power delivery and the resultant 899 hp proves it.
I installed a set of the 350-cc port heads in out-of-the-box form on a mule engine. The engine was hardly a serious build, but it didn’t need to be. The intent was to test some exhaust system parts. For that, I needed an engine that produced at least 700 hp. The heads delivered that and then some by returning 730 hp. Such an output, for the spec of the engine, was good, and if you look at the price of these heads, the performance is even more impressive.
As I write this, I am well on the way to having a finished spec for a ported 350-cc head. Equipped with a 2.35 intake and a 1.88 exhaust, on my bench the head has so far delivered 170 cfm at 0.200 and 416 at 0.700 on the intake and 309 at 0.700 on the exhaust. These are among the best figures I have seen on a conventional 24-degree head. I use the word “conventional” because the real excitement with BMP is the 16-degree heads they introduced in 2012.
These heads accept all stock-style intake and exhaust manifolds but require the use of Jesel or T&D shaft rockers. These heads have been radically “rolled” to enable the production of a chamber of just 80 cc, with the capacity to go to as little as 70. I am currently using these heads because of their excellent performance.
I have tested and had great success with Brodix heads. However, I have not used many of this company’s seemingly limitless offerings. First, the BB1-series heads are offered with a 280-cc runner as a cast-port head or as a 305-cc CNC-machined head (the STS BB-1). As-cast flow is only moderate and thus there is room to unlock more. Fortunately, they are easy to port, and if you don’t want to do the porting yourself, the 305-cc CNC’d port flows some pretty decent numbers. However, the strong point of this head is the fact that Brodix can mill it to produce a chamber as small as about 98 cc. This makes it a great choice if you are going to rework the heads for a serious build effort on a smaller inch engine.
Another notable Brodix head design is the oval-port BB-3 XTRA O. I have experience working on this casting with Mark Dalquist of Throttle’s Performance. This head has a 332-cc intake port and similar versions are available with 351- and 365-cc intakes. Although it is called an oval port, the design is best described as a hybrid oval/rectangle port. My only experience was with the 332-cc version but I have heard from several proficient engine builders that these oval-port heads, when used in displacement applicable situations, make big horsepower numbers, especially when used in 572-inch or bigger street engines.
The highest horsepower big-block I have ever built (in 2000) used Brodix heads. The nitrous-injected engine cranked out 1,800 hp, and it was so powerful, it twisted the input shaft of my dyno almost to the shear point!
To date, I have installed Dart heads on about half of the big-block engines I have built, so I am most familiar with these heads. For that reason, I have extensively used Dart 24-degree heads, and these demonstrate many typical attributes of a traditional 24-/26-degree big-block head. If you buy Dart, you are getting heads that have benefited from a Pro Stock heritage. I have had the pleasure of working with Dart’s talented cylinder head designer, Tony McAfee, on several occasions. I see the effort he puts into his work and it shows as strong results on my dyno.
The Pro 1 as-cast heads come in four port sizes. The smallest, at 275 cc, is an oval-port design while the 310-, 325-, and 345-cc heads are basically rectangular-port heads. I have run all of these heads at one time or another, and they all produce stout performance right out of the box.
Fig. 4.22. This is a wetflow test that Dart’s Tony McAfee and I did on a Dart Pro 1 head. The intake valve (yellow arrows) is at 0.600-inch lift. The mixture is set a couple of numbers too rich. This is somewhat common for many newly built engines that are not dyno tested before going to the track for thefirst time.
The green area is liquid fuel, and it is washing all the lubrication off the cylinder walls. That accelerates bore and ring wear by at least a factor of 50.
Quite a few wet-flow benches are used in the performance industry. However, it is one thing to have a wet-flow bench and quite another to know how to use it and interpret the results correctly. The information we learned developing championship winning Pro Stock heads was, where applicable, used to better Dart’s performance street heads.
Fig. 4.23. Note the form of the area behind the valve guide (arrow). There is a strong trend to make this into a sharp edged fin such as seen in Figure 4.24. This can be good or bad.
