An oxygenate (oxygen-bearing compound) like alcohol can dangerously lean out an engine when used in excess in a fuel-metering system not calibrated for such use. However, engines specifically designed and calibrated for alcohol make more power than an equivalent gasoline-fueled engine...so to prevent cheating, sanctioning bodies test for the presence of alcohol compounds in gas. Reputable race gas manufacturers-including Phillips, Sunoco, Trick, and Tosco (the manufacturer of 76 Racing Gasoline, formerly Unocal)-are well aware of this, and produce their high-octane fuels without resorting to doctoring the blend with alcohol. Unlike backyard bathtub blenders the major companies also publish complete specs on the racing gas so you know what's in it.
Modern high-quality racing gas is manufactured using selectively picked, high-octane hydrocarbons as the base stock. These include toluene, isooctane, butane, and pentane, among others. Off-road-only racing gas also receives a dose of good ol' tetraethyl lead to further raise the octane. While tetraethyl lead-bearing fuels aren't legal today for street cars, in addition to its 110, 114, and 118 (R+M/2) octane leaded 76 Competition Racing Gasoline, Tosco also produces 100 octane unleaded race gas that meets all state and federal regulations for street-legal CARB Phase II and EPA reformulated gasoline (RFG). While quite expensive in undiluted form, blending 100-octane unleaded with standard 92-octane gas may raise the octane enough to support engines presently pinging on normal street gas. For example, a 1:1 blend of 100 racing unleaded and 92-octane 76 Super unleaded yields a 96 (R+M/2) octane fuel. Detailed blending tables are available from 76 Lubricants Company.
Unleaded race gas does contain MTBE, an ether-bearing oxygenate. Unlike alcohol, MTBE has no tendency to separate out from the rest of the gas mix. EPA regs also limit max street-legal gas content to 15-percent MTBE, so at worst you would have to fatten up the carb two to three jet sizes from its optimum calibration with Tosco's 76 Competition 110 "NASCAR" leaded race gas (closed-loop electronic engine management systems automatically self-compensate).
In fact, any change in gasoline brand and/or octane may require carburetor recalibration. The governing factor is the gasoline's specific gravity (SG). If moving from a higher SG gas to a lower SG gas, you need to richen the mixture by going to larger jets. If moving from a lower SG gas to a higher SG gas, lean the mixture. Tim Wusz at 76 Racing Gasoline recommends leaning or richening the mixture by one jet size for every 0.010-inch change in SG (assuming the carb was correctly jetted for the old gas). Reputable race gas suppliers publish their fuel's SG.
Note that the presence of any oxygenate in the fuel will cause the fuel to fail the sanctioning body's fuel-check. You have the paradox of street-legal and environmentally-friendly gas that's illegal for most competition use, while toxic lead-bearing off-road gas is OK for racing but prescribed on the street. In other words, everyday unleaded street gas as well as the special unleaded race gas isn't considered "gasoline" at present by NHRA or NASCAR! Unleaded gas is legal in AMA and SCCA competition, however.
76 Lubricants Co
P.O. Box 7600
Brea, CA 92822-7600