Primo information about premium gas

Q: I have a 1992 non-turbo Volvo 240. My manual specifies premium gas, but being frugal, I've asked about changing to regular gas. I receive mixed responses, some dire. What say you?

A: Many vehicles specify premium fuel. This may be due to the engine having a higher compression ratio, a slower burning combustion chamber design, or the use of a turbo or supercharger. Using regular instead can cause:

• Detonation, or knocking: The air-fuel mixture explodes rather than burns properly. This can burn a hole in or break a piston, or damage the head gasket. Detonation may or may not be audible.

• Preignition, or pinging: The air-fuel mixture self-ignites prior to the spark plug firing. The extended burn time and flaming can also damage pistons, valves, or the spark plug. Pinging is usually audible.

Some engines employ a detonation sensor. The engine control computer reacts by retarding 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 (summer's not the time to take this gamble), with short trips, or with a detonation-sensor-equipped engine. It's risky, though, as the cost of preignition or detonation damage can be huge.

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.

If this helps, the greater engine performance provided by a premium fuel engine may offset the greater cost of the fuel.

Accelerating a bit more gently and anticipating, then coasting to slowdowns and stops ahead can make a big difference in fuel economy. I've found slowing from 70 to 61 mph (my vehicle's most efficient speed) results in an increase of 4 mpg.

E-mail Brad Bergholdt at under-the-hood@juno.com.