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.
Does premium gas clean my engine or improve fuel economy?
Some premium gasolines may contain higher levels of detergent additives compared to some regular gasolines, which can help keep fuel injectors and intake valves cleaner. However, this is not a universal benefit exclusive to premium grades, and many regular fuels also contain effective cleaning additives. Any potential improvement in fuel economy or power from using premium gas is almost entirely dependent on whether your vehicle's engine is designed to require it to prevent knocking. If your car runs fine on 87 octane, switching to 91 will not improve mileage or cleanliness.