L92 Cylinder Heads Install - It’s All In Your Heads

Putting Modified L92 Cylinder Heads on a 4.160-inch Bore Block

Ro McGonegal May 24, 2011 0 Comment(s)
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I was advised to leave the engine combination as it was built, because each piece had been selected for

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its specific contribution to the whole. That was years ago, ancient really, in 2003. Arguably, this engine built by Katech at the behest of then-Chevrolet Special Projects engineer Mark McPhail, was the largest of its type (4.160x4.160) at the time, 452.3 ci to be exact. Today, LS engine technology is a quantum leap from 2003. Through the use of deck spacers and/or tall-deck aftermarket blocks, 500 ci or more is stone reality at a considerably cheaper rate.

View this engine, based on the hideously expensive C5R aluminum race block (currently tagged at $6,400 to $9,250, depending on aftermarket vendor), as one built from a 9.260-inch low-deck GMPP LSX Bow Tie block that can be had for as little as $2,000. Since it is iron, it has been designed to withstand tons more power and torque than the optimum, aluminum C5R—which was originally envisioned as a road race entity that likely would not exceed 427 ci or 700 hp.

But a 427 wasn’t big enough, McPhail reasoned, so what we really needed was a 454. The crankshaft in this engine is a prohibitively expensive billet piece as crafted by the legendary Hank the Crank Bechtloff (now retired). Today it would fetch on the far side of $4,000. By comparison, a GMPP LSX or Lunati long-stroke forging can be had for thousands less. The connecting rods are Carrillo H-beams; the pistons are CP forgings, again built especially for this project.

On the Katech dyno this engine produced 554 hp at 5,600 rpm and 577 lb-ft of torque at 4,400 rpm, providing more than enough power to propel a 4,000-pound vehicle to an untuned, tire-spinning 12.20 at 115 mph. On the street, however, the B-body was uncannily able to dodder around in Fifth or even top gear without so much as a whimper or a shudder, smooth and linear.

But greed hooked us. We surely wanted to see at least 500 hp at the wheels. Enter LS engine builder/tuner Mike Norris, a Corvette doctor and fabricator who welcomed the accommodation and expanse of the ’66 Biscayne that this engine is in. We’d read about the affordable but highly productive qualities lurking in the L92 truck cylinder heads, ones that were posed on a 4.00-inch bore. We’d read about gains of 40 hp or more, but how would these castings enable an engine with a larger bore size and how much of an improvement would they make compared to the early 64cc combustion chamber GMPP CNC LS6 heads?

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As it turned out, not enough to justify the time and expense but happily better than the flywheel claims we’d read about. Under throttle, the engine responds better than it did before in a smoother, more linear exhibition. By the seat of our shorts, the sled doesn’t feel demonstratively quicker. We’d seen a bit more than 19 mpg on an 800-mile scoot from Memphis to Tampa (0.62:1 Sixth gear, 3.89:1 axle gear, 2.41:1 final drive) keeping the revs at 1,900 rpm or approximately 70 mph. Though torque peak has been raised a few hundred rpm, throttle response at low revs in higher gears hasn’t diminished one iota.

At the rear wheels, the changes produced a gain of 31 hp and 15 lb-ft of torque. We thought that the Scoggin cam would pick up even more, but it did not. The upshot? Even though they’ve been extant for at least eight years, the “obsolete” CNC LS6 cylinder heads (commonly $400 more per set than CNC L92s) and matching intake manifold are extremely capable devices. If you’ve already got the stuff, keep it. If you’re building from scratch, you should definitely investigate the CNC L92s.




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