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.
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:
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.
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.
Both have less volatility, so engines burning
gasolines laced with high concentrations can be more difficult to start
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
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
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-
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
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.
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
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.
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
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..
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.
dyno test of the 1995 ZR1 showed the best gasoline choice
for a computer-controlled car that sees a little
detonation on hot days is a mix of racing gasoline and
pump gasoline. Image: author.
It is good for engines with superchargers of modest
boost, moderate nitrous oxide injection systems or any modifications
that increase cylinder pressure somewhat over stock levels.
We asked Tim Wusz about 76 Competition 100 and he said,
"This product has been used in endurance (sports car) racing in
four-inch bore engines with 14:1 compression ratios and aluminum heads.
With cast iron heads, compression ratio should be limited to 12:1. It
can be used in current performance street cars. Historic muscle cars
can also benefit from this product which easily services the 11:1
compression ratios of many engines from that era."
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.
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.
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.
Some think unleaded race gas is
prohibitively expensive but, for a given octane, compared to pump gas
spiked with pour-in additives; its economics are more favorable. Don’t
think you have to use racing unleaded at full strength, either. If your
engine only needs 94.5 octane, mix it 2:4 with 91-oct. pump gas. It’s
relatively easy to "math out" the proportions of the mix you want and
some race gasoline suppliers have mixing charts, such as the ones from
76 Performance Products reproduced here, to make the process easier.
While it is true
that your engine only needs as much octane as is necessary to keep it
out of detonation, there are cases were a particular engine will run
better on a high-percentage mix or straight 100 unleaded in spite of
not needing additional octane. There’s no set rule on this and we
suggest, once you’ve determined the ideal mix of pump gas and 100
unleaded to keep you out of detonation that you, also, run a test of
straight 100 unleaded. In some cases the different components in the
100 unleaded and its slightly faster burn speed will improve
performance even more.
A few of these
unleadeds–Competition 100, for example–are street-legal in all 50
states. Another attractive feature of some, like the Competition 100
sold at selected 76 and Circle-K stations in urban areas, is the
convenience of buying high-octane, street-legal gas at a service
station–no searching for distributors, no hauling cans around and no
storing gas in your garage. Just drive up to to the island, stick the
nozzle in the tank and pump yourCorvette full of 100-oct. race gas.
The bread-and-butter of the racing gasoline business
is leaded fuel. It is the best choice for racing engines requiring more
than 100 octane. All vendors marketing leaded race gas have several
Here’s another guy who depends on and wins with 76 Competition 110:
NASCAR 2002 Rookie of the Year contender, Jimmie Johnson. Throughout
his stock car career, he’s run with 76 racing gasoline and it helped
him drive to victory at the NAPA 500 at Fontana, California in 2002. It
was his first win and came after only 15 Winston Cup starts. Johnson
drives the # 48 Lowes Chevrolet Monte Carlo owned jointly by Winston
Cup Champion Jeff Gordon and Rick Hendrick. Image: author.
"We have four leaded race fuels:" Tim
Wusz continued, "The first three are Competition 110, Super
Stock 114 and ProStock 118.
"110 is our highest volume racing gas. It’s the
gasoline used in Winston Cup Racing and all other NASCAR series. It’s
used by lots of ‘Saturday night’ racers in all types of competition
using all varieties of engines.
"114 is more for bigger engines. Drag racers with
big motors in SuperComp and SuperGas like it. It’s good because large
engines often need more octane. It’s for medium supercharged
"118 was originally developed for ProStock and
worked well until so-called ‘fast-burn’ gasolines came along. The 118
is ideal for high-boost, supercharged applications. It’s also good in
engines running a lot of nitrous oxide. The 118 has a little faster
burn characteristic than the 114, but not as fast as our fast-burn gas.
"We have another fuel, ProStock+, that’s really
better than the 118 for performance in certain applications but,
because of different burn characteristics, it’s around 114 octane. It’s
a very fast-burning gas for use in very high-rpm, normally-aspirated
engines. It was developed for NHRA ProStock and ProStock Truck.
Because of their high
rpms, they need the faster burn speed, otherwise you open the exhaust
valve and loose a lot of unburned hydrocarbons. If you increase the
gasoline’s burn speed, you use that stuff, rather than flushing it out
the exhaust, and the engine makes more power."
Most of the other vendors have between 3
and 5 grades of leaded race gas and one company, VP Racing Fuels, has a
mind-boggling, 14 different grades. If you are having difficulty making
a decision about racing gasoline, contact the suppliers of these
products for detailed information and recommendations.
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.
test work over two years ago with unleaded gasoline in Busch
Grand National cars without any significant adverse effects. For
current, 12:1 compression ratio engines required in NASCAR–Winston
Cup, Busch and Truck Series–our hundred octane unleaded is
adequate, but we have to convince the racers and, to do that, we
need the support of the sanctioning bodies. A few years ago when
compression ratios were 14:1, we tested unleaded in a Winston
Cup engine that had pressure transducers in all eight cylinders.
We found the 100-oct. unleaded ran with no detonation." (Ed
note: this test was under wide-open throttle conditions and
included no part-throttle, lean mixture operation typical of
running in caution periods or of engines not calibrated properly
for part-throttle operation) "This engine was optimized with
34-degrees spark advance. Then, we started bumping the timing
up. We got up to 42-degrees before there was any indication of
detonation. Normally, two degrees is an octane number so we had
a cushion of about four octane numbers."
76, we have the technology to make a 105-octane unleaded
but right now, the demand for it just isn’t there."
Tim Wusz commented.
In NASCAR and other forms of racing that put similar
demands on gasolines, the change to unleaded may be more of a cultural
challenge, however, unleaded gas might be a significant technical
problem in other parts of the sport. It would be a tough sell in some
categories of drag racing, as well as a few other motorsports that use
gasoline engines with very high compression ratios, high levels of
supercharger boost, big shots of nitrous, large combustion chambers or
any combination of these.
there is a conversion of motorsports to unleaded gas, perhaps the first
place we’ll see it is in NASCAR Winston Cup racing. The huge
appeal of NASCAR would be a major factor in marketing the switch to
unleaded gasolines to motorsports, however, at this point in time
(mid-2002) it doesn’t look like a switch will happen anytime soon.
Most of the research for this article along with the
initial interview with 76 Performance Products’ Tim Wusz took place in
February of 2001. In April of 2002, during an interview at the NASCAR
NAPA 500, we once again talked with Tim Wusz and took up the issue of
the conversion of major motorsports to unleaded gasoline.
"The Clean Air Act has always exempted motorsports
from a mandated conversion to unleaded," Wusz told us, "so all the EPA
and other governmental entities can do is discourage the use of leaded
gasolines and request a change to unleaded." He went on to say that the
November 2001 election of President George W. Bush changed the
landscape as far as the EPA’s interest in seeing a conversion to
unleaded racing gasolines was concerned.
While in early 2001, it seemed an environmentally
activist, Gore administration would come to power and push NASCAR and
other sanctioning bodies to unleaded fuels, the more conservative Bush
administration is conforming to the intent of the Clean Air Act and
seems content let motorsports continue to use leaded racing gasolines.
While in early 2001, the switch seemed a probability, in mid-2002, it
now seems a distant possibility.
Nevertheless, if the winds of change start blowing
again, 76 Performance Products will be ready with an unleaded racing
gasoline suitable for NASCAR and many other types of competition.