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9:1+
It should have a minimum of 70 psi.
Actually HP can only be estimated . he HP depends on a lot of factors about the motor.
A diesel engine requires at least a 20:1 compression ratio, or about double a gasoline engine. The PSI this translates to depends on the size of the cylinder. You will need to consult the manufacturer's service manual.
On a exmark lazer z e-series 250 cc motor
10:1 - 12:1 through that range is fine
250 cc does not correllate to HP. Little 250 CC bikes are the fastest class of motorcyles--turbocharged, 10,000 rpm, high compression fuel injection running nitromethane--yikes! The most efficient engines would be very tiny but have all the ups and extras. Different 250cc engines put out anything from 1-100 horsepower. Essentially, it depends on how the engine is designed and what its intended purpose is.
14:1 compression
120psi
60 p.s.i.
Alot.
possibly a lot of carbon build up in the combustion chamber but in a lifetime I have never had a small engine with to much compression.