Tech Tips: Aviation Gas vs Racing Gas
Saturday, 10 February 2001 08:00

Tech Tips: Aviation Gas vs Racing Gas

Legal Stuff: Any Damage that results from using or employing any of the tips outlined below are NOT the responsibility of Drag Racing Pinoy. Use the tips with caution. You are responsible for any damage or injury that may occur. Use caution at all times!

written by, and thanks to
Tim Wusz
76 Products Company

I am going to attempt to address the controversy of aviation gasoline verses racing gasoline for use in race cars. Some racers use aviation gasoline which is fine for some applications but does have shortcomings. There are several grades of aviation gasoline (avgas) that we must identify before going any farther.

1. Avgas 80/87: this product is used in low compression ratio aircraft engines, contains little or no lead, is red in color, and should not be used in any automotive engine due to a low motor octane number of about 80.

2. Avgas 100/130: this product that can be used in some automotive engines. It has both research and motor octane numbers slightly over 100. Avgas 100/130 is green in color, contains four grams of lead per gallon, and is becoming harder to find.

3. Avgas 100 LL: the LL stands for "low-lead" which means two grams per gallon, low compared to the avgas 100/130 that it was designed to replace. It has research and motor octane numbers very similar to the 100/130 product previously discussed. The color is blue. This product sometimes has a high level of aromatics which can contribute to lazy throttle response and dissatisfaction of the consumer.

4. Avgas 115/145: this product was developed for high performance piston aircraft engines used in world war II and in the Korean war. It is very hard to find anymore due to lack of demand although it is of very high octane quality. The color is purple.

The remainder of this discussion will assume that our basis for comparison with racing gasoline is avgas 100/130 and/or 100 LL since they are both available and have acceptable octane quality for limited applications. When the word "avgas" is used, it will refer to avgas 100/130 or 100 LL.

Avgas is less dense than most racing gasolines. Instead of weighing about 6.1 to 6.3 pounds per gallon like racing gasoline, it weighs 5.8 to 5.9 pounds per gallon. The racer must compensate for this by changing to richer (larger) jets in the carburetor when changing from racing gasoline to avgas.

The other major difference is octane quality. Avgas is short on octane compared to most racing gasolines. Many racing engines with "quick" spark advance curves or with no centrifugal advance have more spark advance at low rpm than avgas and some racing gasolines can handle. The result is detonation, especially during caution periods in circle track racing because all of the spark advance is "in", rpm is low, and part throttle air fuel ratios are too lean for the operating conditions. If the driver does not "work" the throttle back and forth, pistons can be "burned" which melts away part of the aluminum piston material. Inadequate octane quality is one of the quickest ways to destroy an engine. Pistons can be severely damaged during one acceleration where detonation is present and the racer may not know what is happening until it is too late.

For maximum performance and power from a racing engine, racing gasoline will normally provide better performance than avgas. Avgas can be a good gasoline for some applications, but since most racers do not know the octane requirement of their engines, they would be better off with a "real" racing gasoline that will give them the overall resistance to detonation that they need to protect their investment. If someone has spent from $15,000 to $50,000 or more on their racing engine, it is foolish to cut corners on gasoline be sure you have a gasoline with adequate octane quality.

Steve Jack
Southeastern Goodguys Rep

As an avid hotrodder and aircraft mechanic, I dispute what you have been told about AVGAS! First, the octane rating for the avgas that you mentioned is much higher than used in general aviation for the most part, so my first question is who or what told you that? I really question the validity of the octane rating. General avgas has a 100LL rating....not enough for 12 to 1 race motors I might add.

Secondly, the first part is moot because AVGAS has a different specific gravity than that of regular gasoline or racing gas for that matter. With this in mind what happens is the avgas leans out your engine tremendously and is really formulated for a whole nother purpose and certainly doesn't have the BTU content of pump gas...a real problem for everyday driving! Simply put..I don't recommend it and most avgases are only good for up to 11.+ CRs (mechanical) in cars anyway even if you jet them correctly. I suspect(I know) that the combination of not being the octane that you though it was, plus creating a lean condition is what sent your engine hammering down the street.

Now, having said that I will tell you a dirty little secret of Racing Fuel. The only difference between racing fuel and pump gas is TOLUENE! Yep, I said toluene!!!!!!(and a little bit of xylene, but it's insignificant here.) The paint/hardware store shelf stock is what makes racing gasoline what it is today. Toluene is approximately 27% of the makeup of racing gas by volume whereas its only 9% of 92 pump gas! Toluene by itself has a RON+MON/2 of 118....pretty slow burning stuff..and a great octane booster.

