Modern Vespa

Hey guys,
I understand compression ratios are based on volume.
Does this translate to a compression test in terms of psi? Could one multiply their atmospheric pressure by a given ratio (my case, 10.2 x 14.7) and expect that result?

When oil-testing piston rings, what psi difference says the rings are shot?
I'm getting 15.

Theoretically, yes. Make sure your compression gauge has a schrader valve in the end of the hose. Without one, these small displacement engines will test very low.

Theoretically yes but actually no. 2 stroke compression ratio to pressure only works using the trapped ratio, which is the volume above the exhaust port. Another reason compression testers are just for rough guidance on 2 strokes.

I have no input about a formula for translating compression ratios to cylinder compression, but they do overlap each other. Ideally you want them both individually to work together in 2 stroke harmony.

I see a compression tester as vital to find out a baseline for ring condition and cylinder PSI. Ideally, you would do a compression test when your top end is first broken in, then you can go back and compare the PSI readings once you put a bunch of miles on the piston and rings. A compression test would also let you know if you have too much compression. I cylinder with 180 PSI will run MUCH more hot than one that's closer to 120 PSI!

Also keep in mind that a 14.7 is the atmospheric pressure at sea level. A 5000' elevation will have an atmospheric pressure of 12.2. This will cause a significant lower PSI reading on a compression test. That means that an engine that is tuned for higher elevations, additional means of modifications will need to be made in order for the cylinder to have as much compression as an equally built engine at sea level.

15psi difference is a fair amount when performing a "wet test". I'd definitely take a closer look at the rings.