Understanding Octane – Yes

From 3arf

The question "Is regular-grade gasoline as good as premium gas for your car" is inadequate to encompass the role that octane plays in your car's engine. Too much depends on your particular engine's design. But since most people will be putting regular gas in cars that are just fine with it, and cars designed to work best with premium can usually tolerate regular, the least inaccurate answer is yes.

That's because most cars do not have a problem with pre-ignition, the tendency of the fuel to start burning before the spark fires, when running on regular gas. The fuel's octane number represents its resistance to that pre-ignition, which could damage your engine, since the resulting explosion would begin while the piston is still rising in the cylinder in the compression stroke of the Otto cycle. That octane number is a percentage of resistance to prei-gnition in comparison to a standard fuel, called "octane." If the fuel resists pre-ignition 10% better (fires at 10% higher pressure) than octane, it has an octane number of 110. If it's 10% worse (fires at 10% lower pressure), its octane number is 90.

Engineers take advantage of that resistance to preignition to improve the efficiency of their internal combustion engines by raising the compression ratio. When they do that, at the beginning of the power stroke the engine's fuel/air charge is squeezed into a smaller space, and the pressure at ignition is therefore higher. At the end of that stroke, the volume in the cylinders of otherwise identical engines is the same regardless of the compression ratio, so the mean effective pressure in the cylinder is higher in the higher compression engine. More pressure means more power.

That can't work if the octane number of the fuel won't support the added pressure.

Raising the compression ratio not only improves power, but since it takes the same amount of gas to cause a higher cylinder pressure, it also takes less gas to give the same performance. So higher compression engines give better fuel economy than low compression engines of the same power.

The result of all this is that while a modern high compression engine will protect itself from damage caused by using a lower octane gas than the manufacturer recommends, it does so by initiating the spark earlier in the power stroke ("advancing the spark") and adding more gasoline into the incoming charge ("richening" the fuel mixture). Both of these strategies reduce efficiency, burn more gas, and lower performance.

So you won't necessarily save money by using lower octane gas in an engine designed for premium. It actually lowers your gas mileage. To know for sure, you'd have to do instrumented tests to figure out whether the lower cost of regular (typically less than 5% here) justifies the loss in fuel mileage. If you lost even 1 mpg, the math doesn't support the switch to the lower grade.

And if your engine predates computer controls (sixties muscle cars, for instance), don't take a chance. You could end up with a blown head gasket, a broken piston or rod, or even a hole in the cylinder wall.

The other half of that equation is that feeding a low compression engine premium gas is like feeding a couch potato Gatorade. Since both the high octane fuel and the sports beverage serve to cure a problem you don't have (pre-ignition in the engine, and electrolyte depletion in the couch potato) neither will be of any benefit. And in these days of high gas prices, the added cost of premium gas is wasted if your car doesn't actually need it.

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