High-Performance Gasolines

ducatimeccanica.com - home

by Scott Parkhurst
from Popular Hot Rodding January 1998

It all comes down to fuel. You can build the hottest, most throw-down thumpin’ big block that ever existed, but it’s gotta have good gas.

What is good gas anyway? What separates the killer stuff for your NMCA Pro-Streeter from the slag your lawnmower barely runs on? After consulting experts in the field, we decided to check into the various street fuels available, various types and grades of octane booster, aviation gasoline (AvGas), and racing fuels. By comparing the different options available to you, it may be easier to choose the best grade for your ride.

What is octane anyway? Octane is a measurement of a fuel’s resistance to ignition. Ideally, the air/fuel mixture will ignite at the proper time and burn smoothly through the power stroke. The idea is, one powerful combustion of better than several. randomly-ignited small flame fronts. When you can precisely control the point at which the fuel will ignite, maximum performance of the engine can be achieved, and power-robbing knock and ping will be eliminated. Knock and ping are a result of abnormal ignition, or multiple flame fronts colliding within the combustion chamber during the compression stroke.

All reputable fuel manufacturers determine the octane rating of their gasoline in the research lab using a special, dedicated single cylinder engine. Comparing the gasoline to a series of standard reference fuels in the test engine results in either a research octane number (RON) or a motor octane number (MON) depending on a set of operating conditions. The RON is determined with the test engine operating at 600rpm, at standard barometric pressure, and the intake air temperature set at 125 degrees Fahrenheit. RON is primarily used to address part-throttle knock and ping problems. The MON addresses wide open throttle operation and is determined with the test engine spinning at 900rpm, also at standard barometric pressure, and the intake air temperature pumped up to 300 degrees.

The best predictor of a fuel’s performance in a street/strip machine is the Anti-Knock Index (AKI). This is simply the average of the RON and MON numbers, or (RON+MON)/2. Most all octane ratings posted at the pumps are determined by this AKI formula, and are the minimum values you could expect to see. The minimum octane requirement of your engine is determined by several variables besides the compression ratio. The engine and cylinder head configuration, air/fuel mixture, timing, coolant temperature, atmospheric pressure, relative humidity, and ambient air temperature will also affect the octane required to make your mill produce maximum power.

The burn rate of a fuel is a measurement of the time required for complete combustion of the air/fuel mixture. The notion that octane ratings affect the burn rate of fuel is about 180-degrees from reality. Burn rate is a function of several variables, and the two are completely independent, although there is generally a correlation between octane ratings and burn rates.

To give you a good example of this, we contacted Jim Wurth from Sunoco Race Fuels. He explains, "A perfect example is Sunoco Maximal, which is our fastest burning fuel, and coincidentally one of Sunoco’s highest octane fuels at 116 (R+M)/2. A lot of Pro Stock teams rely on Maximal for those sub-seven second runs. When they are turning 9,000rpm or more, the fuel has to burn pretty quickly to achieve complete combustion."

Octane boosters offer little help in the quest for higher octane. Most popular street-legal octane boosters claim increases in octane ratings up to five points, and those boosters intended for off-road use only claim up to seven points. That’s a lot of octane to hope for simply by pouring an additive in a tank. Sunoco told us that before they launched their GT-100 Unleaded retail pilot program, they wanted to be sure that a 100 (R+M)/2 octane street-legal fuel would be of value, and that enthusiasts would not be able to get the same (or better) results using an octane booster. Nine of the most popular retail octane boosters were put through a series of tests to determine where the consumer could get the most bang for the buck. The test results were verified by an independent testing facility, using several brands of regular unleaded and premium gasolines, just to make sure everything was legit.

According to Mark Borosky, Vehicle Test Engineer for Sunoco, "Of the nine octane boosters tested, none showed a significant increase, and one actually lowered the octane number of the test gasolines." Testing repeatedly showed a maximum increase in octane of 3.5 points by only two of the six street-legal octane boosters when the recommended treatment rate was blended with lower base 87-octane gasoline. The best the remaining four products could muster was less than a one point increase. "While clearly no one would actually use an octane booster in a low base octane fuel, we wanted to give the manufacturers the benefit of the doubt relative to their claims of five-to-seven point increases" explained Borosky.

When tests were performed using 98 and 94-octane fuel, even the two best products from the previous tests produced a disappointing 1.5 to 2 point maximum increase. The remaining four street-legal octane boosters showed less than a .5 point increase. Those products designated for off-road use only didn’t fare any better than the street-legal products. Subsequent tests where the dosage of octane booster was doubled, tripled, and even quadrupled produced only minimal improvements in octane, regardless of the base octane hum-ber of the test gas. In fact, quadrupling the treatment rate of the most powerful additive produced only a 3.5 point increase in octane when added to 98 premium, resulting in a cost of $3.25 a gallon.

