Using 91-octane premium fuel instead of 87-octane regular fuel will only increase your engine's power if your car's manufacturer specifically designed the engine to require it.
Higher octane fuel resists knocking or 'pinging' under compression, which is a premature ignition of the fuel-air mixture. Engines designed for higher octane can take advantage of this resistance to operate at higher compression ratios or with more advanced timing, which can lead to increased horsepower and efficiency. However, if your vehicle's owner's manual specifies regular 87-octane fuel, using premium 91-octane will likely offer no discernible performance benefit and is generally not recommended. Always consult your vehicle's manual for the correct fuel grade.
Fuel octane requirements are determined by the vehicle manufacturer and using a higher octane than recommended typically provides no benefit.
How is the octane rating of gasoline determined?
The octane rating of gasoline is determined through a standardized laboratory test that measures its ability to resist auto-ignition, or knocking, under compression. The most common method is the Cooperative Research Council (CRC) method, which involves running the fuel in a special test engine under controlled conditions. The octane number itself is a value derived from a mixture of two reference fuels: isooctane (assigned an octane number of 100, representing high resistance to knock) and n-heptane (assigned an octane number of 0, representing low resistance). The percentage of isooctane in a blend that behaves like the test fuel gives the octane rating.