Hello: I know that the force in a combustion chamber of a gas engine is around 1100 psi. lets say the ratio is 9.0 to 1.

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Hello: I know that the force in a combustion chamber of a gas engine is around 1100 psi. lets say the ratio is 9.0 to 1.

Do you meant 'pressure'?

Well,

Pressure = \frac{Force}{Area}

If you have the area of the piston, you have the force on it :)

Hello: I know that the force in a combustion chamber of a gas engine is around 1100 psi. lets say the ratio is 9.0 to 1.

To add to what Unknown said - the compression ratio is immaterial to your queston. The pressure when the air/fuel mixture is compressed 9:1 is much, much less than the pressure once the mixture is ignited. So if you aleady know that the pressure after ignitionis 1100 psi, just use the formula that Unknown gave you to determine the force on the piston.