As most don't know, fuel octane additives are useless to boost octane levels significantly. 8 oz bottles just won't move 15 gallons of pump gas very far, maybe a point at most. I don't think you need much past 98 - 100 octane (aluminum heads..add 5 points for iron and/or a short duration camshaft), if that, to correctly get the hammer down in your engine. If you think that you are going to go faster on higher octane save your money and buy a blower...simply not true, in fact can be the opposite.

You can mix your own racing gasoline by adding toluene to your tank at fill-ups. Let me say that this stuff is nasty and you should read all the precautions on the can. Don't get this stuff on you or the car. It's tuff on paint and tougher on you (carcinogen!!!). Start by adding 3 pints to 5 or 6 gallons of pump gas. See how it runs there. If it runs okay then start experimenting with backing off until it pings again or visa versa. Keep gallon(s) in your car. Actually you can get this stuff from a wholesale paint provider pretty cheap in gallons.

I have a friend who buys the stuff at 4.00/gal and adds a gallon and a half to his 11.5 to 1 big block every fill up (about 12 gallons). This equates to approx 21% toluene by volume (entire tank) and an estimated octane of over 102. The cost to do this with a $1.70 base for 92 comes up to average of less than $2 for the whole tank. Not bad!!......and really works.

Good luck in the laboratory!

ABOUT AVIATION FUELS - Aviation gasoline (or "av gas") is blended specifically for use in small aircraft. It's also commonly used by many high performance engine owners because of it's high stated octane rating (usually 100-110) and the relatively low price compared to racing fuel. Unfortunately this fuel is not all it appears to be. Av gas octane is rated on a different scale than gasolines intended for ground level use. What is 100 octane "av", is not necessarily 100 octane "ground level". Besides this, there is also a big chemical difference. Normal ground level race fuels are made up of gas molecules that have a "light end" and a "heavy end". The light end of the molecule ignites easily and burns quickly with a low temperature flame (as a piece of thin newspaper would burn). The heavy end of the molecule is not so easily ignited, but it burns with a much more intense heat (as an oak log would). This heavy end of the gasoline molecule is responsible for the hotter, more powerful part of the combustion process.

Small aircraft are constructed as very weight conscious vehicles. That's because their somewhat weak engines often have difficulty taking off with any extra weight. To help reduce this weight problem, av gasolines are blended with no heavy molecule end. This makes a gallon of av gas weigh substantially less than a gallon of ground level fuel. Since small plane engines turn very low rpms and produce so little power, the omission of the heavy end is not a horsepower issue. However, for high output pwc racing engines, there is defiantly a compromise in power. This, despite the fact that many pwc owners experience the desirable cooler operating temperatures that "av" gas offers. In addition, some blends of av gas will quickly separate from some oils used in premix situations. For the above reasons, we do not recommend the ongoing use of 100% av gas, and we will not prepare any "av gas" engine kits.

Despite all this bad news, running av gas (accepting the slight power loss) is usually a better choice than
burning down a high output engine on regular pump gas. In this situation, the best choice is usually a 50/50 mix of pump and av gas. That provides "some" heavy molecule ends for the engine.

Av. gas or race fuel?

A: The higher the octane rating, the slower and colder the fuel burns. If you run too much octane in your engine, it won't run very well because the burn is way too slow. If the octane is too high, the piston might already be at bottom dead center (BDC) and the fuel might still be burning! If the octane is too low, the fuel will burn too fast and too hot which causes detonation and leads to sure-fire engine damage! Aviation fuel is another no no. A famous engine builder (top fuel engines) told me a story about an engine that was in his shop that had major melt down in the cylinders. He said, "Arron, av. gas is for air planes! Where do you see airplanes? Up in the sky! Do you see cars up in the sky? No! How does your car run when you're up in the mountains? Yeah, like crap! There's no oxygen up there. Aviation fuel is designed to be run in a low oxygen atmosphere. What happens to that cutting torch flame when you add oxygen to it? Yeah, the flame gets hotter and turns blue! What do you think happens to an engine in a high oxygen atmosphere burning aviation fuel? Look right here at this engine and you'll know!"

He made a good point.