Octane?

I'm a bit confused on the "requirement" for some cars to run on premium grade fuel over the standard 87 octane.  Some people tell me yes, some no.  A Mitsubishi mechanic said that using 87 in a 2004 eclipse (which is soppse to run on premium) will only decrease horse power by 2 or 3, and mpg by about the same.

Some of you guys blow my mind with your knowledge of engines and cars so could you please help me out?

adrenalinejunky wrote: but cars have knock sensors that detect this and readjust the ignition timing to compensate, which is what results in the slight loss of power/milage.

This is the key.  Almost all (all?) modern high-compression engines also have sensors that will compensate for lower octane fuel, and the only negative effect is slightly reduced power and mileage.

BUT - on older cars this may not be the case.  Some older cars cannot adjust, and will experience knocking, which can cause a variety of problems.

Bottom line - if your car starts knocking on lower-octane fuel, switch back.  Otherwise you should be fine.