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

A Short Review Of Corvette LS Cylinder Heads

Richard Newton Aug 4, 2010 0 Comment(s)
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More power. That's always been the cry of Corvette owners. We may never use all the power we have, but we still want more.

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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.




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