Technical Articles

When a lower octane fuel with a higher volitility is better at controling detonation

    On the spec sheet issued by makers of racing gasolines, you will see volatility referred to twice; as the 10% point and the 90% point. The 10% point is that temperature by which 10% of the fuel has come over in the volatility test. This is a measure of the fuel’s ability to cold-start; the lower the 10% point temperature, the easier the cold-starting becomes. The 90% point is that temperature by which 90% of the fuel has come over. This is a measure of how easily or quickly the fuel as a whole will evaporate in a running engine. For example, two fuels of different 90% points may run identically in a 6000 rpm engine; both have plenty of time in which to vaporize adequately. Yet the fuel with the higher 90% point may chop a few hundred revs off the top of a 12,000 rpm engine.

    This effect is most often seen when a racer buys fuel by the 55-gallon drum, and leaves the drum open all the time he is at the track. Over time, much of the fuel’s front end evaporates, raising both the 10% and 90% points. Such a user often finds himself using stranger and stranger jetting as the drum gets down to the last third. Throttle response deteriorates and peak-rpm performance gets soggy. Finally, in desperation, he opens a fresh new drum and all his missing performance comes back. Keep race fuel capped and in a cool place. Every time you unscrew the cap and hear that “PSSHH”, that’s part of your volatility, throttle response, and higher-rpm performance escaping. If you are a small user of race fuels, buy in five-gallon tins instead of in drums – just as you would buy soda water in 8-10 ounce bottles instead of in liters, to keep it from going flat.

    In high-rpm racing engines, the time that elapses between the fuel’s entering the airstream and the spark that ignites the mixture is extremely short – sometimes too short for adequate vaporization. The result is lean mixture, just as in cold-starting. Trying to compensate by richening-up works, but it sacrifices power because the extra fuel passing through unburned robs heat from combustion. In some cases, a more volatile fuel can solve the problem (as above). Octane number is not the only important variable in racing gasoline. Look for a fuel with a lower 90% point and try it. Often, especially with two-strokes and their absurdly high compression ratios, a lower-octane fuel of a higher volatility will cause less knock than will a higher-octane fuel of lesser volatility. Failure to vaporize leads to leanness, which in turn can lead to knock.
A Favorite Subject
by Kevin Cameron