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 but regularly driven, identically fueled street engines. As miles on a street engine pile up, combustion chambers accumulate deposits that increase the engine's need for higher-octane gas by raising both 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.

Octane creep is more readily apparent on an old engine with a distributor and carburetor, but it also affects late-model, electronically managed, computer-controlled EFI engines. You just aren't as aware of it because the engine computer crutches the problem for a while by retarding ignition advance when the knock sensor "hears" the engine pinging. But retarded timing means the engine won't make as much power as it used to.