Fig. 4.24. Cutting a knife-edged tail behind the guide can be a mixed blessing, depending on the quality of the air/fuel mixture. The air on the roof of the port tends to follow the route shown by the red arrow.
Problems develop when the mixture is a little on the wet side because of less-than-optimal booster function and/or rivulets forming because of various factors, including shiny finish of the ports. If either condition transpires, the knife-shaped trailing edge of the guide tends to encourage fuel shearing at this point so it produces at least a small degree of correction.
However, if the mixture is well atomized and evenly distributed within the air charge, the knife-shaped trailing edge can be rounded off so that air can follow the path indicated by the red arrow more easily (as shown on the small-block Chevy head). Air on the port floor on the cylinder wall side tends to flow along the path indicated by the yellow arrow.
Fig. 4.25. The arrow indicates the remachined port of the Dart heads. Originally, they were 335 CNC-spec heads, but for the purposes of our tests Dick Maskin’s guys remachined them to 355. In this spec, we have so far attained 1,080 hp from a 565.
Fig. 4.26. Porting a set of Dart Pro 1s takes a while. Using the right tools, it’s about a 40-hour job, but the results are worth it. These were used on a streetable 572 that cranked out 830 hp on pump gas.
Fig. 4.27. You can see the obviously small 100-cc chamber variant (top) of the Edelbrock RPM Performer. Not only does this chamber prove to be very effective with a low-dome piston, it also has great potential for modifying to achieve some really good torque-per-cube numbers. At the end of the day, this means big horsepower without so much need for a big cam. The intake port (bottom right) comes with a CNC port-match cut so it’s easy to align with the intake manifold.
The flat floor of the exhaust port (bottom left), in conjunction with the chamber shape, lends itself to a much more even port velocity top to bottom and a greater resistance to low-speed reversion. This factor helps both low- and high-speed output. The E-Street and Performer RPM series deliver exceptional out-of-the-box performance and great potential for the informed head porter.
Fig. 4.28. The dotted lines are the stock port flow figures, and, as shown here, are the average of the good and bad port flow numbers. The solid line represents airflow from a basic do-it-at-home porting job. In addition, the quench pad has been relieved around the exhaust valve as per the AFR head shown in Figure 4.19.
I have also used both of the CNC 335- and 355-cc runners. Mark Dalquist helped me in my last foray with Dart’s 24-degree CNC-ported heads. A set of 335-cc heads was used on a fairly serious 556 that made a shade more than 900 hp. After several seasons of drag racing, this engine was rebuilt as a nearly max-performance 565 build. Budget was the only constraint, and although far from unlimited, we had enough money to buy good parts. For this second go-around, we persuaded Dick Maskin to take the heads back and remachine the intakes to the then-current 355 design. This, in combination with a new valve- seat job, resulted in 1,080 hp and 848 ft-lbs of torque.
When I first used the E-Street and Performer series of heads, I consistently heard grumbles that they were somewhat down on performance compared to other brands. My first experience with them was helping out with a couple of friends who had bought them because of the low price. Naturally, I recommended the cam to suit the valve size and displacement of these builds.
E-Street Heads: In each case the heads were bought complete and used on a 468 bottom end with pistons to give a 10:1 CR. The recommended cams were not the typical 112 LCA cams that are usually peddled to the enthusiast engine builder, who has no dyno-proven advice to the contrary. Go this route with E-Street heads, and you can kiss a minimum of 50 ft-lbs and 50 hp goodbye.
As a consequence, I cannot recommend buying an Edelbrock cam to go with your Edelbrock heads because their cam philosophy results in cams that are too conservative, both in terms of mechanical dynamics and the preservation of idle and low-speed manners. Dr. Rick Roberts is the cylinder head and manifold guru at Edelbrock. He has led the company to produce cylinder heads that represent some of the industry’s best. If you are going to tap into Dr. Roberts’ cylinder head talents, I encourage you to take my dyno-result based camshaft spec’ing advice very seriously.
