How ’bout AvGas?

The only aviation gasoline of interest to Corvette enthusiasts is "low-lead 100" (AvGas LL100). Because of its antiknock rating and low price compared to leaded racing gasolines, AvGas might seem a good choice for a moderate octane boost, but closer study raises some doubt.

Because of its lead content, AvGas must not be used by engines fitted with modern emissions controls. It renders catalysts useless in short order and will eventually plug them. AvGas quickly damages most oxygen sensors, too.

The sale and use of aviation gasoline is heavily-regulated. Most aircraft fuel dealers refuse to put AvGas into anything other than an aircraft fuel tank. There is a legal gray area that has some vendors willing to dispense AvGas into approved containers if they believe the end use of that AvGas is fueling an aircraft engine. This loop-hole is how some people obtain AvGas for automotive use.

Aviation gas is formulated for large-bore, long-stroke, low rpm engines which run at high altitude. While AvGas’ higher octane is useful, smaller-bore, shorter-stroke, high-rpm, non-cat, Corvette engines requiring 92-98-oct. will perform better on racing gasoline. AvGas has lower volatility so, used in proportions higher than about 40%, part-throttle drivability and cold starts may be compromised. AvGas has a lower specific gravity so it will require a change in air-fuel ratio calibration for the engine to perform at its best. LL100 is blended with a high percentage of aromatics. That reduces throttle response–not really an issue with an aircraft engine but certainly an issue in a high-performance automotive engine.

The "Aviation" antiknock rating system is different than the MON rating. 100-oct. on the aviation scale, equals 98.8 MON. The biggest limitation of LL100, when used in very high-compression or high-boost, race track applications requiring leaded gas is octane. "For those applications, AvGas," Tim Wusz told us, "is short on octane compared to most (leaded) racing gasolines. Many racing engines have more spark advance at low rpm and/or during lean, part-throttle operation than AvGas and even some (unleaded) racing gasolines can handle. The result is detonation."

Bottom line: AvGas is ok in off-road situations were a leaded fuel of no more than 98 octane required, ultimate performance is not important and you can accept possible drivability quirks.

Most small airports that cater to general aviation will sell AvGas through fueling facilities such as this. If you decide to accept the compromises of aviation gasoline, make sure what you buy is "100" or "100LL". Do not use "Jet A". That’s jet fuel, a form of kerosene, and burning it in an automotive engine will cause serious damage. Image: author.

Blend Your Own Race Gas? Not.

If you’re a regular reader of the VetteNet mail list or visitor to the techie boards on the Corvette Forum, you’ve heard of other do-it-yourself additives said to improve gasoline. Unfortunately, a lot of that is urban legend. The executive summary of "DIY race gas" is: mixing it can be dangerous. You sometimes loose performance. You don’t save money.

Some of these DIY additives are: aniline, benzene, toluene, xylene and propylene oxide. Forget the first two. Both are highly toxic. Aniline is absorbed through the skin and impairs your blood’s ability to carry oxygen. Handle aniline improperly and you die. Benzene is a carcinogen, so you’ll die after improperly handling it, too–it’ll just take longer. Their toxicity and that they are used in making drugs has aniline and benzene Federally-regulated and not available to the public.
The aromatic hydrocarbons ("aromatics"), toluene and xylene are octane improvers. Significant amounts of toluene and lesser amounts of xylene are already in pump and racing gasolines. Both are available from automotive paint suppliers. Both are mildly toxic. Work with them wearing chemical-resistant gloves and in a ventilated area. If there’s any question about ventilation, wear a respirator.

In California, law restricts aromatics to 30% of a gasoline blend. Elsewhere it may be as much as 40%. The effect additional toluene or xylene has on pump gas is unpredictable for two reasons: 1) the octane boosting ability of both is less effective on premium pump gases than on regular grade gas because of the aromatics premium gases already contain, 2) toluene and xylene have high octane ratings alone but lower octane when blended with other gasoline components.

Toluene and xylene have specific gravities higher than pump gas so the more of them you add, the leaner you need to calibrate the engine’s air/fuel ratio. Once you calibrate for toluene- or xylene-spiked, DIY racing gas; don’t go back to running conventional gasoline until you recalibrate to a richer mixture or you’ll be burning pistons.

Both have less volatility, so engines burning gasolines laced with high concentrations can be more difficult to start when cold.

In addition to handling, mixing, calibration, drivability and performance problems associated with DIY race gas, it has a lousy business model, too. A late-model Corvette with a medium-boost, aftermarket supercharger kit at the drag races on a warm day might need 97.2-oct. to keep the engine out of detonation. Toluene, used as a blending component, is 103.5-oct. To make 10-gal. of 97.2-oct., DIY race gas (1:1, 91-oct. unleaded and toluene) costs $42.80. Do it with 91 and 100 unleaded gasolines, you mix 3:7 for $32.05. Because a 1:1 mix of toluene and pump gas costs you performance and throttle response due to slow burn speed; not only is DIY race gas a lot more expensive, but it won’t perform as well, either.

The economics of xylene are worse than toluene. Xylene from industrial sources is "mixed-isomer" and has less octane boosting ability than toluene and a higher unit cost. The higher octane, single isomer varieties of xylene, typically obtained through science and laboratory supply businesses, are obscenely expensive, upwards of $100 per gallon.

Misunderstanding surrounds propylene oxide. Common uses for it are pesticide and fumigant. While the EPA lists it only as a "probable carcinogen," ingesting propylene oxide will at least make you sick and can cause coma or death. Use care when handling it. Some racers are under the impression "P.O." is an octane booster, but it is not. It is an oxygenate that works like nitrous oxide but not as well. "It will improve performance," Wusz stated, "but the mixture must be richer to take advantage of that. PO is more effective than MTBE but less effective than nitrous. The downsides of PO are: 1) it attacks plastic and rubber parts in fuel systems and 2) its low, 95 deg. F boiling point gives it a tendency to easily escape from a blend leaving the DIY race gas blender with a gasoline which he thought contained a certain amount of PO, but in reality, may have retained far less of it. This makes tuning exceedingly difficult."

Bottom line: brewing your own race gas a foolish move for a lot of reasons. You’re better off buying it ready-made.

"Adding more toluene," Tim Wusz told us, "will increase the octane numbers of the gasoline, but when you get above 45 or 50%, throttle response is poor and the flame speed is reduced to where increasing amounts of fuel are still burning as combustion gases are forced out the exhaust valve. Once that happens, power is lost, not gained." Image: author.

Some racing gasoline vendors make available charts like this one, for 91-oct pump gas and 100-oct unleaded racing gas, to assist users in mixing unleaded racing gas with premium unleaded pump gas