Under the Hood

There are more numbers associated with cars than nearly everything else except baseball.

One of the numbers drivers are frequently confronted with is fuel octane. It's staring you in the face every time you fill up.

This is a number steeped in myth and confusion, and because it's tied to your wallet, something you might want to know about.

Myth No. 1: Higher octane fuel is more volatile. It isn't. Octane refers to a fuel's resistance to preignition and detonation.

Myth No. 2: High octane yields more horsepower. Beyond the grade required for proper ignition, this is untrue. Octane is like a fuse; higher octane is a longer fuse, not a stronger explosive.

So, octane ratings describe the speed at which gasoline burns.

Cars that require higher octane fuel to run properly need the speed of combustion to be a little slower. Under certain circumstances, the fuel-air mixture in the cylinder will ignite too soon, an effect known as pre-detonation or pinging.

Most of what determines the need for higher-octane fuel is the engine's compression ratio--how much it squeezes the mixture of air and gasoline.

The harm takes place because the detonation is trying to push the cylinder down as the crankshaft is pushing it up. Long-term damage can be as serious as a hole in a piston's crown.

The culprits in pinging and/or detonation include:

* Fuel with too low an octane rating.

* Ignition timing, which can be adjusted within limits.

* Valves that are too hot.

* Spark plugs that are too hot.

* Compression ratio of the cylinder, which can't be easily changed (read: pricey engine work).

* Carbon deposits, which are not easy to get rid of.

Hot weather aggravates the condition no matter the cause, and some engines that run well in 70-degree weather won't in 90-degree weather.

The first one is simple to fix. Buy the next-higher-octane fuel and if the pinging goes away and performance or mileage improve, you're done. If only the pinging seems to go away, move up to premium.

If ignition timing is advanced too far, higher-octane fuel won't prevent problems. The factory setting should be followed in most cases.

Hot valves can be caused by wear or cooling system problems, so flush that radiator at the recommend intervals.

A hot-plug condition could be caused by using one that's outside the proper heat range. The right plugs should be used.

Carbon buildup on the pistons, valves or top of the combustion chambers can't be driven or treated away, contrary to popular belief.

Automobile manufacturers are required to state what octane fuel is recommended. A properly tuned, stock engine should not ping under hard acceleration in normal conditions (read: flat road, mild weather).

If your car consistently pings under those conditions, and you're using the right octane, a tuneup is in order. Suffice to say that a new car that pings needs to be looked over. Ask the mechanic or service writer if octane is an issue. You might have to move up from regular to mid-grade or mid-grade to premium.

Your car may ping in hot weather under a heavy load, such as pulling a trailer up the Grapevine in August. Try gassing up with premium before those trips to see if that lessens or remedies the problem.

Finally, keep in mind that if your car says it needs premium but you use regular and don't hear anything, it doesn't mean all is well. Detonation isn't always loud enough to be heard, but the effects are still bad.

Flag Down Dr. Gear Head

Do you have a question or suggestion for Dr. Gear Head? Write to Dr. Gear Head, Business Section, Los Angeles Times, Times Mirror Square, Los Angeles, CA 90053. Or fax us at (213) 237-7837, or e-mail us at highway1@latimes.com.

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