Fig. 4.29. This 468-ci engine with Edelbrock E-Street heads produces 598 ft-lbs and 599 hp. It has a 9.5:1 CR and runs on 87-octane gas. With the right cam, this combination delivers outstanding results. In this instance, a Terry Walters COS-Cam selection service was used to arrive at a 268 hydraulic flat-tappet cam. This cam was on a 106-degree LCA, which was about perfect for the valve size, displacement, and compression ratio. See page 133 for the results produced when these heads are given a basic porting job.
One of the many aspects I like about the Edelbrock E-Street heads is the price. It is about the least costly of any heads, and they have American-made quality. Another aspect of value is the fact that many of Edelbrock’s entry-level street-oriented heads have much smaller chambers than the competetion. Edelbrock’s chamber size is usually 118 to 121 cc. However, when you are looking for pistons for these heads to achieve a certain compression ratio, take note that the chambers are usually a couple of cubic centimeters bigger than Edelbrock’s literature indicates.
I helped out with the initial builds that used E-Street heads, but these builds did not go on the dyno. That was outside the budget. I suspected that these heads were better than rumored because the only problems that these two initial builds had were with the tires. Even though they were using high-grip tires in each instance, they lit them up in first or second gear of the Turbo-400-equipped cars.
The first E-Street heads I received were bare. After a three-angle seat job plus a 75-degree bottom cut they were fitted with 2.19/1.88 Fer rea valves. Refer to the flow numbers in Figure 4.28. They were installed on a TWPE 468 dyno mule. This engine made 598 ft-lbs and 599 hp using a COS-Cam (computer optimized spec) 268-degree cam with 219 degrees duration at 0.050-inch lift using a single- pattern hydraulic flat-tappet Lunati- ground cam.
If this looks merely average to you, just go online and check out the torque and horsepower figures claimed for a few advertised pro-built engines. Most 496 builds with much bigger roller cams are not making this sort of output yet cost thousands of dollars more. If this build and out-put looks good to you, see page 133.
Performer Heads: After achieving excellent results from the regular 110-cc chamber E-Street head, I turned my attention to the RPM Performer head (PN 60489), which is the bare casting. It has identical ports to the E-Street head but it is rolled 1.5 degrees. This results in a combustion chamber near 100 cc, and that means (given the compression ratio sought) you can use a piston with less of a dome for a more effective charge burn. The shape of the chamber means checking the piston-to-head clearance and trimming material at any collision points.
The flattop Icon piston in a 468 with the 100-cc chambers gives a 9.3:1 ratio. Although that’s okay, a good target here is 10.5:1. On a properly set up engine, these heads have good detonation resistance so a 10.5:1 CR for the street is not an issue. If you use Icon’s 18-cc dome piston, by the time it is detailed and clearanced, it delivers about 10.7:1 CR. If you want to simplify your life, my good friend Roger “Dr. Air” Helgesen tells me that he ported a set of these heads and his client had Wiseco manufacture a piston to suit them, so that might be an avenue to pursue. How well did the 100-cc heads perform? In short, very well (see page 134).
And while I am on the subject, when Roger Helgesen ported this 100-cc chamber variant of the Performer RPM heads, he equipped them with 2.250-inch valves. Although still retaining the small port by virtue of minimal metal removal, he increased the high-lift flow to a little more than 350 cfm while improving the low- and mid-range flow very substantially. The results were in line with the flow increases, and Roger’s pro engine builder client was apparently very well pleased.
Race Heads: As for the more race-oriented Edelbrock heads I have only experience with the rectangular-port Victor 340-cc runner-style versions. I spec’d out one set that, on a 540 build, performed well. The other set was acquired for cents on the dollar, bare, at a swap meet. The engine had not been well maintained, and the heads took a beating. But they were, with some welding and machining, restored to a functional state.
At that point, I ported them and revalved them with bigger Ferrea 2.35-inch intake valves instead of the more usual 2.3-inch units. The exhaust valves remained the 1.88-inch size. Flow at 0.700 inch was a healthy average of 412 cfm between the good and bad intake ports. That was a pretty good result on my bench but the trick valveseat job, using a Goodson cutter (PN FT 004B-HP), improved the low- and mid-range flow. In addition, the bigger valve produced a flow of 175 cfm at 0.200-inch lift and 258 cfm at 0.300-inch lift. These are strong flow figures, and the output shown on a budget 12:1–CR 525-ci engine (0.060 over 454 with 4.5 stroke crank) resulted in 754 ft-lbs and 851 hp.
