So what make, model, year car are you talking about. Most cars built in the last ten years have detonation sensors, and most OEs have gotten away from the dire warnings in the owner's manual about using less than the recommended octane.

Case in point. The 2005 C6 Corvette. The new LS2 6.0L 400 HP V-8 has a quoted 10.9:1 compression ratio. The fuel recommendation is "premium recommended but not required".

If you use regular the detonation sensor will pick up detonation and the ECU will select a less aggessive timing map and adjust it from there as required. This may be noticeable to sensitive drivers as less power, particularly at the low end of the rev scale.

Nearly any car with a detonation sensor will operate without signficant detonation on regular fuel even if the recommendation is premium. but the later the model year, the more sophisticated the algorithms to "tune" the engine to the octane number it is consuming at the instant.

Duke

Duke