Cylinder pressure to determine dynamic C.R.?

[John Chesher]

Hey engine building experts,
The performance of my newly rebuilt engine is not quite what I expected. I'd like to confirm that the compression ration is not terribly low, so I'm curious if I can determine the dynamic compression ratio of the engine by taking the average pressure of each cylinder, measured with a gauge in the spark plug hole, then dividing this number by 14.7, which is average atmospheric pressure in PSI. Or, is the daily difference in barometric pressure so much that I'd need to factor that into the equation, or is this just not a correct way to determine the dynamic C.R.? If you have to consider the day's barometric pressure, anyone know how to factor that into the equation?

[Duke W.]

Re: Cylinder pressure to determine dynamic C.R.?

No, cranking compression pressure will not yield an accurate estimate of compression ratio, which is actually a volume ratio, not a pressure ratio. Heat transfer during compression and inlet valve closing characteristics render use of adiabatic formulas meaningless. Local ambient pressure variation is a minor influence.

The trouble is that most "engine builders" just throw together a lot of mismatched components like high overlap cams and low compression pistons with thick head gaskets, so you end up with poor torque bandwidth and mediocre top end power. Such "builds" have ZERO engine system engineering content.

See my discussion of CR in an earlier thread today - "C-l head gasket..."

Duke

[John Chesher]

Re: Cylinder pressure to determine dynamic C.R.?

Thanks Duke! Yeah, it was that post that got me thinking about this. As always, you are a wealth of technical information. I specifically search for your posts, as they have taught me a lot. I appreciate it!