[COBRAIIW]

I was told that you could pretty much tell what a motor's compression ratio is by the numbers you get during a compression check.
Is there a rule of thumb, or formula on this?
I'm trying to determine which octane I should be using, got a small amount of clatter under load.

[T. Hadley]

if its stock you can look up what the cranking compression should be, for the stock CR.

But if after market parts are being used their are to many factors that change cranking compression.

pistons
cam
how the cam is installed, adv/rtd
ring type
ring seal

Is it at part throttle rattle, If so try tuning the vacuum advance