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Big Block Cylinder Heads - Big-Block Cylinder Head Test

You Don't Need Sewer-Sized Intake Ports To Make 600 Hp With A 496ci Rat Motor As We Discovered In Our. . .

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'There's something about big-block Chevys that demands big cylinder heads. Maybe it's some sort of deep, genetic, missing-DNA-link thing between car crafters and Chevy Rat motors that insists on monstrous, rectangle-port cylinder heads. It's like some kind of baritone inner voice that won't be content with anything less than massive 360cc intake port heads on a 396.

We learned long ago not to listen to that caveman voice. Instead, we decided to look into how much power we could extract from a set of oval ports. That led us to stacking up all the available aluminum oval-port heads on a Rat and pushing 'em hard on the dyno. For a testbed, we used our existing cast-cranked 496 Rat motor built back in the Mar. '07 issue with plenty of displacement to really challenge these heads. We also included a set of production iron peanut-port heads just to see how they would do as a baseline. We really wanted a set of true iron oval ports, but we couldn't find any that hadn't already been ported. We also tamed the camshaft with a hydraulic roller so we wouldn't have to spin the engine as high to get the peak horsepower numbers. The results of this shootout were more than a little surprising. But be sure to read this entire story, not just glance at the power numbers, because we evaluated these heads in a bunch of interesting ways. Taken as a whole, the oval-port Rat market offers heads that are a lot stronger than you might think.

The Mule
If you recall, we built a 496ci Rat in the Mar. '07 issue that made a stout 707 hp on pump gas with a set of as-cast 305cc AFR rectangle-port heads and a big Comp mechanical roller cam. We had to twist that combination up to 6,600 rpm to achieve that power, which created more than a few Internet blog predictions of imminent engine failure focused on the cast crank and stock rods. Despite those doomsday prognostications, the engine is still very much alive even after this latest thrashing. But acknowledging those realities, we know it's just tempting fate to continue to spin this thing that fast.

To remain consistent with a conservative oval-port street concept, we decided to pull back on the cam timing to keep the peak horsepower below 6,000 rpm. This milder Comp Cams hydraulic roller cam timing is also a better fit with the entire oval-port package, including the Edelbrock RPM Air-Gap dual-plane intake manifold and 850-cfm Barry Grant carburetor. If peak horsepower had been the goal, we would have chosen any one of many single-plane intakes that would have easily kicked it up over 600 hp. Taken as a whole, this engine package turned out to be extremely successful considering that the engine idles at 950 rpm with 12 to 13 inches of manifold vacuum and offers tire-shredding torque. That idle quality is also more than enough to support power brakes and perhaps even an A/C compressor. We'd call that very streetable.

Cam: Comp Cams XE274HR-12 Hydraulic Roller
Advertised Duration Lift Lobe
Duration @ 0.050 (inches) Separation
Intake 274 224 0.555 112
Exhaust 282 232 0.565
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