Types of Ports
The LS engines have gone through a variety of ports, though they fall into two basic types:
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.