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Tuning a Carbureted Street Rod Engine - Part 1 of 2

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Racing Gasoline vs. Aviation Gasoline

By: Tim Wusz

Many racers see Aviation Gasoline (Av Gas) as a way to reduce the cost of their racing operation. This may be true, but if you want the most out of your engine and want to avoid problems, Av Gas may not be your first choice. Av Gas is a good gasoline for low speed aircraft engines that run at 2700 to 2800 RPM at 10,000 feet or higher. This does not mean it is a good gasoline for racing engines operating at 8,000 to 10,000 RPM. Av Gas is also illegal to use in anything except aircraft engines. Violations can carry a potential penalty of $25,000 per day of violation.

Av Gas octane numbers are determined in a different test than motor gasoline octane numbers. Do not be confused by the big numbers from the Av Gas test method. They are not comparable to motor gasoline test numbers. The 111 octane version of Rockett Brand Racing Fuels tests at 160 on the Av Gas scale.

Av Gas is held to tighter requirements than street gasoline, but not nearly as tight as is the entire line of Rockett Brand Racing Fuels. Some racing gasoline blenders use Av Gas as a blending component to save money. For Rockett Brand Racing Fuels, we do not. We use only the highest quality components since Av Gas is far too inconsistent to use as a blending stock for Rockett Brand Racing Fuels.

Av Gas has a lower specific gravity than most racing gasolines. This means that if a racer tries Av Gas and has not re-jetted, he can burn a piston because the air-fuel ratio is too lean and/or the engine detonated. To make a good comparison between two gasolines, the air-fuel mixtures must be the same. Even after re-jetting, the racer can experience burned pistons with Av Gas if the Motor Octane Number (MON) is lower than what his/her engine needs.

Another potential problem with Av Gas is that there are several different octane grades. The 80/87 grade is red in color and can get you in lots of trouble because of its very low octane number. The 100LL is blue and the 100/130 grade is green. Both of these have much lower Motor Octane Numbers than most racing gasolines and will detonate when the engine octane demand is greater than the octane number of the gasoline.

The bottom line is: Feed that high dollar racing engine a good grade of gasoline so it will deliver the maximum performance for you. Don�t use �low bidder� mentality. This is not the space shuttle.