347

Is there any way of telling roughly what comp ratio I have from doing a compression test given I know the size of the comp chamber and the stroke?

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blue66coupe

Depends heavily on cam specs also and what lash (if any) adv/retarded etc. Short cam might have 230psi big cam 170psi with same static comp ratio. I know there are formulae to work out dynamic compression ratio, maybe use these and factor it out. There is probably a simple formula to rough it out.. anyone?

xpc66

The ovious question is "why not just work it out the conventional way?".

To work it out using actual compression is very difficult. You need to know your leak down rate and volumetric efficiency at idle, plus your inlet valve closing angle, stroke and rod length to convert from dynamic CR to static CR, to name a few, but......

having said that if you run the engine at idle and measure the cylinder pressure and the manifold vacuum:

e.g compression pressure = say 42 PSIA, manifold pressure = 18"Hg;

18"hg = 8.8 psi, so absolute pressure going in (14.69- 8.8) = 5.89 PSIA

DCR = 42/5.89 = 7.26

with a little trig on the inlet valve closing angle of say 70°, stroke 3", rod length 5.09".

Dynamic stroke length =2.2"

CR = 9.5

Like I said it's easier and more accurate to work it out the conventional way:- using a reasonable calculator that takes into account toplands, dishes/domes, gaskets, deck to deck heights, etc.

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