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IHRA Chevy Big Block - From Street To Strip

Prepping The ZZ 502 Crate Motor For IHRA Top Stock Competition

By Barry Kluczyk

Kit In A Crate
The Top Stock rules demand a crate engine package, which Bogan's ZZ 502 certainly is, but McLaren Engines started with an unassembled 502 crate motor kit (the 502 is available from GM in kit, long-block or fully assembled versions), PN 12371171. Because many components, such as the pistons and rods, would be replaced, starting with the kit saved time that would have otherwise been spent disassembling the long-block. The photos that accompany this story provide a thorough description of what it takes build the street-friendly Chevy 502 into a competitive race engine.

In a nutshell, the engine will spend most of its time between 5,000 and 7,000 rpm, and it's built to "live" there. Like almost every other race engine, this 502 draws its breath through a single-plane intake manifold, which helps the high-rpm velocity. Interestingly, IHRA is pretty tight about the carburetor, requiring it to have a choke. So, an out-of-the-box Holley 850 Double Pumper mixes the air and fuel.

Here Are Some Of The Engine's Other Basics:
*Stock, but balanced, crankshaft
*Stock, cylinder heads that are cc'd and treated to a three-angle valve job (per IHRA rules)
*IHRA-legal, heavy-duty pistons (with full-floating wrist pins) and connecting rods
*Custom-grind camshaft (with IHRA-mandated stock lift specs)
*Heavy-duty valvetrain
*IHRA-mandated, SFI-approved balancer
*Custom, windage-reducing oil pan
*Numerous balancing, deburring, and other block preparation details.

Rules set down by IHRA for Top Stock competition are designed to keep racers within reasonable budgets. This is why, for example, the choke is mandatory for the carburetor; it dissuades the owner from spending tons of money on a completely custom carb. Same goes for the off-the-shelf intake manifold and cylinder heads.

For engine builder Halvorson, such restrictions aren't a hindrance, they're just the rules of the game, and he uses them to define his game plan. Though he can't port the heads, gaining high-rpm flow is still a goal which can be attained by using a custom cam (as long as it retains the stock specs).

But since the rules stipulate things like stock camshaft lift, stock combustion chamber volume, and stock piston design, all of the obvious horsepower-increasing tricks seem out of reach. For example, a three-angle valve job is allowed, but angle-milling the heads and any port work is strictly off limits. So, Halvorson turned to the engine's finer details. By swapping the street-based, dual-plane intake manifold for a high-velocity single-plane part, he was able to feed the engine at high rpm and, correspondingly, spec a cam with lots of duration. At the upper end of the rpm range, this combination really makes the big-block come alive.

But Halvorson is quick to point out the power gains don't come from just one place and says that several horses were picked up by swapping the engine-driven water pump with an electric pump. Also, great gains were realized by designing an oil pan that virtually eliminates crankcase windage. These are parts that wouldn't work on a street engine, but they're race-legal and give the engine a competitive edge.

Indeed, on the dyno, this engine produced 657 hp-about 30 percent better than stock-with a stock-lift cam, stock-style pistons, and virtually the stock compression (9.84:1 vs. the stock rating of 9.6:1). All that, and an off-the-shelf, four-barrel carb, too.

Other changes, such as the connecting rods and the addition of a camshaft thrust bearing, were done more for racing durability and high-rpm accuracy than horsepower gains. Most surprising, though, is the number of parts not changed in the build-up. Halvorson credits the crate motor's inherent strengths. "We could have changed the crank but didn't have to," he says. "The GM crank from the kit is already a forged, balanced part that's as strong as anything needed for Top Stock."

By Barry Kluczyk
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