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Understanding GM LS-Series Cylinder Heads

Before I get into the the meat of the book—the spec charts—there are a few things you need to know. They include information on converting from a cathedral port to a rectangular port, some thoughts on the merits of cathedral versus rectangular ports, and a bit of cylinder head theory.

 


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THIS TECH TIP IS FROM THE FULL BOOK :

HIGH-PERFORMANCE GM LS-SERIES CYLINDER HEAD GUIDE

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 The intake manifold is the most important element in swapping from a cathedral to a rectangular port head. To match the runners, this factory LS3 (PN 12610434) or an L76 manifold are the most popular. It is important to note that there are some minor differences between the L76 and LS3 intakes, such as a screw-in MAP sensor instead of the Gen III–style clip-in; though none are of any real consequence to swapping over. These factory manifolds come as shown here with a 90-mm electronic throttle body, injectors, fuel rails, and gaskets. (Photo Courtesy General Motors)

 

Converting from Cathedral to Rectangular Port

Since the factory L92 cylinder heads were released, one of the hottest topics has been how to convert older (cathedral port) LS-series combinations. Thankfully it is a pretty simple head and intake swap, which lends itself well to fairly high horsepower combinations on a modest budget. And since all LS blocks have the identical (four-per-cylinder) head-bolt pattern, all LS or LSX heads are compatible. (Some aftermarket blocks have two additional head bolts on the outside and inside of the standard ones). The only exceptions are when certain heads have valves too large to fit in a particular bore. The L92/LS3 heads, for example, must be used on a 4.00-inch-or-larger bore, and the LS7 requires at least a 4.125-inch bore.

In order to utilize such mammoth intake valves in such scarce real estate, General Motors went with an offset intake rocker on the L92 head, which means converting it also requires sourcing at least a set of 8 intake rockers (if not a full set of 16). The LS7 uses a unique rocker system due to its offset intake valve and 12-degree valve angle, which is not compatible with either the LS1/LS2 or the L92/LS3. Beyond this point, the rest of the conversion largely depends on the previous chosen components and the application.

With factory valve covers, some grinding may be required to clear the offset intake rocker. This is particularly true on older LS1 designs (on the passenger side mainly), though LS2 and LS6 or later-style valve covers should have no issues. Aftermarket valve covers are usually made to clear bulky aftermarket rockers; if they don’t clear, a simple set of spacers from UMI can be used to rectify this problem.

The much different intake runner shape also necessitates a different intake manifold. It is important to note that the factory intakes come complete with injectors, fuel rails, gaskets, and a 90-mm electronic throttle body. However, neither the injectors nor throttle body is compatible with a factory LS1 wiring harness and computer. This is most commonly rectified by purchasing injector plug adapters (such as those from FAST or Caspers Electronics), a 90-mm cable throttle body, and a cable bracket.

 

To utilize the Gen IV–style injectors that come on an LS3, L76, or LS7 intake manifold with an LS1 wiring harness you need injector adapters. The Gen III injector style is known as EV1 and the Gen IV is known as EV6, so look for an EV1 to EV6 injector adapter kit from Caspers Electronics or FAST.

 

 

Because the LS3/L76 now places the MAP sensor at the front, just behind the throttle body, a MAP extension such as this one from Caspers is also needed. You may choose to extend the wires themselves, but for $34 you can save yourself the hassle.

 

Because the valve sizes and combustion chambers are different, it may be necessary to check piston-to-valve clearance in applications with larger camshafts. Without changing pistons there is a slight mismatch in the pistons’ valve reliefs, which can affect clearance.

 

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All rectangular port heads use offset intake rockers to accommodate the massive valves. With factory rocker setups you can get away with just changing the intake rockers; however, changing the whole set or going with an aftermarket variation are also options. In either case, extra clearance may be needed on the valve covers.

 

Generally speaking, cathedral port heads have much better exhaust flow than rectangular ports, which is why cam grinds tend to have more exhaust duration with a rectangular port. For optimum performance, changing cams is recommended, though not necessarily a must. (Photo Courtesy Comp Cams)

 

It is also worth mentioning that the vacuum line to the rear of the manifold sometimes requires lengthening, as do the wires to the MAP sensor. Those using a carburetor simply need to transfer the old carb to the new intake, assuming its size is still appropriate for the new combination.

