I have a 91 Z28 5.0L TPI and I was wondering which fuel to use, either 89 or 91. My engine as about 130k miles.

 

If you're stock and haven't advanced the timing at all, regular (87 octane) should work just fine, and I think thats what it was made for from the factory. If you run 87 and it knocks, then youre going to want to run a higher octane.

 

You only need enough octane to prevent detonation under load. The only way to see the knock counts is with a scan tool. If the computer sees too many knocks, it will retard the timing until the knock count drops. This will feel like a lack of performance. Going to a higher octane will feel like more power but all it will be is that the computer isn't retarding the timing.

Using too high of an octane is a waste of money and can actually slow you down because the fuel will be slower to burn.

If the engine doesn't knock with 87, you don't need anything higher. Use a scan tool to find out the knock counts under load.

 

I never heard of that before, but thanks for the recommendation