V8 Engine Swap
- Aug 9, 2007
- Reaction score
- C. Wisconsin
- Vehicle Year
- Make / Model
- Engine Size
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You are Really misinformed here. It depends on which 351W heads he's going to use. As for the bolt size difference, the cylinder head locating dowels determine where the head sits on the block. The bolts have nothing to do with this. And the so called port mismatch is also BS. The heads would have slightly larger ports, so a slightly smaller intake manifold port dumping into a slightly larger head port is a positive as far as flow goes. Even the other way around was proven long ago to not be a hinderance to flow (bigger intake into a smaller head port) it created turbulence in the flow which enhanced mixing of the air and fuel. (351C 4 bbl intake on a 2bbl head and 429CJ intake on a std 429 head) The chamber volumes are all over the place on the small blocks, so the comp ratio may or may not be reduced.You will lose compression as the 351 has a larger combustion chamber, the head bolts for the 351 are larger, 1/2 vs 7/16 for the 302 (as you know) which can cause issues with the heads shifting, making them prone to blowing head gaskets.
The big idea with doing it for "performance" was that the 351 had bigger intake valves and so it could flow better, the problem is that with the mistmatched cylinder and intake port/valve you end up lowering intake velocity, and reducing overall performance.
Someone would have had to convert those heads to screw in studs then. As for the rail rockers, that's easily fixed with a set of aftermarket rockers. I've ridden in a 70 Boss and it was no slouch at any rpm.I also have a set of C8OE heads off a 68 302 that have the same size valves and 63cc chambers, screw in rocker studs, and unfortunately rail rockers. Heads that are too big cause low rpm dogginess and a high rpm screamer. Ever drive a 69 Boss 302?