Let’s start by understanding what octane means, and what it doesn’t mean. It is not a rating of fuel system cleaning ability. Gasoline has detergents. This renders fuel injector “flushing” as a maintenance item almost always pointless, but that will be another topic. Octane ratings do not define the quality or amount of detergent additives. Most everyone has heard of the practice of occasionally running a tank of premium in a car to “clean it out.” This serves no purpose. Octane ratings do not define the energy load in a gallon of gasoline. There is no automatic correlation between increased octane and increased power. So what does octane mean? It has been taught that the gasoline engine produces power by gasoline/air mixture explosions. This is not strictly correct. The burning should not be a violent explosion, but instead a very rapid flame front should travel across the combustion chamber. But under certain conditions, before the flame front has completed, all of the remaining fuel/air mixture will spontaneously explode. This hammers the piston top and the phenomenon is known as “pinging” due to the noise it makes. Pinging reduces efficiency, and, in excess, is damaging to a gasoline engine. This is more or less how a diesel engine always burns, and why diesel engines are so noisy. It is also why they are built with much more massive and rugged pistons, rods, crankshaft, block etc. than used in gasoline engines.

Whether or not an engine pings depends on:

A: Combustion chamber temperature. Higher temperature means more tendency to ping.

B: Incoming air temperature. Higher temperature means more tendency to ping. Hence pinging is more prevalent in hot weather.

C: Ignition timing. The earlier in the cycle the fuel is ignited, the greater the peak pressure developed and the more tendency to ping.

D: Density of the air/fuel mixture. The greater the throttle application, the more tendency to ping. You might hear your engine rattle and ping going up a hill, but not idling or coasting downhill.

E: Compression ratio. The higher the compression ratio, the greater the mixture density and hence tendency for the engine to ping.

F: And…OCTANE RATING OF THE FUEL. The higher the octane, the more the molecular structure of the fuel is RESISTANT to pinging. That is what octane means and all that it means — the fuel’s resistance to burning in a violently explosive mode.

So, traditional informed wisdom has been that one should use the lowest octane where little or no pinging is heard. Possibly lower in the winter than in the summer, possibly lower for getting groceries than for towing your boat. Modern computerized engine controls make this not so simple. For the sake of maximum efficiency — that is to say maximum fuel mileage and horsepower, minimum emissions — the compression ratio and ignition timing of a modern engine are pushed close to the point of producing ping. The modern engine has a “knock sensor.” This is a microphone on the engine tuned to “hear” pinging. If pinging occurs, it sends a signal to the engine computer that says “Whoa Nellie….back of the ignition timing!” Pinging is controlled, but fuel mileage and power suffer with the less efficient timing setting.

So my advice is consult the owner’s manual. You rarely need more than the recommended minimum octane rating. If the owner’s manual for your car calls for higher than 87 octane, but the weather is cool or you drive gently, try using a fuel with less than the recommended octane. If fuel mileage doesn’t suffer and it doesn’t rattle and ping going up a hill, save yourself some money. Buy the octane rating you need and no more. Gasoline is expensive enough without spending extra to no purpose.