is there any sort of relationship between compression and vacuum at idle? would it be correct to assume that a car with low compression, should have low vacuum at idle? say for example... an engine with a compression of 55-60psi (low compression), how much vacuum would i expect to see at idle... or would compression that low even allow the car to start...?

 

an engine with low compression at low running speeds (like at idle, or when cranking the engine for the compression test) won't be as powerful at idle speeds, so it needs slightly more airflow to make enough power to idle. this equates to lower manifold vacuum. this can because of the engine being worn out OR because the engine has modified ports that don't work as well at idle/low speeds.

 

55-60psi is really low. Anything under 80 is pretty rough, with 120 being tops. 55-60 might start with some coaxing..but I doubt it'd run for long.

Peejay was dead on.