Jeff Smith Senior Technical Editor
June 1, 1999

Step By Step

P86438_Large Ford_Mustang Gas_Tank_View
Octane is the most important issue when it comes to selecting the proper gasoline. Octane is a numerical rating that indicates a fuel’s resistance to detonation. The higher the octane number, the more resistant it is to detonation. Higher octane allows an engine-builder to create higher cylinder pressures to make more power. If your engine does not detonate, rattle, or ping on 87-octane fuel, there is no advantage to using a higher-octane fuel. Also, a common fallacy is that higher-octane gasoline burns faster or slower. As octane increases, the components that increase the octane chemically react in the combustion process to merely prevent detonation rather than change the burn rate.
Pump gas and most race gasoline is rated in the (R+M)/2 fashion, which refers to an average of two ways that octane is measured: Research and Motor octane. Research octane uses an industry-standard, single-cylinder test engine run at 600 rpm with an inlet-air temperature of 100 degrees Fahrenheit. Motor octane numbers are generated with this same test engine operating at 900 rpm with inlet air at 300 degrees F. Typically, Research octane numbers are 8 to 10 numbers higher than Motor octane numbers, since higher inlet air temperatures will increase an engine’s tendency to detonate. There is an almost infinite number of ways to create the (R+M)/2 octane rating. According to Wusz, higher Research numbers will reduce part-throttle detonation, while higher Motor numbers are better for detonation resistance at wide-open throttle.
Unocal’s 76 Racing Gasoline offers two different types of racing gas: 108-octane leaded and 100-octane unleaded. The 100-octane unleaded is oxygenated and therefore legal for use in street-driven vehicles. Often, 100 may be more octane than you really need for your engine. You can mix the 100-unleaded with 92-octane pump gas to create the octane you need to eliminate detonation. Wusz also mentioned that unleaded can be mixed with leaded race gas. When you do this, the final octane number will be slightly higher than indicated in this chart, as small amounts of lead will increase the blended octane number. This chart is courtesy of 76 Racing Gasoline.
Recently, the Environmental Protection Agency (EPA) has required adding oxygenates to fuels used in “air quality non-attainment” areas. Mostly, these are large metropolitan areas such as Los Angeles. Oxygenates are exactly what they sound like: additives that blend oxygen into the gasoline. The target is to add 2 percent oxygen to the gasoline, which, according to Wusz, has the effect of leaning the air/fuel ratio of the fuel. This can mean a significantly leaner air/fuel ratio compared to nonoxygenated gasoline. This used to be mandated only in the winter months and served to reduce hydrocarbon emissions. But in Los Angeles, for example, oxygenated fuels are now here to stay. At the start of 1995, approximately 35 percent of all gasoline sold in the U.S. was oxygenated.
You may have heard the term specific gravity in relation to gasoline. Most race organizations specify the type of gasoline by this method. Specific gravity is the comparison of the weight of fuel in relation to the weight of water. If water is rated as a value of one, then gasolines (since they are lighter) are specified as a percentage of 1. Many street unleaded 92-octane gasolines measure from 0.75 or higher, while leaded race and unleaded 87-octane street gasolines can test below 0.75. This is important, since the specific gravity of a particular fuel affects the mass of fuel that can pass through a jet orifice. Specific gravity of fuel also determines how high (or low) a carburetor float sits in the fuel. Therefore, switching from a high specific-gravity fuel to a low specific-gravity fuel will have the same impact as reducing the jet size in the carburetor, since fewer hydrocarbons will be introduced into the engine even though the jets have not been changed.
Many enthusiasts use aviation fuel as a blending agent or as a straight gasoline for high-output engines. According to Wusz, aviation gasoline, such as the green 100/130 octane fuel, is generally a lighter specific-gravity fuel that has good octane and a high percentage of alkylate that tends to boost octane. Aviation fuel will work especially well as a blending agent for pump gas but will tend to lean the air/fuel ratio due to its lighter specific gravity of around .69. Also, aviation fuel octane is not rated the same as street gasoline. For example, a 112/160-aviation-fuel-octane rating is equal to 108-octane Unocal 76 leaded race gas.
One advantage to using unleaded race gas is that it will not poison catalytic converters or oxygen sensors when used in emissions-controlled cars. One way to improve the octane rating of an unleaded fuel is to add what are called aromatics, such as toluene, which is an industrial solvent. Unfortunately, such aromatics will attack rubber, making it swell and become soft. We’ve witnessed this in a street car using high-octane unleaded race gas. Often, high-octane unleaded fuels will especially attack older rubber fuel line or rubber that is not rated for fuel line use. According to Wusz, most quality fuel line made in the last 10 or 15 years should not be affected by these aromatics. But if you are concerned about whether your fuel line will be affected, Wusz suggests cutting a short section of fuel line and immersing it in the race gas in a sealed glass container and allowing it to sit outdoors for a few days. If the fuel line swells in the presence of this gasoline, this is an unsafe condition and you should either choose a different fuel or find a fuel line that will not be affected.

Gasoline is a mystery to most Ford buffs. For the most part, we just fill up the tank and forget about the fuel. The most difficult question is whether to step up from 89 to 92 octane. As you might guess, there's much more to gasoline than just regular or premium. With the generous help of Unocal 76's principal engineer for fuels technology, Tim Wusz, we'll take a quick look at gasoline and how that aspect of hot rodding is undergoing some significant changes.