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Q: What grade of gas should I put in my 2008 grand prix

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My car start knocking when I put 87 in my gas tank

My car has 121000 miles.
My car has an automatic transmission.

A: Your car should run just fine on 87 octane ...

Your car should run just fine on 87 octane fuel unless you have other modifications to your car such as a super charger or turbo charger. The knocking may suggest that you have something else wrong with your engine. Your car's model came in a couple of different engine packages, one is a GT and the other is a GTP which is a super charged model. The GT model will run just fine on 87 octane fuel. The GTP model will need 91 octane fuel in order to keep it from knocking due to the advanced timing of a super charged engine.

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