AvGas is short for Aviation Gasoline. AvGas is of interest to motoring enthusiasts because of its availability, octane rating and low price compared to commercial race fuels. AvGas might seem an obvious choice but closer study raises some doubt.
AvGas LL100 stands for "Low Lead 100 Octane". TEL (Tetra-ethyl-lead) is added to raise the octane. When used in a motor vehicle this fuel will leave a lot of lead deposits in the motor. 100LL has a high lead content (0.5 grams per litre), even higher than leaded race fuels. The deposits left when TEL(lead) is burned are corrosive and damaging to valves, valve guides, valve seats and cylinder heads. Lead deposits will also block oxygen(lambda) sensors and catalytic converters and foul spark plugs even after only a short use. Also, 100LL has a chemical package added to make it perform at high altitude, and that isn't the best thing for motor vehicle performance here on the ground.
AvGas is blended for large-bore, long-stroke, low RPM engines which run at high altitude. While AvGas' higher octane is useful, smaller-bore, shorter-stroke, high RPM engines will perform better on racing fuel or high quality octane boosters. AvGas has lower volatility so when used in proportions higher than about 40%, part-throttle drivability and cold starts may be compromised. AvGas has a lower specific gravity so it will require a change in air-fuel ratio calibration for the engine to perform at its best. LL100 is blended with a high percentage of aromatics causing reduced throttle response which is not an issue with an aircraft engine but certainly an issue in a high-performance automotive engine. These high levels of aromatics will also damage rubber components in automotive fuel systems such as fuel lines, fuel pump seals and injector washers.
The sale and use of AvGas is heavily-regulated. Most aircraft fuel dealers refuse to put AvGas into anything other than an aircraft fuel tank. There is a legal grey area that has some vendors willing to dispense AvGas into "approved" containers if they believe the end use of that AvGas is fueling an aircraft engine. This loop-hole is how some may obtain AvGas for automotive use. Because AvGas has no taxes and duties on it, use on public roads is illegal and if found could result in your vehicle being impounded.
The Bottom Line:
Avgas may be suitable for some race cars that don't have catalytic convertors or oxygen sensors and are rebuilt often enough that the TEL(lead) build-up is not an issue. For other applications use an unleaded race fuel or a good quality octane booster.