Corvette LS Cylinder Heads - It's All in Your Head

GM has tried to meet our obsession for power by continuously making changes to the LS engine. One approach is to make the motor bigger. What began as a 346ci engine in 1997 is now available with 427 cubic inches. Bigger engines make more power. That's why we have a religious experience when we're around the old L88.

The other way to increase power is by getting more air in and out of the engine, regardless of its size. Racers have known for years that cylinder heads are the real secret to power. You can have a huge engine, but if you can't get air in and out of the heads you're never going to make serious power. Real power is all about air flow.

Your Corvette engine is nothing more than an air pump. The air/fuel mixture flows into the combustion chamber, where it is compressed and set on fire. That fire (almost an explosion) pushes the piston down. That's the power stroke. Then, all of the spent gases have to leave the combustion chamber so a new charge can come in.

The more air you can get into the combustion chamber, and then out of the exhaust system, the more power you've created. (We'll skip over the whole issue of when the valves open and close, since cam science warrants its own story, but that's all about air flow as well.)

The Corvette LS engines have gone through a variety of cylinder heads in an effort to satisfy the desire for more power. There have been so many changes that it can get a little confusing at times. We'll break this down into a few categories to get a better understanding of what's going on with Corvette motors.

Gen III and Gen IV
There are really two different Corvette engines. They're usually called the regular engine and the big-block. You can bolt every LS cylinder head ever created onto any LS block ever made. That doesn't mean it actually fits, though. Not every head will match up with every intake. Then we have the problem of the combustion chambers not matching up with the cylinder bores. Even the crankshafts are different enough to present a problem.

All Corvette LS engines designed for the street clamp the head to the block using four bolts around each cylinder bore. The bolt pattern is consistent throughout the LS engine program, and they all use the same 11mm and 8mm bolts. Since 2004, these bolts have been the same length on all the different LS engine variations. Prior to 2004, there were differences.

There were exceptions to the four-bolt rule, including the racing engine used by Pratt and Miller in the ALMS GT2 program and the big dollar drag racing engines, both of which use five bolts per cylinder. Think LSX crate engines here. But keep in mind that these aren't real street engines.

When you get into really high compression ratios, the LS cylinder heads have a tendency to lift off the block. That's the reason for the fifth bolt. The LSX engines, available from GM Performance Parts, use 10 11mm and 13 8mm bolts to hold the cylinder heads in place. This is a 21 percent increase in clamping ability, and a 100 percent increase in the 12 o'clock and the 6 o'clock positions. The LS9, by the way, uses 12mm head bolts in the same length.

The real problem area when swapping LS cylinder heads from one block to another is the cylinder bore. An LS1 small-block engine has a bore size of 3.890 inches, while an LS3 big-block has a bore size of 4.060. This means that the cylinder head combustion chambers simply don't match the bore diameter. You could end up with a head gasket hanging into the combustion chamber, and that's not good.

If you have a small-bore (3.89-inch) block, you're really limited to the LS1, LS2 and LS6 cylinder heads, what we generally call the cathedral cylinder heads (see below). This isn't the end of the world for small-block performance, since these heads flow more than enough air to provide you with serious street performance.

Cathedral: This was the first design, so named because it looks like a cathedral window. It was used on the LS1, LS2, and LS6 engines. Generally, the cathedral heads can be interchanged with any other cathedral heads.

These are usually thought of as the small-block heads. They have the smallest valves of any Corvette cylinder heads-a 2.00-inch intake and a 1.50-inch exhaust. These valves are spaced close together, which means that the use of aftermarket valves is very limited, so you won't get much of a performance gain from new valves. The LS6 valves are lightweight, with a hollow stem filled with sodium. This allows for the increased rpm used in the Z06. If you intend to turn some high rpm, these valves need to be considered.

Rectangular-L92 Style: This type of port is very similar to the cathedral ports, except they're slightly taller and a little narrower. This rectangular L92 design allows for greater air flow than the cathedral port heads, but not as much as you might get with an LS7 cylinder head. This intake design is used for both the LS3 and LS9 Corvette. The intake manifold bolt patterns are unique to this cylinder head, which is another limiting factor.

These heads are very similar to the LS7 heads, except the valves are smaller: 2.165-inch intakes and 1.590-inch exhausts. They're also placed close together, so you can't just swap in a new set of LS7 valves. The valves are also placed at a 15-degree angle. The devil is always in the details.

These heads will work on an engine with a 4.00-inch bore, but they really work best with a 4.060-inch bore. These heads also get the valves really, really close to the pistons, so you need to be very precise when you check piston-to-valve clearance. This is especially critical if you're using this head on an engine that originally had cathedral-style cylinder heads.

In addition to those two main types of ports, you'll also find:

Rectangular-LS7 Style: This is a third-generation port design introduced on the Z06 LS7 engine. This is the highest flowing LS cylinder head ever produced by GM. It features 270cc intake ports, and the ports and combustion chambers are CNC-ported right from the factory.

These heads have the biggest valves available. We're talking 2.200 inches on the intake and 1.610 on the exhaust. The intake valves are made of titanium, and the exhaust valves are sodium filled. The valves are placed at a 12-degree angle in the heads. The huge valve size in these cylinders requires the use of an offset rocker arm for the intake valves. Did I mention that details are important?

C5R: These are really raw castings. The final shape of the ports is determined by whoever does the port work. The bolt pattern is also unique, so these are really a step beyond the normal Corvette market. The important point is that these heads were the basis for the rectangular port design. This was GM's first effort at raw power. Emissions were never a consideration. But we can put heads on our Corvettes that come pretty close to what the race teams use.

Now that you know the basics, your next step is to get the GM Performance Parts catalog. It has all these wonderful charts that explain all the possible variations. It tells you which heads work with which block and provides the parts numbers. Also, spend some time talking to someone who has actually worked with the heads that are available. D.J. Racing, for example, has about 100 LS heads in its shop. Danny Kellermeyer at D.J. Racing actually knows how they work-or don't, as the case may be. That's good information you can use.