What Is Octane Creep?

New engines on the dyno and regularly refreshed race engines run OK with higher static compression ratios compared to otherwise similar, regularly driven, identically-fueled street engines.

- Marlan Davis

As miles pile up on a street engine, combustion chambers accumulate deposits that increase the engine’s need for higher-octane gas by raising the effective compression ratio as well as inner cylinder-wall temperatures. These deposits cause audible spark “pinging” or “knocking” to increase. Engineers refer to this syndrome as “octane creep.” Severe knock can shorten engine life. To a point, slower-burning, higher-octane gas reduces audible spark knock.