An alternative path to octane euphoria is to blend gasolines of different octane levels yourself. It’s easier than you may think, safe, and the results are predictable. The formula for mixing gasolines of the same type is pretty straightforward. When you mix a 50/50 blend of two unleaded fuels, simply average the two octane ratings to determine what’s in the tank. If you mix 94 and 100, you get 97. The same generally holds true for leaded gasolines, assuming the lead content is nearly equal.

Blending a leaded fuel with unleaded, however, pushes the octane up a bit more than the math would suggest, due to the effect of the lead. Just a gram or two of lead blended into the unleaded fuel will raise the octane number significantly. Commercial leaded racing fuels contain anywhere from a trace to six grams of lead per gallon. If you were to mix 50 percent 110 octane leaded fuel with 100 octane unleaded, you would actually end up with an octane number around 106 to 107. Keep in mind that even the smallest amount of lead or leaded gasoline with unleaded, could spell the end of your catalytic converter or oxygen sensor. The same holds true for using octane boosters intended for off-road use only. A word to the wise, check for any lead content in all the additives you might mix with your unleaded gasoline. And check with your state emissions regulations for street use.

We asked Sunoco’s Wurth about using aviation fuel in an automobile engine. He was emphatic when he said, "Don’t do it. Even though Sunoco is a major producer of aviation fuel, this fuel is specifically blended for aircraft engines. Aircraft operate under very different conditions than automobiles, and the fuel requirements are quite different as well. Aircraft engines generally use very small pistons and run within a very narrow rpm range. There’s no need for transient throttle response in an airplane because after the pilot does the initial engine run-up, the throttle is set in one position and the rpm doesn’t normally change until landing. Also, airplanes fly where the air is cold and thin, and the atmospheric pressure is low. These are not even close to the conditions your street machine will see on the ground. Also, since most piston-driven aircraft cruise at 3,000rpm or so, the burn rate of aviation gas is much too slow for any high-performance automotive applications."

What is it that makes race gas so different? What’s it made of? Sunoco tells us their GT PLUS 104 octane unleaded race gas is only 15-20 percent traditional gasoline, and about 85 percent additives! Actually there are about 120 different chemicals in GT PLUS. One reason it isn’t street legal is the high oxygen content. The EPA requires that the oxygen content of a street legal fuel cannot exceed 2.9 percent. GT PLUS is about 3.5 percent oxygen. This fuel is light in weight at only 6.14 1bs-per-gallon. The high oxygen content improves the octane, and when the induction system is properly calibrated, this fuel will help make additional horsepower. The high oxygen content has a supercharging effect, since 3.5 percent oxygen is the equivalent to about 17 percent more air. Different fuels can actually alter horsepower 5-to-10 percent or more.

We wanted to know more about the different types of race gas Sunoco had, and didn’t realize there were five different types of racing fuel alone.

GT-100 Unleaded, is a clear fuel with a pump octane of 100, and will handle compression ratios of up to 12:1, and is street legal in all 50 states.

GT PLUS, is also unleaded, and is rated at 104 octane. It is suitable for compression ratios up to 14:1 and is colored light blue. It will not harm oxygen sensors or knock sensors in computer controlled engines. It is not street legal.

STANDARD, is a leaded fuel rated at 110 octane, is colored purple and is intended for drag racing, road racing, and race boats.

SUPREME, also a leaded fuel, rated at 112 octane, is dark blue. It was developed to help resist vapor lock and meet the demands of sportsman, modifieds, offshore powerboats, and endurance racing where engines regularly run in excess of 7,000rpm.

MAXIMAL, we mentioned earlier, is colored red, has 116 octane, and is leaded. It is intended for exceptionally high performance applications, like Pro Stock, where extremely high cylinder pressures are common. Its extremely fast burn rate is satisfactory where rpm exceeds 10,000.

Now that you’re an expert on gasolines, you probably would like to know where to buy and store the stuff. If you are fortunate enough to live in the mid-Atlantic states, you can take advantage of Sunoco’s GT-100 Unleaded retail pilot program and get 100-octane race fuel at pumps located at select service stations. The rest of us have to purchase from local speed shops, at race tracks, or directly from Sunoco distributors.

When you plan on buying fuel in quantity, say a 55-gallon drum, you’ll be happy to know that racing fuel has a shelf life of about a year, if you store it properly. The container must conform to all safety standards, and should be made from metal or polymer. Make sure the container is opaque and solid in color. The white plastic jugs we see at the track should be used for short-term storage only. They let in sunlight, which will affect the fuel The lead in leaded fuel and other chemicals in unleaded fuel are photosensitive, and will dissipate if they are exposed to the sun. Keep any container tightly sealed to prevent evaporation.

Good gas is a great thing, and we learned a lot from the Sunoco guys. They have been at this a long time (remember Mark Donahue’s Camaro?) and we hope you learned as much as we did. We couldn’t talk them out of a tankful of the killer stuff for Project X, but we had fun. C’mon guys! It’s research!