Rick's Free Auto Repair Advice

What is premium gas?

What is premium gas? —Regular versus high octane gas

What’s the difference between regular and high octane gas?

What is Octane?

A fuel’s octane rating is a numerical value of the fuel’s ability to resists detonation (engine damaging high temperature high pressure fuel burning at the end of the power stroke). Octane is not an indication of fuel’s power. A higher octane fuel doesn’t provide any more power or provide any more cleaning additives to fuel than a lower octane gas (that’s a pretty common myth and there’s no truth to it). A higher octane rating is only an indication of its ability to resist detonation.

What is detonation?

Let’s review the combustion process in an engine. Air and fuel are combined during the intake stroke. Then the air/fuel mixture is compressed during the compression stroke. That creates heat, not quite enough heat to ignite the fuel though. As the piston reaches the top of the compression stroke, the air/fuel mixture is ignited by the spark plug. As the air/fuel mixture burns, a flame front expands away from the ignition point near the spark plug, and expands ever-increasing downward pressure on the piston

The air/fuel mixture was already compressed during the compression stroke, so the rapid pressure rise caused by combustion raises the temperature of the uncombusted air/fuel mixture to the point where it self-ignites. causing multiple flame fronts in the combustion chamber. When those multiple extremely high temperature flame fronts collide, they create audible shock waves known as detonation or “knock”. Detonation robs the engine of power, and if allowed to continue, can destroy the engine by melting the pistons .

Premium high octane prevents the high-pressure unburned air/fuel mixture from self-igniting. So it prevents detonation and knock.

So what’s the difference for the ordinary driver?

If your carmaker recommends a 91 or 93 octane fuel, that’s what you should use. If you use a lower octane fuel, like 87, the computer will try to adjust ignition timing to reduce detonation. However, there’s a limit to how much the computer can adjust, so it is possible to get a check engine light if you use 87-octane in your engine that requires 93-octane.

If your carmaker recommends 87-octane, that’s what you should use. You will gain NO benefit whatsoever by filling up with 91 or 93-octane. That fuel will not provide more power. It does not have added detergents. Although some premium fuels are made without ethanol, that’s not true across the board with all refiners. In other words, filling up with 91 or 93-octane in an engine rated for 87-octane is a complete waste of money.

©, 2021 Rick Muscoplat

Posted on by Rick Muscoplat

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