340 Cam Size

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gabe380 said

I've always been taught that there is ZERO correlation between cylinder compression (tested with a gauge in the spark plug hole) and compression ratio. In fact, changing cams (and nothing else) can result in a different reading on the pressure gauge.

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only partly true; it's a jumping off point.
If your gauge is accurate, and you know what elevation you are at, and what cam is in it,Then you can back-calculate the Scr within a tenth or two.
But to be accurate,you do need all three; psi, elevation, and Ica. And your results are only as accurate as your inputs.
If you don't have all three, or don't have a good calculator; then it is absolutely true.
An error in Ica maths to about just over 1psi per degree.
An error in the elevation can be 5psi per 1000 ft
An error in the gauge, well that can kill the whole deal; garbage in/garbage out.