I have monitored dyno test sessions on engines that used other Edelbrock heads. One such example, a set of Victor CNC heads with 377-cc ports, was fitted to a 556-ci engine with 12.5:1 CR and a Holley Dominator carb. The result was 795 ft-lbs and 901 hp. There is little doubt that Edelbrock heads can produce strong results.
Other than having good manufacturing quality, the GM aluminum big-block heads do not enjoy a particularly strong reputation for making power. That said, they are far better than their current reputation suggests, especially when used for a pump-gas street application. Like Edelbrock heads, these are often used on engines in the 500- to 632-ci range. The cams in these engines often have far too wide of an LCA, and as a result they fail miserably. Using a GM cams does nothing to help here, as all the LCAs are too wide for the job.
With these GM heads, replacing a 112-degree LCA cam with one in the 105 to 106 range on a 572, yields an increase of better than 70 hp, although the cam has as much as 10 degrees less intake duration. Torque and drivability have also taken a big leap.
So if you are offered a deal on any of the GM heads with an appropriate port volume for the build you are planning then, by all means, get them. However, take the time to get the right cam for the application. Your first step here is to absorb the contents of Chapter 9, Camshafts and Valvetrain Events, and Chapter 10, Valvetrain Optimization. As for porting the GM heads, they respond to the same porting techniques in the same fashion as most other conventional 24-degree heads.
Liberty Engine Parts
Liberty is headquartered in Philadelphia, Pennsylvania. It has five big warehouses and services the North-eastern states and is also a trade-only outlet, so you have to order these big-block heads through an engine rebuilder shop. These heads seem to be a combination of offshore and U.S. manufacture. As far as I can ascertain, a company in India manufactures these castings and also those for the OE industry, including General Motors. The castings are then machined in the United States.
My experience with offshore- produced heads has not been good, so upon receiving these heads I scrutinized them very closely. Short of X-raying them, the castings appeared to be as good as many of the U.S.-produced castings. The heads had thick decks and generally looked as if they could take a beating and survive.
Before going any further, however, let me address one negative. During the first-time install of these heads on a 468, the pushrod clearance at the intake port was off to the extent that substantial clearancing was necessary to obtain a functional valvetrain. There were some other fitment issues, but none were beyond fixing with a die grinder and files.
Fig. 4.30. I cleaned up the chambers on these Liberty heads as part of my porting job. The seats have still to be cut. When doing so, I use a Goodson cutter (PN FT 004B-HP). This has a radius lower cut, which, with a 2.190 intake valve, only partly cut the lower radius. If you buy the heads bare and install a 2.25-inch valve, that valve’s under-seat radius, when cut, should be close to its full form.
Fig. 4.31. The guide boss in the exhaust port has had a rework, and the remainder of the port has been cleaned up, which works okay up to about 0.500-inch lift. If lift is more than 0.500, the tight short-side turn stalls the port at about 250 cfm.
Fig. 4.32. The arrow indicates the location of the rocker- thread casting bump, which has already been removed. In most cases, the bump can be removed without suffering a stud failure. When the casting bump has been removed, the airflow at 0.600 and above increased by 15 to 20 cfm. On the dyno, a 600-hp big-block is rewarded with about a 10-hp increase. This is one of the quickest, cheapest 10-hp gains, and applies almost universally to all 24-degree heads.
These heads are available in two port sizes: 319 and 345 cc. Terry Walters and I tested two sets of 319-cc heads. One set was as-cast and I ported the other. We did not do any “A versus B” tests, but we have done more than enough big-block dyno builds and tests to know what is good and what is not. On the dyno, the as-cast heads were okay but not as good as many of their all–U.S.–made counterparts.
