Avgas

Cylinder cracks are nothing new, however, and there is no reason to expect autogas-burning engines to have a higher incidence of head cracking than avgas-users

In the area of autogas volatility, a Reid vapor pressure of 9.0 to 15 is very common, compared to the vapor pressure of avgas, which is 5.5 to 7

Autogas can be obtained for a fraction of the cost of avgas, but its use is not without controversy.