www.ducatimeccanica.com/gas.html
home by Scott Parkhurst from Popular Hot Rodding January 1998 It all comes down to fuel. You can build the hottest, most throw-down thu mpin big block that ever existed, but its gotta have good gas. What separates the killer stuff for your NMCA Pr o-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 gasolin e (AvGas), and racing fuels. By comparing the different options availabl e to you, it may be easier to choose the best grade for your ride. Octane is a measurement of a fuels resistance to ignition. Ideally, the air/fuel mixture will ignite at the proper time a nd burn smoothly through the power stroke. The idea is, one powerful com bustion of better than several. Whe n you can precisely control the point at which the fuel will ignite, max imum performance of the engine can be achieved, and power-robbing knock and ping will be eliminated. Knock and ping are a result of abnormal ign ition, or multiple flame fronts colliding within the combustion chamber during the compression stroke. All reputable fuel manufacturers determine the octane rating of their gas oline in the research lab using a special, dedicated single cylinder eng ine. Comparing the gasoline to a series of standard reference fuels in t he test engine results in either a research octane number (RON) or a mot or octane number (MON) depending on a set of operating conditions. The R ON is determined with the test engine operating at 600rpm, at standard b arometric pressure, and the intake air temperature set at 125 degrees Fa hrenheit. RON is primarily used to address part-throttle knock and ping problems. The MON addresses wide open throttle operation and is determin ed 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 fuels performance in a street/strip machine is t he 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 ar e determined by this AKI formula, and are the minimum values you could e xpect to see. The minimum octane requirement of your engine is determine d by several variables besides the compression ratio. The engine and cyl inder head configuration, air/fuel mixture, timing, coolant temperature, atmospheric pressure, relative humidity, and ambient air temperature wi ll also affect the octane required to make your mill produce maximum pow er. The burn rate of a fuel is a measurement of the time required for complet e combustion of the air/fuel mixture. The notion that octane ratings aff ect the burn rate of fuel is about 180-degrees from reality. Burn rate i s a function of several variables, and the two are completely independen t, 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 Ra ce Fuels. He explains, "A perfect example is Sunoco Maximal, which is ou r fastest burning fuel, and coincidentally one of Sunocos highest octan e fuels at 116 (R+M)/2. A lot of Pro Stock teams rely on Maximal for tho se sub-seven second runs. When they are turning 9,000rpm or more, the fu el has to burn pretty quickly to achieve complete combustion." Octane boosters offer little help in the quest for higher octane. Most po pular 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. Thats a lot of octane to hope for simply by pouring an additive in a tank. Sunoco told us that before they launched their G T-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 bo oster. Nine of the most popular retail octane boosters were put through a series of tests to determine where the consumer could get the most ban g for the buck. The test results were verified by an independent testing facility, using several brands of regular unleaded and premium gasoline s, 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 act ually lowered the octane number of the test gasolines." Testing repeated ly showed a maximum increase in octane of 35 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 clearl y 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 t o 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 15 to 2 point maximum increase. Those products designated for off-road use only didnt fare any better than the street-legal products. Subseque nt tests where the dosage of octane booster was doubled, tripled, and ev en quadrupled produced only minimal improvements in octane, regardless o f the base octane hum-ber of the test gas. An alternative path to octane euphoria is to blend gasolines of different octane levels yourself. Its easier than you may think, safe, and the r esults are predictable. The formula for mixing gasolines of the same typ e is pretty straightforward. When you mix a 50/50 blend of two unleaded fuels, simply average the two octane ratings to determine whats in the tank. The same generally holds true f or 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 octan e number significantly. Commercial leaded racing fuels contain anywhere from a trace to six grams of lead per gallon. If you were to mix 50 perc ent 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 t he smallest amount of lead or leaded gasoline with unleaded, could spell the end of your catalytic converter or oxygen sensor. The same holds tr ue for using octane boosters intended for off-road use only. A word to t he wise, check for any lead content in all the additives you might mix w ith your unleaded gasoline. And check with your state emissions regulati ons for street use. We asked Sunocos Wurth about using aviation fuel in an automobile engine . Even though Sunoco is a ma jor producer of aviation fuel, this fuel is specifically blended for air craft engines. Aircraft operate under very different conditions than aut omobiles, and the fuel requirements are quite different as well. Aircraf t engines generally use very small pistons and run within a very narrow rpm range. Theres no need for transient throttle response in an airplan e because after the pilot does the initial engine run-up, the throttle i s set in one position and the rpm doesnt 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 slo w for any high-performance automotive applications." Sunoco te lls 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 isnt street le gal is the high oxygen content. The EPA requires that the oxygen content of a street legal fuel cannot exceed 29 percent. This fuel is light in ...
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