Originally Posted By: Burt
They can put more butane in winter gas and still meet the reid vapor pressure limit. Butane has less energy content (btu's) per gallon, but more octane.
Originally Posted By: Mar57
I'll add another question ... Why change the blend from summer to winter?
The official line is to raise the vapor pressure so that it evaporates more readily in colder temperatures, thus lowering unburned HC emissions out the tailpipe.
Since that's not even remotely an issue in 90% of the country for fuel injected cars, and it greatly increases the HC emissions out the fuel tank vent system on all cars (especially pre-fuel-injected cars,) I suspect its a convenient way to get rid of lighter, higher vapor-pressure refined fractions at a profit marin that normally happens only for the lower vapor-pressure fuel fractions in the summer.
If it were really about emissions, winter-blend fuel would not exist. My garage REEKS of unburned gasoline when I pull one of the older cars in after a winter drive- you can sometimes hear it gurgling in the tank and fuel lines. One of them will also vapor-lock on an 75-degree winter day when it won't vapor-lock on a 110-degree summer day. Even the 2001 Jeep Cherokee I had would vapor-lock the #3 injector (there's a TSB about it) on a hot winter day on that winter swill. Winter-blend fuel is a re-used waste-product, IMO.