Effective Compression Ratio

The real cylinder compression once boost is added to the static mechanical ratio, the true predictor of detonation risk under boost.

Effective compression ratio (ECR) accounts for boost on top of the engine's static (mechanical) compression ratio. A 9:1 static engine under 14.7 psi of boost is breathing air at roughly twice atmospheric pressure, pushing effective compression toward the high teens. ECR matters because detonation risk tracks the real cylinder pressure the engine sees, not the static ratio stamped on the build sheet. This is why high-compression engines tolerate far less boost before they need race fuel, retarded timing, or methanol injection. It directly drives your safe timing and minimum octane. HP Tuners has no ECR channel, but you estimate it from static compression and logged MAP, then use it to set conservative timing in VCM Editor spark tables and to decide fuel requirements. The discipline holds: command less timing than you think you can get away with, run a wideband, and confirm with knock data before pushing ECR higher with more boost.