compression vs. compression ratio

MOst of the 400s I've dismembered have 452s, but as Mike said, 346s are possible on an early one. I think you are confusing two different measurements here.

Static compression cant be approximated by "pumping" an engine. for two reasons, first, leakage would add volume to the reading, and second, because you'd have to add something and measure what you're adding to get the two volumes to compare. Some racing bodies use this test to determine displacement, but I havent ever done it or seen it done. Compression ratio is a ratio of volume before the piston moves up from TDC, and the volume of the space left after the piston stops at TDC. it is not dynamic at all, and assumes the cylinder is sealed from the moment the piston moves up, and there is no leakage past rings or valves. This is not the case, as cams close the intake after the piston begins up, and very few rings seal at 100%, more like 96% on a new but seated set.

Cranking cylinder pressure is what you are actually measuring by using a compression gage..taken into account by this test it ring seal, valve seal, and the time the cylinder is sealed, and the piston is moving up. That's why the exh valve timing is the only figure that matters. And this is a much more accurate way of seeing how a street engine will respond in the lower rpm band. Depending on the cam timing, the same engine may read 130psi, 150psi, or 175psi.

Dynamic compression ratio is another figure of importance. Think of it as the cranking cyl pressure, but taken at higher engine speeds. It's most useful in determining peak torque ranges, and octane requirements...Theres a link on another board to try to figure that out if you dare..it gave me a headache, but theres good info there.

Basically, I'd look for cranking pressure readings in the 115-130 range for a 509 cam in a fresh 8:1 engine. BTW, this test should be done with a good battery, no plugs in the engine, engine warmed up, and the carb held at wide open throttle.
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