Altitude can affect pressure in the cylinder that could be measured with a compression test, but it does not affect compression ratio. Compression ratio is the ratio of the combustion chamber volume when the piston is at bottom dead center to the volume at top dead center. The ratio does not change when you bring the bike to a different altitude, it is a concrete number.
Actually your definition applies to STATIC (or mechanical) compression ratio for both two and four stroke engines, and the definition Marcus posted is DYNAMIC (or trapped) compression ratio for two-strokes. Dynamic compression ratio on a four stroke is measured from the intake valve close point in the cylinder. The dynamic compression ratio is the only one that has any real value, unless you are in marketing :)
There in lies the partial answer Rich. To my knowledge KTM does NOT post compression ratios in their marketing materials, or their manuals. I do have an engine manual at home that I will also check. So, other than the do it youself measurement, and the type of compression 'ratio' you are trying to achieve, the only quick and easy down and dirty measure that the KTM owner has is a compression test (PSI). The 2-stroke variations that Marcus mentioned is probably why KTM declines to provide any measure.
So KLXsmoker, you slam me because I provide the only measure of compression that I have access to on one of the specific bikes in question, and then you provide a 4-stroke answer to a 2-stroke question. O.K.:silly:
I will post back whether or not the KTM engine service manual listl the Compression ratio. I am fairly certain that 'vanilla' owners manual/guide doesn't have the info.
I don't know much about KTM's and assumed that the 380 was a 4 stroke, so my bad. I didn't realize that it was a 2 smoker until Gunby mentioned exhaust ports.
The original post asked for an engine's compression ratio, and that's what I tried to help him understand. Even if the mechanical or "static" compression ratio is a useless number, it's still what the guy asked for.
Strick, I'm not slamming anybody. You came back at me kinda snotty and I simply answered your question.