Experienced builders, particularly those who are accustomed to building high-compression engines with larger camshafts using tight clearances, may be wondering one or both of two things: how are these heads going to match the valve reliefs on the pistons and how well will the old cam work with the new heads?

If high-compression pistons with LS1-style valve reliefs are in the engine, there is a slight misalignment due to the offset and much larger intake valves with the new heads. In a more street-friendly engine with plenty of piston-to-valve clearance this doesn’t factor in, but those running lots of duration and lift with a smaller chamber may need to swap out the pistons.

Cam specs also see quite a bit of variation between cathedral and rectangular ports, most often to compensate for a more choked exhaust port. Depending on the difference in the two heads, this is not to say that gains can’t be had without switching cams, but experience shows that most often there is a considerable performance increase from doing so.

 

Cathedral Versus Rectangular Port

Believe it or not, there is no clear-cut winner between cathedral and rectangular ports, despite the hoopla that rectangular port heads have gotten. In large-cubic-inch and full-on race applications, it appears that rectangular ports have the advantage, but at least one aftermarket cathedral casting (TrickFlow Specialties) has proven to have enormous potential with additional CNC or hand porting—boasting more than 700 hp on a naturally aspirated 440 street engine. And with the latest designs from Air-Flow Research and Mast Motorsports, we may see even more powerful combinations using cathedral heads.

According to cylinder head design theory, neither head is actually the best design—the sharp edges associated with both create resistance in airflow. However, it is important to note that quality rectangular port designs actually use more of a rounded (oval) shape, though they are referred to as rectangular. This started with the C5R head and continues in most newly designed race heads. Quality head porters even use epoxy to further smooth out these edges.

 

Using the old ET Performance 11-degree, 255-cc cathedral port castings, extensively hand-ported by Gregg Good and matched to a custom Beck sheet-metal intake manifold, Joe Huneycutt’s 2002 Camaro is one of the fastest all-engine LSX combinations ever built. The large, solid-roller camshaft spins the little 403-ci engine more than 10,000 rpm, which makes well over 900 hp. This wild 15.8:1, dry sump race engine proves that cathedral ports can be used in high-lift, high-duration, high-RPM applications. Using a highly modified factory 6-speed manual transmission Joe has gone as fast as 8.69 at 156 mph in the quarter-mile.

 

In reality, neither a cathedral nor rectangular port is optimum. An oval port, such as the Mast Motorsports Mozez canted-valve race head shown here, provides the best overall flow because there are no sharp edges to restrict it. (Photo Courtesy Mast Motorsports)

 

Much like the C5R before it, the LS7 has massive valves and a straight, tunnel-like runner that is effective in producing both low-RPM torque and high-RPM horsepower thanks to its large runner volume, which also manages excellent velocity. The 12-degree valve angle aids in this excellent geometry andthe large valves are accommodated by a Siamesed valve seat. Complete CNC porting on a 5-axis machine makes these some of the highest-flowing factory heads ever. (Photo Courtesy General Motors)

 

Cylinder head porting is somewhat of a black art; gurus can spend hundreds of hours on a single set of heads with a grinder, various grits of sandpaper, welder, and  epoxy. CNC machines, however, allow you to replicate a particular set of well-ported heads over and over again, and in half the time. The LS market has embraced this technology by providing programs for virtually every factory or aftermarket casting. West Coast Cylinder Heads is among several companies that even port LS7 and LS9 heads, which come CNC-ported from GM. (Photo Courtesy West Coast Cylinder Heads)

 

Here is a look at two molds made from Chevrolet Performance’s CNC-ported L92 head (left) and an early Mast Motorsports small-bore LS3 casting (right). They are similar, though greater variation can be seen in Mast’s newer castings, particularly the large-bore and splayed-valve designs.

 

Like any CNC porter, West Coast Cylinder Heads uses CAD (computer aided design) software to create the runner design. Experienced designers know how different shapes affect flow; however, there is even software to predict how various changes alter airflow. Keep in mind that the job of the designer is also to create a “happy” port with good velocity, not just a high-flowing one.

 

This brings me to the point that not all rectangular heads are created equal. There are many different port shapes, the basic two being the L92/LS3 and the LS7. However, even among them, a CNC program or casting design could be vastly different in terms of runner shape, size, and taper. Some boast high flow numbers, but poor velocity and swirl. Others can be a total home run. As you navigate this book, try not to be obsessed or mesmerized by the flow numbers and instead consider how well the cylinder head matches the rest of your combination.

