Friday, February 10, 2012

Don't come a knockin'....

Automotive tip of the week: When it comes to gasoline, use what your car recommends, no more, and no less. I mentioned back a few weeks that additives are generally useless. If you are getting gasoline from Citgo, Shell, BP, Mobil, etc they typically already have a ton of additives and detergents. In some rare cases they only put this in their premium fuels, but it is generally done across the board. So why not use premium, etc? The octane rating - 87, 89, 91, 93, etc only means one thing, the resistance to pre-iginition/detonation/knocking the fuel has. The fuel has no actual extra power or energy per gallon than fuel with a different octane rating, so there is no benefit to the vehicle for MPG or horsepower. The reason part of the myth of more horsepower from higher octane fuel evolved is because high horsepower cars required it. This is a factor of the engine design, not the fuel. A high output vehicle will often use forced induction (turbo or supercharger) or uses high compression pistons etc. These things allow more power to be extracted from an engine, but requires a fuel with high resistance to knocking to be safe. When a low octane fuel is used in those applications the fuel can burn incorrectly and cause severe damage to the engine. The pressure waves from a bad burn (engines burn fuel, they don't explode fuel like people often think, in the rare cases that it does explode it can be a form of very severe detonation and will typically destroy the engine) are what make the "pinging" or "knocking" noise people know detonation to be. So the root of the myth of high octane for more power is the fact that high powered cars employed engines that needed the octane to function, not that there was more power in the fuel. So what happens if you use too low of an octane fuel in a car? In newer cars, not much, but it isn't typically considered safe/good to do. The computers in modern cars know how to adjust the engine programming to use the lower octane fuel safely. You will likely lose some MPG efficiency as well as some horsepower but the odds of damage are somewhat controlled. However, some cars are not well suited to this, and the older you get the more and more dangerous this becomes. If you have a particularly high strung vehicle from a couple decades ago you can easily destroy the engine on a hot day with one bad tank of gas. So as always, follow your owners manual!

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