Originally Posted by Doctor J
Octane rating does not relate to the energy content of the fuel. It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner. Where the octane number is raised by blending in ethanol, energy content per volume is reduced.
87 octane has more energy in it than 91.
As long as his engine is not pinging -- no harm done really.
and to the person that mentioned "LED" (sic) --- there hasn't been any lead in pump gas for years and years
Technically, I believe octane is a measure of how resistant the fuel is to knocking (caused by premature detonation of the fuel). You don't want knocking because it reduces engine preformance and will eventually damage your engine.
However, I agree that putting 87 octane gas will not damage the engine. Most cars today, including ours, have very sophisticated engine management systems that allow them to run on fuels of different octanes. The car's computers will make adjustments to avoid engine knocks caused by variations in fuel octane. However, the manufacturer designed and tuned the engine to run at its highest potential with a certain octane fuel, and if you do not use that octane fuel, the engine will not run at its highest potential. This is why those of us here in CA will never get to enjoy the full performance potential of our cars because we only get 91 octane gas (unless we find special stations that carry higher octane gasoline) and I have read that our engines were designed to run with 93 octane gas.
Thus, while I agree that it's cheap and stupid to save a few bucks and put 87 octane gas into an M3, he was not going to harm the engine. At least the guy was putting tier 1 gasoline into the car. Bad additive packages from cheap or no-name gas stations will hurt your engine more than 87 octane gas ever will.