Thread: COPO heads? Rod Length on 496?

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No, that's not necessarily inaccurate. It's true that too much port volume for a given displacement will slow the speed of the mixture going past the valve and result in inadequate cylinder filling at lower rpm's.

But here's my take on your situation. These heads were designed to feed a 396, probably at an elevated rpm level in a hi-po factory piece. But you don't have a 396, you have a 496. That's a 25% increase in displacement, so the heads will seem smaller to the motor than they did on the 396 and build max torque at a lower rpm level than they did in the 396.

I guess it all depends on what you want to do with the motor, but I don't think I'd go to the time, effort and expense of building a 496 stroker and then trade off some nice heads for oval port units unless I wanted a low rpm stump-pullin' motor. Now, if a stump puller is what you're after, then by all means use the oval port heads.

If it WAS a little slack on the bottom with the rectangular heads, which I don't think it will be, I'd adjust the combination with a looser converter.

I WOULD use a dual plane, 180 degree intake manifold such as this from Chevrolet....
3933163 High Rise Intake Manifold, Rectangular Port
Your big-block Chevy will breathe easy with this high-rise aluminum intake manifold. This manifold's dual-plane design produces impressive low-speed torque and plenty of high-rpm horsepower. It can be used with high-performance cast iron and aluminum cylinder heads with rectangular intake ports. The carburetor mounting pad is machined for a standard flange Holley four-barrel.

and a Barry Grant 850 annular discharge carb. I'd build it 9.5:1 with a 0.040"- 0.050" squish, zero deck, headers and a hydraulic roller cam something like this...
http://www.cranecams.com/?show=brows...tType=camshaft

2,500 converter and a 3.70 gear should do the trick.