Without Premium fuel, your more than likely experiencing pre-detonation when your engine is under boost or under load. That is a bad thing and can cost you a new engine. You should only run regular unleaded if you have tuned your car to run on it. I for one, would never put anything but premium in my tank since it only cost $3 more a tank. It would take a lot of $3 savings to = a new or rebuilt engine.

If your modded at all and have been running regular, I would be surprised that you haven't chipped a piston yet.

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1997 Black GTP 72k (4dr), 3.5 Pulley, Wbody Downpipe, Wbody Shift kit (street), DHP 1.0, NGK TR55ix Spark Plugs, GMP Handling kit, KYB AGX Struts, AT Italia Inox 235x50x18 Bfgoodrich KDWS, GMPP Springs, Corvette C5 Calipers, Blazertech 3200, DHP Powertuner (97-03), Built Trans, Torsen Diff