My cousin from Chicago visited me last weekend so I loaned him my car. Anyways, my car was tuned real good so I didn't really care if he punches the heck out of it..anyways...When he gave it back to me, he had put a new tank of gas in (about 1/4 emptied). I didn't think it at first but then I thought to myself..did he put PREMIUM 93 octane in like the car was tuned for?? Nope, he put PLUS (89 octane gas in)...I test drove the car and it detonated even in partial boost...He doesn't know much about car, needless to say detonation. I'm sure he drove the hell out of it too. The car still drives good though.
My question is...how do I determine the damage (detonation) that has been done? What kind of simple test can I do/run?

 

First, throw in a container or 2 of octane boost or lower boost.

Do a compression test. I think your car is fine. If you have an RX7....rebuild your motor.