what's compression ratio?

[Scribbl3s]

hey guys,
i was just wondering what compression ratio is. i've heard it a million times, but when i did searches on it, i can't actually find the definition. And what's being compared...like when they say 9:8:1 or whatever... what's it mean? thanks

 

[Defiant]

Well, if I can do this without getting it backward, it's the volume of the combustion chamber with the piston at bottom-dead-center divided by the volume of the combustion chamber at top-dead-center. So far as I know, the only true way of determining it is by using a lexan etc. plate over the head and measuring how much (usually) kerosene it takes to fill it, then doing the same with the cylinder. It takes a fairly tiny amount to alter the ratio, such as valve unshrouding, smoothing and polishing the combustion chamber, or a change in piston pin height.
So if your CR was 10:1, the chamber would be 1/10th the volume at TDC than at BDC. Up to a point, a higher CR means higher output, but also higher emissions. Turbo engines start with low CR so that when the boost is on -effectively "raising" the CR, kinda- you can still manage knock, a larger problem with whatever that stuff they sell us as being gasoline is these days.

 

[Scribbl3s]

thanks defiant. i appreciate it, man.