Gen III V-8 Cylinder Head Shootout

The production valvetrain on the Gen III V-8 is non-adjustable, but geometry issues can be addressed by installing different-length pushrods. In general, most Gen III V-8 builders say a stock valvetrain with the proper geometry will require the rocker holddown bolts be turned another 1 1/2 turns beyond the point where the pushrod will not spin between your fingers. This is considered the valvetrain "preload" with the components "hot."

These same engine-builders have found power by running less valvetrain preload. The general consensus is that power exists in valvetrains with a preload of about 3/4 turn of the holddown bolts. On a stock head, this would mean you'd run about a 0.040-inch shorter pushrod to achieve this--but always measure your valvetrain for this measurement. Most of the aftermarket valvetrain companies offer Gen III pushrods in multiple-length increments to allow geometry tuning. You could buy adjustable rockers, but they will just add valvetrain weight (not good for high-rpm applications) and cost that can be rectified with the proper pushrod length.

The LS6 heads were first released in '01 on the Z06 Corvette and are available today in assembled form from GM Performance Parts (PN 12564824, casting number 12564243) loaded with hollow stem valves (these were factory pieces from '02 to today) and LS6 valvesprings.

To learn more about the Gen III V-8, check out www.cartechbooks.com to order How to Build Performance Chevy LS1/LS6 V-8s.

The LS1 (above) and LS6 (below) combustion chambers look different and have slightly different volumes. The port and chamber design work for these heads was done mostly with supercomputer modeling, some flowbench and dyno testing, and a little in-vehicle tuning.

A quick way to narrow down the type of cylinder heads on your Gen III V-8 is to look at the bolts holding the valvecovers on the heads. The '97-'98 LS1 Gen IIIs had "perimeter" located bolts...

...while the '99-and-later Gen IIIs had "centered" bolts. The centered bolts were to be the design all along, but the engineering validation process was not complete when the LS1 went into production in mid-'96, so these engines used the proven perimeter bolt pattern.