Latest Reader Question (Jan. 15, 2018)

Here is the latest reader question, along with my reply:

Roger asks: Is using premium gasoline in a motor not requiring it according the owners handbook, detrimental the life of the motor?

My reply: No, but it is detrimental to mileage – and performance!

Here’s the deal: “Premium” is synonymous with “high octane.” It is not a measure of the fuel’s quality, per se.

Octane is a measure of a fuel’s inclination or tendency to burn – either sooner or later – when subjected to pressure and temperature.

High octane fuels are more resistant to burning prematurely due to pressure and heat, before the spark plug fires. This makes them ideal for high-performance engines with high compression ratios or high cylinder pressures, as in turbo/supercharged applications.

Lower octane fuels are less resistant to pressure and heat and will burn prematurely if used a high-compression engine, resulting in an uncontrolled explosion which often occurs as the piston is still traveling up and not yet in firing position. The resultant uncontrolled/premature explosion tries to force the piston down as it is still mechanically going up – placing enormous strain on it, the bearings, crankshaft and so on.

Most modern engines have knock sensors that prevent mechanical damage to the engine which would otherwise result from this pre-ignition (also called “engine knock”) but the power/performance and mileage delivered by the engine will often be noticeably reduced. So you do not save money by using regular/low octane fuel in an engine designed for high octane fuel.

And using high-octane “premium” in an engine that does not need it? You also get less power and worse fuel economy, because the octane/burn rate and the engine are mismatched. An engine designed to be most efficient on regular/lower octane fuel will be less efficient if you feed it premium.

The bottom line? You wont hurt anything – but you are wasting money – when you use high-octane fuel in an engine that doesn’t need it. And when you use low-octane fuel in an engine that does need it.

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