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Fred Lum/The Globe and Mail

I just read your article about Toyota versus Lexus on what type of gasoline to use. I drive a VW with a 2.0-litre turbo engine. The manual says it requires premium fuel. Will using regular or mid-range gas hurt the engine? I have written to VW and its answer is yes, but it writes the manual. What do you think? – David

This is one of the most popular question I get.

The use of regular fuel in a modern engine where premium is recommended will cause little harm. It will result in a slight decrease in performance and fuel efficiency, however.

Story continues below advertisement

Before the arrival of powerful computers and sensitive sensors, the use of regular fuel in an engine designed for high test would result in what is known as detonation, engine knocking or pinging. This occurs when pockets of the fuel-air mixture in a cylinder ignite as a result of heat and pressure rather than a spark (plug).

This can lead to catastrophic – expensive – failure through holes punched in the pistons or damaged cylinder heads.

The cures are higher-octane fuel or the prediction and detection of "knock" and subsequent change in ignition timing. Today's sophisticated engine management systems – made necessary by the need to have engines police themselves for exhaust emissions – are quite capable of reducing the risk of engine damage from lower-octane fuel.

In the case of turbocharged engines – which force fuel and air into the combustion chamber under greater pressure – the need to detect and prevent knock is even greater. In this case, not only does the ignition timing have to be altered, the amount of boost has to be reduced as well. Many companies that sell vehicles in a wide variety of global markets have to make allowances for really poor quality fuel, thus the insistence on high test.

Having said that, premium fuel commonly contains additives not used in lower grades, chemicals that help clean the combustion chamber. And, don't forget, that while it appears using regular fuel saves money, you are using more fuel or going fewer kilometres on each litre of fuel.

Send your automotive maintenance and repair questions to Globe Drive experts at globedrive@globeandmail.com

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