This is often where cathedral heads prove to be invaluable. Although most can’t claim 330-plus-cfm, they do provide torque, tuneability, and drivability. On average, cathedral port heads have smaller runners (in terms of volume). Until recently, the largest intake valve you could fit was 2.08 inches. By normal standards this is considered quite large, but not compared to the 2.20-inch intake valves in an LS7 head or the 2.16-inch LS3 valves.

The enormous difference in valve size has a lot to do with the difference in flow between typical cathedral and rectangular port heads. As the result of these factors, very seldom are we comparing apples to apples. So again, I urge you to consider each cylinder head individually.

 

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Cylinder Head Theory

As previously mentioned, matching air speed (velocity) is almost always more important than just flow. This is why higher flow numbers on a given combination do not necessarily mean more power. At the end of the day, it is up to builders to use their experience to determine what the engine’s airflow needs are.

The other parts of the induction system target horsepower and RPM range, and several other factors must be considered. For example, AirFlow Research’s 230-cc LSX Mongoose head may make a perfect match for a 402- to 408-ci, naturally aspirated street combination. However, this head may also work well in a race-only setup, such as a solid-roller 346-ci LS1 that spins to 8,500 rpm or sees a lot of boost.

As much experience as it takes to select the right cylinder head, it takes even more to know how to port one. Top-notch cylinder head porters are few and far between, especially when a CNC machine does the same amount of work at a fraction of the cost. Meticulous hand porting, using epoxy to add material back in where needed, easily yields the best results, and is most often how a CNC program is designed.

Done properly, the CNC machine replicates the port shape and flow within 5 cfm. And best of all, there is no variation—each head is identical, unlike with hand porting. This is, of course, not to say all CNC heads are outstanding. A CNC head is only as good as the program and the casting it was designed on.

Head geometry, as it pertains to valve angles, valve seat angles, and port location, is particularly important and often overlooked. Changing the valve angle, when it comes to LS heads, may not provide nearly as much improvement as it has in the past (such as going from a 23-degree to a 15-degree small-block Chevy head). On the other hand, changing valve angle does allow for larger valves and a higher port, which can greatly improve performance.

 

 

Generally speaking, GM has done its homework when it comes to combustion chamber design. However, its designs were originally meant for emissions-controlled vehicles running either regular or premium pump gas. Optimizing for a hot rod or race car sometimes requires updates such as WCCH’s L92 design, which even helps stuff a 2.20-inch intake valve on its Stage 3. Just like the runners, the design is created with CAD software and the work is done on a 5-axis CNC machine.

 

Few companies have spent as much time going back and forth between the flow bench and engine dyno as Lingenfelter Performance Engineering. Thousands of hours went into designing its LS3 CNC program, which has been completely dissected here to demonstrate wall thickness as well as runner height. Though rectangular at the opening, here you can see what the runner shape actually looks like. (Photo Courtesy Lingenfleter Performance Engineering)

 

LS7 heads, for example, are praised for their raised runners because air does not have a sharp angle at the short-turn radius as the result. The short-turn radius is directly related to torque, mid-range power, and top-end power. There is also something to be said for having shallower combustion chambers, which flatter valve angles allow, as they can decrease detonation and burn time, while increasing efficiency.

The combustion chamber is one of the most overlooked aspects of the cylinder head, yet its design directly affects spark timing and the distribution of fuel in the cylinder—the two other elements needed for combustion. When it comes to chambers, quench is the main focus of attention.

The quench area “squishes” the air/fuel mixture into the flame (spark plug) while also homogenizing the mixture. This flatter area of the head is crucial in meeting the piston to create necessary turbulence; however, the chamber must also complement the port design and layout. The chamber design should be heavily tailored to either lower-lift street applications or higher-lift race  applications.

Street applications must stimulate torque production, while also de-shrouding the valves at lower lift. Race chambers must promote airflow stability at high lift using steeper valve seat angles.

The valve job is crucial to smoothly getting properly mixed air and fuel into the chamber, and the specs are again highly dependent on the intended lift and use. Higher valve seat angles, such as 50 to 55 degrees, are better for higher-lift flow and have less durability.

There are many factors that go into designing a good cylinder head, so please use this information wisely and look beyond just flow numbers.

 

Written by Dave Grasso and Posted with Permission of CarTechBooks

 

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