Engine compression test values

why does the compression gauge read in lbs., but the specifications are in ratio, and how do I convert my readings to a ratio?(and/or versa-visa?)

I don’t know if it is possible to convert the ratio to PSI.

Why do you ask? Are you testing compression and all you can find for a spec is the ratio?

When I look under engine specs I find the compression ratio but if I’m looking for the compression specs I look under engine testing and inspection. I have ALLDATA at work and that is how compression ratio and compression specs are listed.

The two items are related in a tenuous way. Mechanical compression ratio is the volumetric relationship between the cylinder and the combustion chamber. If, for example, a cylinder has 500cc of volume and a combustion chamber has a volume of 50cc, then the mechanical compression ratio is established as 10:1. Compression pressure measured at starter motor cranking speed, however, is determined by a combination of the mechanical compression ratio and the intake valve closing timing which will vary among engine brands.

Another way of saying it is that the mechanical compression ratio is different from the dynamic compression ratio. Going on from there, the dynamic compression ratio will vary depending on engine speed, throttle position, and even more with variable camshaft timing.

In an absolute perfect world the compression pressure would be the (ratio -1) times 14.5 pounds. That would be the absolute high end.

Don’t worry about the relationship. Any Haynes manual on your car or other source will gave the required readings is lbs. The importance is the variation between the readings. If the car wears out uniformly, all readings will go down by the same amount. A variation of 15% or more between cylinders indicates you should repair the engine.

On one of my cars, at 320,000 miles the compression on 6 cylinders was still 100%, while the remaining two had 94% and 96% indcating the engine was still sound.

In large industrial engines, an average of 85% of new compression means an overhaul must be performed.