I went to check a 97 supercharge engine this morning, the owner told me that he used a mix of 87 and 91 fuel in the past 7 years (some periods for 87 while others 91). In that time I even did not realize it could be a problem.

In the test drving, I did not hear any noise from engine, it worked perfectly and smoothly, speeded up very quickly. But I did find a little carbon accumulation from exhaust pipe, I just thought it might just need some tune-up. The owner explained he parked this car in garage for 2 or 3 months, and only in recent days he added another 3000 KMs on it.

I am not sure for this situation, what may be the possibility of engine knocking because of using low octane fuel... Engine has 12,9000 KMs = 80,625 miles on it. It looks that I need to give up this car. Or is there anybody who would think this is not a big issue....

Well.. it should be fine to run 87 ( even though it'* not recommended ) pendin on how he drives.. if he'* hard on the gas and lays down on the throttle.. he may run into Knock Retard and predetonation.. but if he grandpas it.. it should be ok.. if you're worried that it may have predetonation and such.. check out the spark plug..

I find it amazing how ppl ignore the 'Premium Fuel Only" disclaimer right on the cluster.
Jr posted a long schpeel about the dangers of not using at least 91 on SC'd cars. Hardy, if I were you, I'd get the car extensively checked out. But the kms aren't too bad. Seems like highway driving to me

They are nice cars tho eh?

Yup, it says premium fuel in the cluster gauge on the gas cap and in the owner'* manual. You would think that they get the point with all that advice!

Ooops... I really feel confused about your conclusions....
Not sure anybody has some real experience or know somebody using 87 gas causing problems?

We've all run it from time to time in a pinch, and some have run it because they didn't know. I've never seen a long-term problem from it. Only short-term performance problems. Like Drifter said, your plugs would be victimized. Probably not anything else. The car has an amazing way of protecting itself.

The car should be fine. Have an independant mechanic do a pre purchase inspection before you buy it though. How much does he want for it?

Up here the majority of "premium" is only 91 octane. We do have 89-90 octane with 10% ethanol in it, as well as some places with 92 and 10% ethanol. My question is that when using ethanol, is there a problem? I have heard good and bad with it, and I don't want to use it if it'* not worth it.

It'* fine really, and anythign in a consumer gas pump is fine for any unleaded car.