Just what *does* all that octane stuff mean at the pump? Will airplane gas really make your engine run better, longer and more efficiently? Will you be more popular, attractive, suave and debonair when people see you filling up with premium???? Put your organic chemistry books (and your wallet) away as it's not that complicated. Really....


Mythbusters - Octane

Submitted by Ted Stehr
1958 Chevy Apache Panel
Bolter #35658

  July 29, 2013

 

No octane in my gas??!!

 
This story doesn't pertain to the price of fuel but I couldn't resist using this picture here ... thought you would enjoy ~ Editor  

Contrary to popular belief, there is no octane in the gasoline you buy at your local station. Octane is a measurement of how resistant the fuel is to spontaneous combustion. In a high compression engine, gasoline can spontaneously combust before the spark ignites it, simply due to pressure and heat in the combustion chamber. In fact, that is the basic principle of a diesel engine and why it doesn't need spark plugs. But in a gas engine, if part of the fuel combusts before or after the plug ignites it, the resulting collision of "flame fronts" will cause knocking. So high octane gasoline resists knocking and ignites in a controlled manner.

The Octane Rating of gasoline is a comparison to a mix of two gasoline-like petroleum products: isooctane and heptane. Isooctane or octane has great combustion properties, and heptane has very poor qualities.

Octane Rating

Here is how they determine octane ratings:

First they run the fuel they want to test in an engine with a variable compression ratio. They crank the compression until it knocks.

Next, they mix octane and heptane until they get the same knocking at the same compression in the engine. The ratio of octane to heptane (e.g. 90% octane, 10% heptane) is the octane rating, e.g. 90 octane, of the fuel they are testing.

There are two methods to get this rating. The one I described is the Research method.

The Motor method is also run with a special engine but varies the timing and preheats the fuel to simulate load. If you look on a gas pump, it will list the octane as R + M / 2. That means take the Research method rating, add it the Motor rating and divide by two to get the average.

So there is no octane in gasoline.

Many other properties are assigned to high octane fuels that also don't apply. It will not cure much more than knock or ping. It may result in slightly better gas mileage because it can start burning sooner (advanced timing) and will burn in a more controlled manner so more of its heat can be used by the engine. But it is hard to say if the difference would be noticeable and almost certainly would not outweigh the extra cost of the gasoline.

All that being said, it sure feels great filling up the Stovebolt tank with premium!

Ted

 

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