Gen III V-8 Cylinder Head Shootout

The stock pushrods are designed for durability, weight, and cost, so they aren't the best for performance applications. If you are installing a performance cam and valvesprings, replace these with more robust aftermarket pushrods. Many companies offer smaller-than-stock oiling holes on the pushrods to limit the increased oil flow (you did port the oil pump outlet and add a stiffer bypass spring, right?) from drowning the top of the engine in oil.

"Lost" Power

For the sake of mass production, the production Gen III V-8 cylinder heads were designed to fit the smallest-bore Gen III V-8 engines--the 4.8L and 5.3L, 3.780-inch-diameter cylinder bore SUV and truck engines. The displacement difference between the 4.8L and 5.3L engines is created with the stroke. The 4.8L engine uses a 3.27-inch stroke crank, while the 5.3L engine has the common Gen III V-8 3.622-inch stroke crank. Since the 5.7L and 6.0L Gen III V-8 engines have 3.898- and 4.00-inch bores, respectively, this means there is room for larger valves and better valve placement in the larger bores.

The '02-and-later LS6 valvesprings (PN 12565313) are by far the best production valvespring for performance Gen III V-8 applications. GM identifies its valvesprings visually by colors or tints on the springs--but the colors have changed over the years. Essentially, the yellow and light green springs are the LS6 springs, available from GM.

Why did GM do this? For every LS1 or LQ9 engine built, there are about two 4.8Ls or 5.3Ls built. The 5.3L is GM's bread and butter, so the heads had to be common to work with all the designs.

Virtually every GM engineer who worked on the Gen III V-8 knows there is power in moving the valve spacing to install bigger intake valves and achieve improved flow into the engine. They also know this change would require a new intake port design, offset rockers, and other changes to accommodate the design, but the power potential is great enough to warrant this work.

Obviously, a cylinder head with all these changes requires considerable work to create and manufacture, but it's possible it might appear in the near future--the multiple-digit power potential is too great for hot rodders to pass up. For those not capable of creating a cylinder head, just know there is power left on the table there!

To make sure the compression ratio doesn't go to the moon, it is recommended you determine the exact chamber volume of the heads you remove from your stock engine and the ones you intend to install on the engine. In most cases, for normally aspirated engines, the LS6 64cc chamber heads are the popular choice, while boosted (super- or turbocharged) engines often use the LQ4/LQ9 71cc combustion chamber head--as both heads have the LS6 intake and exhaust ports. In the CNC head business, a popular move now is to start with the 5.3L 61cc head, as the chamber can be fully CNC-machined and still net roughly a 64cc chamber.

If porting a set of Gen III cylinder heads is in your future, the circles indicate the thin wall areas to avoid. This cutaway shows the rocker bolt bosses that protrude into intake ports. Notice the valvespring pad--if this is machined to a larger diameter, this will likely break into the intake port or the water jacket.	This photo shows the backside of the intake port (where there are power opportunities). The wall is not very thick, so be careful opening this up.
This photo shows the backside of the intake port and where opening its cross-section up or the backside of the port at the bowl requires caution as the wall thickness is less than 3/16 inch thick.