When high-compression motors came on the scene, the increased pressure in their own tanks occasionally caused gas to explode too early. That preignition, known as spark knock, was an irritating noise (although certainly that); it caused 8hp Outboard Engine harm. The alternative was to add materials to the fuel that could slow the ignition until the piston was prepared for the spark plug in to activate, which could avoid damage and make efficient use of that higher compression. So today, the higher the octane rating, the better the fuel can handle compression. Sounds good. However, the truth is high-octane fuel -- generally 92 or 93 octane -- is a waste of money for the vast majority of outboards. Some producers -- such as Outboard Motor 40 Hp-- do advocate midrange fuel, usually 89 octane. Check your owner's manual, and gas up with what it prescribes. Proceed above the recommended octane and you're giving away about 20 to 30 cents per gallon. Fuel Truth • Only high performance engines require high-octane fuel.
• High-octane fuel in engines created for low-octane fuel nets no better performance, but larger fuel bills.
• In a few circumstances, motor makers may prescribe mid-range gas, typically 89 octane.