The July 2006 issue of Sport Compact Car Magazine features an article on octane rating of pump gasoline, specifically comparing the effectiveness and cost of off-the-shelf octane booster treatments versus the addition of toluene as an octane booster.

The magazine editors utilized Saybolt LP, an independent octane test facility.

The magazine sums up their findings with:

"There is a limit to how far you can take toluene, though. According to Tim Wusz (of Rockett Brand Racing Fuels), there is an optimal window of effectiveness and beyond 30 percent things get ugly. Eventually the benefits of higher octane are outweighed by the poor vaporization and slow combustion of toluene.

Toluene-laden fuels burn slower and make less power on high-revving engines. So much, in fact, that much of the fuel/air mixture is still burning as the charge exits the exhaust port. For these reasons, true race fuels don't just use toluene or other active ingredients to boost the octane. Instead they use better-refined dydrocarbon chains to raise octane while retaining optimal combustion characteristics."

Earlier in the article, a toluene/91-octane pump gas combination was mixed at 10%, 20%, and 30% to evaluate how much these concentrations would raise the octane. 99% pure gasoline-grade toluene was used for their tests. The quality of toluene that is readily available from hardware stores will vary but will not be as good as what was used in during these tests. At the 30% toluene concentration, it raised 91 R+M/2 pump gas to 95.5 octane. Unocal 100 octane unleaded racing fuel uses 25% toluene but the rest of the improvement comes from additional refinement of the gasoline.

"Step back and look at the cost-per-octane-point-per-gallon, and the off-the-shelf boosters look pretty good. Ignoring the 8 cents-per-point-per-gallon rating of our home-brew (since the results are withing the margin of error, the number is meaningless) all the boosters are similar. Bottom line? Octane gains and costs are comparable between the off-the shelf boosters and our home-brew boosters. But the convenience of a little bottle of booster, compared to 4.5 gallons of toluene is obvious. Which would you rather carry in your trunk?"

I've attached a chart published by Union 76 that shows the result of mixing 91-octane pump gas with their 100-octane racing fuel in various concentrations. I've also included a modified version of the chart that takes into account a vehicle that has a 16-gallon gas tank. I arrived at the extra data points in the chart by extrapolation, not by testing, so the results may not be as accurate as lab testing but should suffice for most purposes as a rough calculation.

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