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Under certain conditions (high load, warm day), you'll start hearing a 'pinging' sound on high-compression engines; that's the air/fuel mixture pre-igniting before it is supposed to (before the spark fires) essentially turning the engine into a wannabe diesel and ruining performance/efficiency.

There are a few other undesirable things about high-compression.

And I just read your message about knocking. same thing. Nevermind.

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Modern engines (as in, post 1989) will sense an engine knock (or a misfire) and go into open cycle ECU mode, ignoring the lambda sensor inputs. It'll run really rough and rich in order to avoid any more knocking, which could seriously damage the engine.

And to those curious on how it senses it, they literally stick a microphone in the engine calibrated for the ping (knock) frequency.

that actually sounded like such a low tech solution that I wasn't sure if it was some kind of joke that was going over my head.

Thanks for sharing, that's actually really fascinating.

the patent for anyone interested: https://www.google.com/patents/US4040294

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