Fuel economy and additives

rgatijnet1 wrote:
Your engine has a sophisticated computer management system. If you use the MINIMUM fuel octane as recommended by the owner's manual, which is probably 87 octane, the computer will make the necessary adjustments to give you the best performance/mileage. Using a higher octane fuel will not improve the mileage on an engine that has the computer calibrated for 87 octane.

This is absolutely correct, what is worse is if the ECU (Engine computer) is calibrated for a certain minimum octane and a higher octane fuel is used then fuel economy can actually suffer i.e. get worse. Octane is resistance to pre-ignition or knock. This occurs when the air-fuel mixture ignites before the spark plug is fired by the ECU. Gasoline engines are homogeneous ignition combustion engines where the ignition is controlled by the ECU sending a spark to the spark plug vs. stratified charge compression ignition in diesel engines that rely on injecting fuel at precisely the time the ECU wants ignition to occur. It should be noted that with gasoline engines that do rely on a higher octane fuel such as boosted engines e.g. Ford EcoBoost or higher performance engines such as the GM 6.2L V8 can and will use 92 octane fuel and will have better fuel economy on higher octane fuel. The reason for this is the higher compression ratios and or high boost pressure. In these engines, the ECU will back off the timing and or actually inject more fuel if engine pre-ignition (knock) is detected. This would necessarily cause poorer fuel economy than using the recommended octane fuel.