Volume III, Issue 11, Page 19

High-flow, low-buck L92 truck heads on a 454ci C5R block

This 454 project goes back a few years. Now, you can buy an affordable iron LSX block to get those coveted cubic inches on your side. These blocks are designed to poke and stroke all the way out to 500ci if you’re so inclined.

When we began this build up, however, the only way to do it was with an ungodly expensive C5R race block designed for the Corvette American LeMans Series (ALMS) road race program and an ungodly expensive billet crankshaft (4.160 x 4.160 inches). Today, the block is even more expensive ($6,300) than it was seven years ago. Most of them returned as 427s, thus preserving cylinder wall integrity, as 4.160 bore approaches the ragged edge of structural integrity.

But for a pure street application, this bore size still leaves plenty of meat and will definitely not compromise longevity. Race tests (at 9,000 rpm) on iron LSX blocks with a displacement of 454 cubic inches generated considerable crankshaft whip, enough to wipe out the center main bearings in short order. Since this street engine will never see the far side of 6,300, I can assume with some certainty that this engine will never have that problem.

When I was contemplating the L92 swap, I talked with ex-GM Special Projects Manager Mark McPhail. Mark was the person responsible for the 454ci C5R project in the first place. He was adamant about not fussing with the torque-laden combination I already had (460hp and 498lb-ft, at the tire). He suggested the possibility of maybe a FAST intake manifold but to do no more. “This one’s finished,” he said. “Move on to your next project.”

On the other hand, Nickey Fowler at Scoggin-Dickey really twisted the burner, dangling those big carrots right in front of me. Nickey: “I see no reason why the L92 heads and corresponding L76 [Holden passenger-car] intake manifold wouldn’t increase output by about 100 units on both sides of the equation.” Then he sealed the deal. “Whatever you can imagine doing to this engine, well, I’ve already done it, so don’t be afraid to ask.”

The L92 was developed for a 4.00-inch bore, so it will also fit the 6.0L truck block as well as the 6.2L application in the Denali and Escalade. It has raised port floors and increased port width. Dennis Gerdeman, Project Engineer: “…it [the L92 casting] had to go down our existing production lines. We took everything [from the C5R head] we could feasibly apply to the program, but then that’s where at lot of lines got drawn in the sand: we had to maintain certain pushrod lengths, [15-degree] valve angles, and so on.

“We pushed the roof up just about as far as we could without wiping out the valve train, then we brought the floor up to accommodate the raised roof. The other big constraint for us has always been the width of the port: if you compare its width to a Gen-3 (cathedral port), the L92’s is significantly wider and right near the pushrod has always been our constraint. So we incorporated an offset inlet valve rocker arm and that allowed us to pull that pushrod off to the side. Now it’s at more of angle to the lifter, and when we did that we were able to widen the port substantially.”

Whereas Mark McPhail was instrumental in acquiring the cylinder block and hastening the build-up at Katech, Hank the Crank Bechtloff was responsible for filling its insides with the best stuff available. He built the billet arm with Honda 2.10 main bearing sizes, incorporated corresponding Carrillo forged steel connecting rods and hung CP pistons on the little ends. The result was a 10.7:1 long-block, with CNC LS6 cylinder heads, a ZO6 intake manifold, a GM Hot Cam (219/228 degrees at 0.050-inch, 0.525-inch lift, 112 degree centerline), and SLP 1.85:1 rocker arms. At the flywheel, It produced an average of 554hp at 5,617rpm and 577lb-ft of torque at 4,399rpm.

At first, the L92s looked like the answer to a question that wasn’t on everyone’s lips. The main concern is bang for the buck. A pair of these beauties in full dress and in as-cast form, is about $800, less than a third of the price of assembled LS7 heads and about the same as one complete CNC LS6 cylinder head. 

Matt Hilton’s HPS Cylinder Heads, made something out of all the parts we threw at him. Using his well-trained eye, intuition, and a flow bench, he was able to get some cherry numbers from the el cheapo castings. Matt: “Although they appear a slight bit high, remember that the CNC process leaves small ridges that tend to interrupt flow somewhat. More and more racers are bringing me CNC’d heads that I smooth for them.”

Consider this story a teaser. We’ll test these heads on the 454 shortly and get you some real numbers right off the back tires and some intuition from the seat of our shorts.