Compression ratio? 787

[scooper77515]

Compression Ratio = 12.00:1
Corrected CR = 5.9:1

HUH?

I know in my car, the calculated and "true" running compression ratio is different, but that is by just a fraction (due to cam overlap, etc.). What is up with losing over half your compression when corrected?

Something to do with being 2-stroke?

 

[Chester]

Yes, it's because it's a 2 stroke.
The 12:1 compression ratio is calculated as the difference in cylinder volume between top dead center and bottom dead center. That number is irrelevant when discussing 2 strokes. The piston doesn't start compressing the air/fuel mixture until it is about half way up the cylinder when it closes the exhaust and transfer ports. The Corrected CR is the true compression ratio.

Chester