I finally got the bike back and it's running like a charm. My mechanic mentioned that compression in the cylinders is 185 (left) and 190 (right). I read online that the way to calculate compression ratio is to divide the psi by the atmospheric pressure (14.7). Is that correct? If so, looks like I have between 12.6:1 and 13:1?
BTW what was the stock psi/compression ratio on the CB77?
Compression Ratio vs PSI
The cranking pressure is a result of the static compression ratio and the intake valve's closing point (along with the valve and ring sealing quality, stroke, rod length, atmospheric pressure, etc.). You can have a 14:1 static compression ratio racing engine with a late closing intake, or an 8:1 street engine with a very mild cam, and they both could pump 185 PSI. No direct correlation. Published figures show 9.5:1 compression ratio for the earlier pre-1965 CB77's, and 8.5-8.7:1 for the later ones. My 358cc Bore Tech engine also happens to pump 185 to 190 PSI with stock cams. Regards, Chase
Last edited by cknight on Tue Sep 26, 2017 4:23 pm, edited 1 time in total.
compressionHi Vince
Glad to hear that you got the bike up and running. As for the way to calculate the compression ratio I don't think it's that simple. The ratio of V1/V2 is based on the difference in volume between BDC and TDC. I checked my manual and the stock compression pressure is 149.34 psi and a compression ratio of 9.3 to 9.7. Merz 67' CB77
66’ CB77 65’ bomber 68’ hellcat
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