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04/29/2003 Archived Entry: "WHAT GRADE OF GASOLINE IS BEST?"

WHAT GRADE OF GASOLINE IS BEST?

This came this morning from Dan Allard, a former Ellsworthite now living in Texas. I found it informational and a good thing to know.

Many people believe using premium or mid-grade gasoline in their vehicle is always the best choice. They pay the extra money believing the terms “premium,” or “mid-grade,” signify the fuel is better than “regular” unleaded. In fact, all the word premium actually describes when referring to gasoline is the “premium price” you are paying for the higher octane you probably don’t need.

Octane is a term used to describe a gasoline’s ability to resist knock. That formula you see on the side of the pump, R+M/2, represents the average of the Research Octane Number (RON) and Motor Octane Number (MON). RON relates to low to medium speed knock and MON to high speed or under-load knock. Once R+M/2 is figured, the result is the Anti-Knock Index (AKI), or pump octane rating. Added to this alphabet soup is the “ONR,” or Octane Number Requirement. The ONR is what your engine requires to operate knock-free and is determined by the compression ratio, timing, air/ fuel ratios, type of combustion chambers, and other design elements of your engine as well as environmental factors such as humidity or altitude.

What makes your engine run, and your vehicle go, is a series of controlled explosions occurring at precise times within the combustion chambers of your engine. Inside each of these chambers a moving piston squeezes a mixture of air and gasoline to just the right amount of compression before being ignited by the spark plug. Knock occurs when any of these explosions happen before they’re supposed to, and the sound you hear is the force of an early explosion against a moving piston causing it to slap against the wall of the chamber (cylinder) it’s moving within. The problem with this, in addition to the obvious mechanical shock to engine parts, is if the early explosion is severe enough it will force the oil out from between where these metal parts touch and cause them to wear.

Fuel with a higher octane number, such as premium, burns slower and because of this can be compressed more in the combustion chamber without causing an early explosion. This makes it perfect for engine designs requiring high compression, such as found in high performance sports or luxury cars. In engines not designed with high compression, the higher-octane fuel is not burned completely and results in excess carbon build-up and fouling of spark plugs. Tests have also shown that using a premium or mid-grade gas in an engine not designed for it may actually reduce your gas mileage, thereby wasting money in more ways than one.

Many people believe they need premium or mid-grade gasoline because they occasionally hear their engine knock when passing or under a load. Modern engines are designed with knock sensors that will cause a change in the engine’s ignition timing to stop this knock or its severity. Only when the knock is continual, or severe, should a change to a higher-octane fuel be considered in a car with a modern “computer” management system.

Just remember, the higher price you pay for premium is to offset the added cost of refining to achieve the higher octane, not to make a better gasoline. Follow the fuel recommendations found in your vehicle’s manual and if your engine doesn’t need a higher octane, not only are you wasting money at the pump by buying premium, you may also be wasting money down the road.