That is the downside, but on the upside the price of these heads is almost half the cost of their nearest U.S.–made competitor. This means you can purchase these heads at a price that is marginally more than the cost of doing a top-notch job of reconditioning a set of factory iron heads.
Out of the box, these heads flowed a little less than I expected. However, applying the basic porting techniques, I produced a bigger gain than with many of the other heads in this chapter. With a 2.19/1.88-inch valve combo, the ported Liberty heads delivered 340 cfm at 0.700 lift, and with my seat job, 157 cfm at 0.200 lift. These are respectable figures for what is essentially a very low-cost head.
On the exhaust side, I found that the short-side turn was a little tight starting about 1/2 inch below the valveseat. Applying basic porting techniques does not fix this issue. I was loath to reshape this to fix it, as I had no idea how thick the casting was. This limited the exhaust flow to about 250 cfm. The flow reached this value at about 0.500 lift, and from there up to 0.700, the flow remained unchanged.
I ran the heads on the 468 previously used for the as-cast Liberty heads. It resulted in the classic “too-much- port-volume.” Even though the flow had increased considerably, the intake port was now about 10 cc bigger; too big for the job. The ported heads only delivered about 2 to 3 hp more, and this minimal gain in output did not start to show until about 4,000 rpm. Below that RPM, the output was as much as 5 ft-lbs less. I have to say, with a nearly 330-cc port, this was not an entirely unexpected result.
At this point the ported Liberty heads were stored and the stock heads were used on a 496. What a difference. The port size fell in line with this displacement and the results showed it. This prompted a swap to the ported Liberty heads. Same result: It produced good numbers all the way up the RPM range with the 274 flat-tappet hydraulic cam. The key to success with these heads is: A) Don’t use them on an engine that requires smaller parts, and B) If you are specifically looking for performance, don’t use less than a 10:1 CR.
To sum up here the principal question is: Are these heads a viable option for the financially constrained engine builder? Answer? Yes! Although not the strongest power producers they do represent good value for the money. If the budget was really tight, I would feel confident buying a set.
I have experience with three sets of Pro Comp’s big-block cylinder heads. Initial inspection indicated the decks were thinner than I would have liked. I ported a set of them for a nonnitrous street application for a friend right after they became available. Just as I finished porting these heads, which, by the way, flowed much better than I expected, Terry Walters had two sets come into his shop, each with less than 1,000
miles on street builds, and both sets had dropped intake seats. Needless to say, this caused a great deal of expensive-to-repair damage. The set I ported is being used, together with a polished camshaft, as the legs for a three-legged glass-topped coffee table.
Pro-Filer Performance Products
You may not have heard much (or anything) about Pro-Filer. If so, let me give you a few pertinent facts concerning their performance pedigree. Darin Morgan is Pro-Filer’s premier big-block head designer. As you may know, he is also the cylinder head guru at Reher-Morrison, a company famous for numerous Pro Stock championship-winning engines.
To date, I have had experience with Pro-Filer’s oval-port 290 and 320 as-cast heads. Just for the record, on the first set of 290 heads I had the ports cc’ed out at about 300. I was unconcerned about this as they were going on a 565-ci engine, so the question of a port being too big was moot. Terry Walters built the 565 at his shop, and the owner was okay with it being used as a dyno mule. This allowed us to see what refinements we could add to the original specs. After some significant tweaking of the Dominator’s calibration by AED Performance carb guru Jeff Harris, plus cam and ignition timing, plug style, and exhaust system, it produced impressive results.
In fact, these results were so strong that even Darin Morgan, with all his experience with high-output engines, was very impressed. So how much did this 87-octane-burning 565-ci street build make? How about 770 ft-lbs and exactly 800 hp at 6,250 rpm on the, if anything, under-reading TWPE dyno? (See page 141 for the build specification.)
Fig. 4.33. The Pro-Filer big-block Chevy head’s principal design feature is the oval port. Some computational fluid dynamics studies by my good friend David Woodruff of Design Dreams indicate that even if the port is sized so that no additional flow is produced, the velocity and port energy are often higher.
