[mrtexas]

"Mogas has no such regulations and it's volatility changes from summer to winter" EPA mandates the vapor pressure of gasoline. In summer it is quite lower than it used to be somewhere around 6psi vs 10psi which means no butanes in the summer. In winter with cold weather it has to be higher like 15psi with butanes so the gasoline will vaporize in the cylinders. The colder the region the higher the vapor pressure required.