The higher octane fuels will not harm your motor, but would be a waste of money if you car was not tuned for them, either from the factory or with an aftermarket tune. The higher the octane rating of the fuel, the more resistant to detonation the fuel is. Detonation, commonly referred to as knocking or pinging because of the noise it makes, is when the gas ignites from the heat and pressure inside the cylinder while the piston is still on the up stroke. This can be extremely harmful to your motor as the engine is pushing the piston up at the same time as the combustion is pushing the piston down.
When tuning, much of the power lost on the factory tune comes from the manufacturers having to make the factory tune safe to run on the poorest quality 87+ octane you will come in contact with, without causing detonation. This is why tuners will always recommend 91+ or higher since they can make the tune much more aggressive. The higher the octane you tune for, the much higher the gains will be.
This is also why you should always use a quality name brand gas, because believe it or not, all gas is not created equal! Low quality, generic brand-x fuels, convenience mart, and discount wholesale fuels could cause pinging in a tuned car, even if you are running the octane it is tuned for. Saving 5 or 6 cents a gallon at the pump of a low quality gas station will cost you in the long run. Studies have shown that even in non-tuned cars that gas mileage from 2 gas stations right next to each other can vary by up to 60 miles per tank! Basically you should always use a quality name brand gas such as Sunoco, Mobile, Exxon, Shell, etc.
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