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WHAT OCTANE RATING OF GASOLINE SHOULD I USE?


The type of gasoline to use is one of the most misunderstood areas of vehicle ownership. I am going to offer some ideas that I hope will save you a few bucks on gasoline.

The first rule of thumb is that higher octane gasoline is not necessarily better for your vehicle.

WHAT IS OCTANE RATING?

Octane, by definition, is the resistance to burn or detonation. The higher the rating, the slower the burn when ignited during the compression burn cycle of the piston. The higher octane allows for better control of burning for high compression engines.  So we want to match the correct octane rating of the gasoline to the engine design to ensure complete burning of the gasoline by the engine for maximum fuel economy and clean emissions.

I THOUGHT GASOLINE WITH HIGHER OCTANE REDUCED ENGINE KNOCK?

It did in older engines using carburetors to regulate air/gas mix They cannot as accurately regulate the air/fuel mix going into the engine as a computerized fuel injector. Carburetors need adjustment, as a part of regular maintenance, to keep the air/fuel mix as accurate as possible. So many times, these adjustments were not made regularly causing too much fuel to be mixed with the air. When this happened the gasoline would not burn completely soaking into carbon deposits. This would cause a premature ignition of the gasoline due to the intense heat in the engine cylinder creating "engine knock." When this happened, people would change to the higher octane/slower burning gasoline to resist the premature burn, thus minimizing the knocking problem. And it worked. Good solution.

However, since the middle to late 80’s, engines are designed to use fuel injectors with computers to accurately control the air/fuel mix under all types of temperature and environment concerns. However the accuracy of the fuel injectors and computers is based on using the recommended gasoline for that engine.

Most cars are designed to burn regular unleaded fuels with an octane rating of 87. If the vehicle needs a higher octane rating of 89-93, there is documentation in the owner’s manual,  as well as possibly under the fuel gauge and by the fuel fill hole. Usually you will see this rating for high performance engines only.

WHAT IF I PREFER TO USE GASOLINE WITH HIGHER OCTANE RATINGS?

You can, but there are no real benefits, other than the gasoline manufacturers making more money off of you. When you use a fuel with a higher octane rating than your vehicle requires, you can send this unburned fuel into the emissions system.  It can also collect in the catalytic converter.  When you over stress any system, it can malfunction or not do what it was designed to do properly.  In the early 90's, an early warning symptom was a rotten egg smell from the tailpipe. Easy fix, go back to using regular 87 octane gasoline. The rude odor usually disappears after several tanks of regular octane gasoline. Now that fuels use reduced sulfur content, that odor will not happen anymore, but you are still wasting expensive fuel that is not necessary for your engine.

DOESN'T HIGHER OCTANE GASOLINE HAVE MORE CLEANING ADDITIVES THAT ARE GOOD FOR MY ENGINE?

No. Government regulations require that all gasoline contain basically the same amount of additives to clean the injectors and valves.  The only differences are the type to help create the different octane ratings.  All gasoline burns at the same rate, it is the additives that create the different octane ratings for the different types of engines.

REFORMULATED GASOLINE

In some major cities with air pollution problems, reformulated gasoline is required.  It is an oxygenated fuel, that burns really clean but can slightly lower fuel economy and engine performance.  If your engine is really dirty with carbon deposits, it will also cause pinging or premature burn.  In these types of situations, you may want to consider stepping up to the next grade of gasoline.

The bottom line is to use the type of gasoline recommended for your engine.  In some cases, like towing, or other stresses on the engine, you may find a higher octane fuel helpful.

NOTE: I do not proclaim to be an expert in these matters, but am only presenting an overview of what I have discovered in my work in this industry with the different auto manufacturers.

Copyright, 1997, J. Daniel Emmanuel

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