July 28, 2004

Should you use high-octane Premium gasoline?
by Jim Kerr

With the price of fuel high, drivers want to save money whenever they can, so the natural tendency is to fill up with the cheapest gas. For many of us, this works just fine but some car manufacturers specify that higher fuel octane is required. Do we need to use higher octane fuel? Maybe - maybe not!

Octane rating is a measurement of the fuel's ability to resist self-ignition. Fuel can be ignited prematurely by high compression ratios or hot deposits in the engine cylinders. When premature ignition (pre-ignition) occurs, extreme pressures build up in the cylinder. This sudden rise in pressure causes vibrations that can break piston rings and pistons, damage engine bearings and break crankshafts. Obviously, we want to avoid this damage so we need to use fuel that will not cause problems.

Pull up to the pumps and you typically find three grades of fuel - regular, mid-grade and premium. Although the octane rating varies slightly from different suppliers, regular has a minimum octane rating of 87. Mid-grade is commonly rated at 89 octane and premium is rated at 91 or higher octane. Alcohols blended with fuel raise the octane rating of the fuel. For example, premium grade alcohol blended fuel may be rated at 95 octane.

These octane ratings are an average figure based on two methods of testing fuel. The Research Octane Number (RON) is a calculated number, while the Motor Octane Number (MON) is a measurement of pre-ignition resistance in a specified research engine. Check the labels at the fuel pumps and you will often see the formula RON + MON divided by 2 = Octane rating.

If your owner's manual specifies that your vehicle needs regular fuel, then there is no real benefit of using higher octane fuels. The only exception is if you are experiencing pinging or pre-ignition in your engine. Modern fuel injection engines use knock sensors on the engine to measure the severity of any engine knocks. If a knock is detected that could cause engine damage, the computer will retard ignition timing until the knock is no longer present or until the base minimum ignition timing is reached. As soon as the knock stops, the computer advances the ignition timing again for better performance.

On older cars without computerised engine controls, a driver may be able to hear pinging or pre-ignition in the engine and could reduce it by lowering the load on the engine. With the use of modern knock sensors, by the time you hear any engine knock, the timing has been retarded so much that both fuel economy and engine performance are seriously affected. If you suspect that your vehicle's fuel economy has dropped off or performance is sluggish, try a tank full of a higher octane fuel. If the economy and performance improve, then you have likely been experiencing pre-ignition. Sometimes adjustments can be made to enable you to go back to regular grade fuel, but if the problem is caused by deposits inside the engine, then using a higher grade fuel is suggested and could even be more economical by improving your fuel mileage.

Open the gas door on some vehicles and you will find a sticker that states "Premium Fuel Required" or "Premium Fuel Suggested". High performance vehicles have higher engine compression ratios and often operate hotter, both of which can cause pre-ignition to occur. Even though these vehicles typically use knock sensors to detect engine knock and protect the engine, you are probably better off to use the fuel specified.

There are exceptions. For vehicles where "Premium Fuel is Suggested" a lower grade of fuel can be used is some situations. Operating the vehicle in cold or cool weather or under light loads at low speeds reduces the possibility of pre-ignition occurring. You can probably use mid-grade or even regular fuel, especially during the winter months. The vehicle will not have the performance potential it would with premium fuel, but if you are not using the performance, why spend the money on fuel to support it?

If you are not sure what fuel to use, start with the fuel grade specified by the manufacturer and keep track of your fuel economy. Then go up or down a grade of fuel. If fuel economy stays the same, then stay with the cheapest grade of the two. If fuel economy drops, return to the original grade of fuel. Even high performance cars can be driven with regular grade fuel but only if the driver is aware that the vehicle should be driven sedately.

Many drivers believe that using a premium grade fuel is better for their vehicle and produces more power. Not true. As long as the fuel does not pre-ignite, any grade of fuel will produce the same amount of power. Save some money at the pumps and optimize your fuel economy by selecting the correct grade of fuel for your operating requirements.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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