After a result like that, I decided to port the Morgan-inspired Pro-Filer heads. After a very thorough investigation of the ports’ flow characteristics and some very carefully executed detailing work, I managed to improve on the flow from 0.400 lift on up; not much was left on the table. After about four days of work, I improved the top-end flow by just 20 cfm. When these heads went back on the 556-ci test engine, the power increased to 825 hp at 6,400 rpm, and peak torque climbed to 791 ft-lbs.
The Pro-Filer 320 heads also delivered a very creditable performance on the less-than-highly-developed 572-ci dyno mule. Walters, Jack Sain, and I built the engine, but it was not a budget build. Jack Sain did 90 percent of the assembly, while my job was mostly parts selection and cam spec’ing.
However, I did help build a 712-ci engine using a set of out-of-the-box Pro-Filer Hitman 12-degree heads. At 0.700 valve lift, they showed just 460 cfm on my bench and 502 at 1.1 inches lift. That last number was relevant because we had planned on a valvetrain that would deliver at least 1.00 lift on the intake. Exhaust flow was also strong at 325 cfm at 1.00 lift. So how did all this work out? Well, the resultant 16.0:1 compression and AED Dominators on a tunnel ram intake contributed to the 1,098 ft-lbs of torque and 1,346 hp. How well did it perform in competition? The owner won almost every round of racing and the championship. But he said that he had never been able to make a clean pass with this engine even with that winning record. (See page 142 for more on this head, a build spec, and power curve.)
Racing Head Services
RHS has some excellent Chevy big-block heads. Not only do they flow some great numbers right out of the box, but also, when certain porting techniques described here are applied, they produce among the best flow numbers of any of the 24-degree heads I have covered, but only by a small margin. I gave one of my top students at University of North Carolina, Charlotte (who, at the time, had zero porting experience), a set of these RHS heads to rework. This means that I let a complete novice start a possible porting career with a set of heads costing four figures. It could have been either courage or folly on my part, but it all ended well!
After following the instructions in my previous Chevy big-block book, the student produced intake ports that, at 0.700 lift, flowed 415 cfm (for the good port) and 409 cfm (for the bad port). In addition, the exhaust port delivered 303 cfm. These heads were about as easy to port as they come. Even the first time around, someone with average manual dexterity should be able to use a die grinder to rework a set of these heads in about 20 to 25 hours.
As for making power, they do the job well on street engines targeting high output. To date, though, I have never used them on a max-performance engine but I see no reason that they could not return very competitive outputs.
Trick Flow Specialties
TFS is probably best known for their small-block Chevy and Ford Twisted Wedge heads. These heads don’t need intake valve cutouts and possess great potential for producing high output on street small-block engines. Their success has tended to overshadow the design and production prowess TFS has had with their big-block Chevy heads.
In terms of power potential, quality of manufacture, and cost they are right there with the best. The castings are available with 280-, 320-, or 360-cc ports. Some are the best the industry has to offer, and this makes for good out-of-the-box performance. It also means they are easy to port.
Let’s compare TFS power right out of the box compared to Dart, one of my favorite brands. I had the opportunity to test a set of Dart 320-cc runner heads against a set of TFS 320-cc runners on a stout, street 572. This test engine was in the 750-hp range. The average of three runs from each set of heads produced an output over the test range that was within 1 ft-lb and 1 hp. When I compared the cost, the TFS heads were hundreds of dollars less expensive. TFS has improved chamber-casting accuracy to the point that it is almost as good as a CNC chamber. See Figure 4.40 to see just how well the as-cast chamber blends into the valveseats.
Fig. 4.34. The intake ports on the RHS 320 heads have very little taper. This continues almost to the bowl beneath the valve, which is good for performance and simplifies porting. All that needs to be done here is to cut the port entrance as per the scribe lines.
Fig. 4.35. A finished RHS combustion chamber and intake port should look like this. Note that the tail on the trailing edge of the guide has been rounded off (upper right arrow) to accommodate the flow pattern. Also reference the shape of the vane/ splitter (lower right arrow). With close inspection, you can see that it veers off to the left of the port. This tends to push the high-speed air traveling on the port’s roof to the cylinder-wall side of the port (at right). This typically adds both swirl and flow while directing any wet fuel toward the hotter part of the combustion chamber (left arrow) on the exhaust-valve side.
