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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.

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.

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.

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.

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.

Many think racing gasoline is just for racing but, in recent years, the term has also come to mean high-octane, unleaded used by street cars. The obvious reason to use race gas is higher antiknock rating, but just as important is: it’s an optimum, balanced blend of hydrocarbons and additives intended to produce the highest possible performance from a high rpm, automotive engine. It’s better than pump gases fortified with octane boosters, better than AvGas, better than pump gases spiked with store-bought aromatics and the only alternative if you want your engine to perform best. There are 10 vendors of racing gas in the United States. Most sell through distributors, however, in a few urban areas, some refiners sell racing unleaded at retail service stations..

"Entry-level" racing gasolines are usually unleadeds of about 100-oct., R+M/2. Eight of the 10 companies sell it. Octane ratings vary from 93 to 99 MON, a significant gain over the approximately 88 MON of the best pump gas. Racing unleaded is the perfect choice if the engine in your Corvette needs a moderate octane boost. 

When late-model Corvette is run hard on hot days, as shown in our tests on the K&N Dynojet with the ZR-1, its engine computer will often enable spark retard. This is because the car’s engine controls are calibrated for aggressive spark advance to get best performance in cool weather, then "save" the engine with spark retard when mild detonation is sensed during high-load/hot-weather operation. Because of the retard, their performance is reduced slightly and coolant temperature is slightly higher.

On cool days there’s no problem, but in warm weather, the engine will get a little detonation, the knock sensing will retard spark and performance will drop slightly. A small amount of unleaded race gas mixed with premium pump gas (start at 1:5, then work up) is the best way to eliminate this problem, short of engine modifications, changes in calibration or moving to a cooler area.

Does this work in practice? At our shop, we drained our test ZR1’s fuel tank, added 10 gallons of 76 Competition 100 then drove 50 miles to K&N for a second session on its Dynojet. The IAT was 106°F and the Dynojet read 9.5hp more than the first test on straight unleaded premium. After lunch, we "thinned" the remaining Comp 100 down to 94.7-oct. by mixing it 1:2 with 76 92-oct. unleaded and ran again. The IAT was now 109 and the power was up 7.5hp over straight pump gas. In all runs of this second round of tests at K&N, our Vetronix Mastertech showed no detonation. Clearly, in hot weather, unleaded racing gas works well as a detonation fighter.

The United States EPA announced in late 2000 it intends to discourage off-highway use of leaded gas. EPA is putting pressure on the general aviation community to accept unleaded AvGas and it also wants motorsports sanctioning bodies, particularly NASCAR, to adopt unleaded gasoline. You can bet, if NASCAR goes unleaded, other sanctioning organizations will follow. We asked Tim Wusz what plans 76 Performance Products has if NASCAR goes to unleaded fuel.

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