340 Static Compression Numbers and Test Procedure
HF, US General 0-300 PSI Gauge, I suppose it could be off?
Yes! See if you can rig the compression gauge to a compressed air system with a decent gauge (NOT a cheap HF compressor with their tiny junky gauges) and compare. (A large shop compressor will more likley have a decently accurate gauge.)
As for backing into the CR... for a
stock-ish cam, (SCR-1)*20 gets pretty close to cranking compression. But you're not in that boat. As noted.. 200 is quite high for NA, and pump fuel. Explains the detonation as noted.
There is a less invasive way to do this in-car. Pull the intake and measure the cam duration at .050" and .006" lifter lift, plus valve lifts, and check cam timing, so you will have that cam info. With the intake closing angle measured (lifter at .006" from the intake valve being fully seated on closing), and an accurate cranking compression number and your altitude, you can back into the Static CR with the Wallace Dynamic Compression Calculator or with several other such tools. (The Wallace one accounts for you altitude so is useful in that way.)
And you would get a pretty decent cam duration numbers and timing, including that .006" lift intake closing angle,
for a solid cam if you measure carefully at the top of the pushrod, so you could avoid taking the intake off if you just wanted a decent idea of things. But for best accuracy do it right at the lifter. And if it's hydraulic, then it pretty has to be done at the lifter body; pushrod movement when not running will be inaccurate with a hydraulic lifter.