Fig. 4.36. Here is another view of a finished intake port clearly showing the removal of the guide’s trailingedge tail.
Fig. 4.37. A close look reveals that the splitter behind the exhaust port guide leans toward the cylinder center instead of being in the more normal vertical position. This is because the exhaust does not exit the port straight down from the valve but at an angle from the center of the cylinder in the direction indicated by the arrow. This RHS exhaust port shape also has a low-velocity gradient from top to bottom. This is a good attribute as it enhances output throughout the entire RPM range.
Fig. 4.38. Here is a TFS 280-cc oval-intake port. Even though it is far from large, the flow, as-cast or ported, is good. (See Figure 4.42.)
Because it is so often an issue, a point worth noting is that the 280 heads come with a 113-cc chamber. With just a slight-angle mill I have cut these down to 107, but it looks as if they could go a few cubic centimeters smaller yet. In short, my thoughts here are that these TFS heads should be ranked high for consideration on any serious build, and especially so where the budget is really tight.
At the time of this writing, I have categorized some heads as “super heads.” The reason is that they are a definite step up in terms of flow, and you need to know about them if you’re building a large-displacement short-block of 550 inches or more. These heads all flow in the 480- to 500-cfm range.
First, Pro-Filer is about to launch a brand-new 24-degree casting that uses 2.4-inch intake valves and is said to flow just over 500 cfm.
Then there is the Race Flow Development (RFD) 24-degree Victor Pro developed by Curtiss Boggs and based on a set of Edelbrock castings. These have chambers down to 96 cc.
Fig. 4.39. TFS rectangular port heads have a generous corner radius. They also have a CNC-portmatch cut going about 1 inch into the runner.
Fig. 4.40. The current TFS head chambers have a smooth precision-cast finish. Although TFS offers CNC-machined chambers, the advantages of having just the chambers machined is small.
Fig. 4.41. The TFS exhaust delivers 245 cfm at 0.700 lift as is, but with simple porting, airflow increases to almost 290 cfm at that lift. The favorable velocity distribution top to bottom is also good.
Fig. 4.42. Here are the TFS 280 heads’ flow curves. The lower dotted gray curve is the exhaust. The upper two dotted gray curves are the good and bad intakes. The blue curves are the good and bad intakes ported; the red curve is the exhaust ported.
Fig. 4.43. This test shows the 280-cc TFS heads in both stock (blue curves) and modified form (red curves). It also shows a set of reconditioned 088 factory heads (black curves) with an average port volume of 312 cc.
The factory heads had worn guides and seats, so valves with an oversize stem and 0.015 oversize heads were installed. The seats were cut to a three-angle performance spec with a fourth-angle bottom cut at 75 degrees. The head faces were also milled to more closely match the chamber volume of the TFS heads (114 cc).
The reconditioning undoubtedly helped the performance of the 088 factory castings because they showed better performance to the tune of 10 to maybe 15 hp as well as a similar amount of torque.
Although the 088 heads increased the engine’s output, the TFS heads outperformed the 088 heads everywhere in the RPM range. The extra top-end output of nearly 75 hp (red curve versus black curve) showed as a marked improvement on the track, but the additional low-speed torque is instantly noticeable in day-to-day street driving or towing if this engine is used in a truck.
Also, there is the rolled SR20 head casting produced by Brodix.
“Slick Rick” of Slick Rick Racing heads in Magnolia, Texas, designed these heads, and they look as if they pack a lot of potential so be sure to at least check them out.
Finally, there are heads from Reher and Morrison done by Darin Morgan. Morgan’s ported SR20 heads are 500-cfm-plus designs and although they are slightly more expensive, you should also check out his 12-degree Raptor heads, which have 550 cfm at 1-inch lift.
Written by David Vizard and Posted with Permission of CarTechBooks