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Octane ratings have nothing to do with fuel economy or power. The octane rating only indicates how much the fuel air mixture can be compressed before it ignites without the spark plug firing in a hot running engine. Some really high compression engines require higher octane fuels so they don't knock when running. If you don't have a sticker under the filler lid that says you have to use one of higher octane fuels, then don't use high-test.
It used to be that using a low octane fuel in an engine designed to use high octane fuels could damage the engine.
Nowadays, engines are all computer controlled and most high octane engines will re-tune themselves on the fly so that they don't knock if the wrong grade of fuel is used.
Low-octane "regular" gas is usually all that is required. Octane is only a rating of the fuel's resistance to engine-damaging pre-ignition ("knock") in high-performance engines (that few people have). Low octane gas is less expensive and a better value if that's all your engine requires. Best case scenario you're wasting money by filling up with a higher than recommended grade of gas. Worst case scenario that high octane fuel is building up damaging carbon deposits in your engine because it's not being burned as completely as lower octane fuel would be. Check your owner's manual to be sure. Modern high performance cars will sometimes recommend higher octane fuels because they are engineered to use those fuels. The use of lower than recommended octane will not make your car explode, the ECM (Engine Control Module, aka:computer) will adjust the fuel injectors and spark timing to save the engine and compensate for you cheaping out at the pump. Those adjustments will consist of retarding the spark (reducing power and efficiency) as well as dumping lots of extra fuel into the cylinders to cool them, potentially costing you more than getting mid grade or premium IF that's what your car requires.
NOTE: Remember that engines need less octane at higher altitudes. If your engine does not "knock" on regular, paying more for a higher octane rating is a waste since the increased octane makes no significant improvement to gas mileage and it is no better for your engine. All available fuels have detergent and additive packages.
This will help.good luck.
It may not be bad gas, but it could be the wrong octane rating for that engine. Check your owner's manual and see what octane is recommended for that engine. If you're running 87 octane and it requires 91 or higher octane, then you need to be running the higher octane fuel. By running lower octane fuel in an engine that requires higher octane, your getting pre-ignition, the knocking sound you hear. What's happening is that as the piston moves up on the compression stroke, the gas/air mixture is compressing and as the pressure goes up, so does the temperature of that mixture. It's possible that temperature can get so hot that the gas/air mixture ignites from the high temperature and not from the spark plug. Higher octane fuel will help this since higher octane fuel is harder to ignite. I say try running some 92 or higher octane fuel in your car and see if you still hear the knocking.