Air molecules (nitrogen, oxygen, etc.) at altitude are less compact (more spread-out) so, doesn't this effect the compression on our engines?

If I have a 12:1 compression piston, what will the compression be at 4000' elevation where I usually ride?

If it goes down by 12 % (4% / 1000 feet is common de-rate for a boiler burner) I am now at 11:1 (12X.88)

Does this math work? I am just guessing here.

Or, is compression a ratio to outside pressure and the only loss is the loss in the number of air (and specifically, Oxygen) molecules available for combustion?

Thanks for your input.

 

compression is a ratio and hence doesn't go "down" as alt increases. But, as you know, air density does, so the effective engine compression is lowered. Got bansheeHQ.com and track down "meats" compression table. I think he includes altitude. Its quite thorough.

I always use a 3.5 % HP loss per 1k of alt, and would suspect that correlates pretty well to air density.