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© 2009 by Hib Halverson
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In areas which are below 3000 feet altitude, on cool days and on 93-octane fuel, there's no problem, but in warm weather, or in just about any weather when running 91-octane, 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 is the best way to eliminate this problem, short of engine modifications, changes in calibration, moving to a cooler area or moving to the mid-west or eastern seaboard where 93-94-octane gas is widely sold. We suggest starting at 1:5, pump gas to 100 unleaded, then work up until the detonation or knock retard goes away.
Does this work in practice? We took our C5 Z to the AutoDyn at Westech, ran some baseline tests and observed knock retard with the Mastertech. Back at our shop, we drained the fuel tanks, added 10 gallons of Rockett Brand 100 then drove 50 miles back to Westech. This time, the IAT was 106°, the engine ran with no knock retard and the dyno read 9.5hp more than the first test on straight premium unleaded. After lunch, we "thinned" the remaining Rockett 100 down to 94.6-oct. by mixing it 2:3 with Chevron 91-oct. and ran again. The IAT was now 109 and the power was up 7.5-hp over straight pump gas. In all runs of this second round of tests, 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, as we proved earlier, for a given octane, compared to pump gas spiked with pour-in additives; its economics are usually more favorable and, considering the possibility of engine or emissions controls damage some concoctions can cause, it's always a better value. Don't think you have to use racing unleaded at full strength, either. If your engine only needs 94.0 octane, mix it 1:2 with 91-oct. pump gas. It's relatively easy to "math out" the proportions of the mix you want using the mixing charts some race gasoline suppliers make available, such as the ones from Rockett Brand Racing Fuel, reproduced here and there are more, along with a list of retailers, on Rockett's web site.
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 a 100 unleaded and its slightly faster burn speed will improve performance even more. With our Z06 test there was a 2-hp improvement at high rpm.
A few of these unleaded fuels are street-legal in all 50 states. Another attractive feature of some, like the Rockett 100 sold at Rebel stations in southern Nevada and at Gas City stations in the Chicago area, 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 your Corvette full of 100-oct. unleaded race gas.
"We also have '112,' our new, E85 product," Wusz told us. "We decided to introduce it because some of the 'green racers', which are mostly sport compact drag racers and some road racers, have asked for it. It's attractive to them because they consider it a renewable fuel. Some green racers who used to run on methanol and have switched to ethanol and some of them are interested in E85 because they think it's 'greener' than E98 (98% ethanol/2% gasoline) and a better contribution to cleaning up the environment. It's really not, but advertising by GM and others has been successful in convincing people of that. We had a lot of input at trade shows from green racers running in classes where fuel is unregulated or allows E85, asking for a racing E85 and they're now buying our '112'.
"Where pump E85 can be anything from 'E65' to 'E85', our E85 is consistently 85% ethanol and 15% gasoline, plus the gasoline component is a high-quality, unleaded racing gasoline similar to our '100 Unleaded' so it's performance, throttle response, burn speed and other properties are very consistent in their performance. At this point, we see E85 as a growth area in racing fuels."
Some racing gasoline vendors make available charts like this to assist users in mixing unleaded racing gas with premium unleaded pump gas. Rockett Brand Racing Fuel has charts for 92- and 94-oct., too. Graphic: Rockett Brand Racing Fuel.