Q: I have a 1992 non-turbo Volvo 240. My manual specifies premium gas, but being frugal, I asked both my automobile association and Volvo about changing to regular gas. I receive mixed responses: "There might be performance issues, but it won't hurt your car," or "You will need to have an expensive ring job if you use regular gas." What say you?

A: Many vehicles specify the use of premium fuel to obtain the best performance. This might be due to the engine having a higher than typical compression ratio, a slower burning combustion chamber design, or the use of a turbo or supercharger.

Higher than typical cylinder pressure and a slow-burning air-fuel charge, under certain conditions, can lead to detonation - also called knocking - if premium fuel isn't used. Detonation is a harmful condition where the air-fuel mixture explodes rather than continuing to burn properly. Detonation causes momentary extreme temperature and pressure, which can burn a hole in or break a piston, as well as damage the head gasket. The part of a piston often broken is around the upper ring lands (grooves). Detonation might or might not be audible, depending on engine design.

Preignition - also called pinging - is another harmful condition where the air-fuel mixture self ignites prior to the spark plug firing, usually as a result of a hot spot in the combustion chamber. The resulting extended burn time and flame front collision can also damage pistons, valves or the spark plug. Pinging is usually audible, and is more a jingling than the deeper knock of detonation.

High-octane (premium) fuel resists detonation and pinging to a greater degree than lower octane fuel. The most likely time for pinging and detonation to occur is when the weather and engine are hot and the engine is accelerating or under continual load. Some engines employ a detonation sensor, a microphone of sorts, which detects detonation. Should detonation occur, the engine control computer retards ignition timing until it stops, and adapts ignition advance for the best detonation-free performance.

Using lower octane fuel than specified might be OK, particularly during cooler weather, with short trips or with a detonation sensor equipped engine. It's a gamble though, as the cost of preignition or detonation damage can be huge. Replacing pistons requires a full engine teardown and could reach thousands of dollars. Also, if the ignition timing is frequently being retarded due to sensed detonation, fuel economy and engine performance will suffer, perhaps neutralizing the cost savings of the cheaper fuel.

Summer isn't the time to take this gamble. If this helps, the greater engine performance provided by a premium-fuel engine might offset the greater cost of the fuel, if you keep your foot out of it. In other words, a similar engine, without the design features requiring premium fuel, would likely go one or two less miles per gallon, turbo equipped engines excepted.

Accelerating a bit more gently and anticipating, then coasting to slowdowns and stops can make a big difference in fuel economy. I've found that slowing down from 70 to 61 mph (my vehicle's most fuel-efficient speed) results in an increase of four miles per gallon. Cold starts and idling murder fuel economy. Consolidating trips and trying to arrange them to avoid peak traffic times can also make a difference, as well as maintaining correct tire pressure and removing unneeded heavy items.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose. E-mail under-the-hood@earthlink.net. He cannot make personal replies.