By the common method: ECR = CR X (ATM + PSI)/ATM
For 9:1 CR and 6 psi: 9 X (14.696 + 6) /14.696 = 12.7:1
This does not work.

To approximate the effective compression ratio from the static compression ratio and boost, where:
ECR: effective compression ratio
CR: static (nominal) compression ratio
A: atmospheric pressure, in pounds per square inch (psi); 14.696 is approximately correct for sea level and 70° F

ECR = CR × ((B + A) ÷ A)^.74074 (.74074 exponent, the reciprocal of a sample ratio of variable heats at 1.35)

For 9:1 CR and 6 psi: 11.6:1
For 9:1 CR and 12 psi: 14.0:1

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