Summer, Winter, Bad Gas?

[Jay Laifman]

When do they switch the gas and when is the gas worse?

Both my son and daughter have been complaining that their cars seem to be running a little off. And now I'm thinking I'm noticing both my 912 and 911 being a "little off." On the way to work in the 911, I was just remembering about the switching of the gas. I don't know if the schedules match or when it goes from good to bad (or bad to worse) gas. I'm in So Cal.

Either that or all the cars need something - which would be a bummer.


Ok, I just did some more research on line. I see that the winter gas has more butane, which has a negligible impact on making it easier to start in cold mornings. Butane apparently evaportates too quickly during the summer. So the forumula in the summer is "more pure." Gas milage is worse with the butane by 2-3%.

I also saw one reference that the change is usually around September 15th. So that would be pretty close to when my kids started complaining. But, I don't know that this is the date for California and I don't know if by that date all gas stations already have the winter gas, or that's when the refineries have to be sending it out.

But, I still don't know if the car would actually run worse with the Winter gas. I also don't know if they also upped the corn content recently.

[Flieger]

Neither one is worse. Horses for courses you know. If you live in Minnesota your life depends on the extra lighter fractions (gasoline is a soup of hydrocarbons from butane and propane to heptane and octane to dodecane (12 carbon)) because you need them for cold starting. Summer gasoline won't vaporize in the depths of winter unless the car is warmed up first. The lighter fractions are rarer/harder to come by so they save them up for the winter since they are not needed in the summer. So, if anything, winter gasoline is better and costs more as a result.

Now, the stoichiometric air/fuel ratios are different for each type of molecule, so if you have more lighter fractions you need more mass of air per mass of fuel. If you don't have closed-loop EFI (oxygen sensor) then your car will run rich if you had the same amount of air coming in. Colder air is denser, though, so you could swing the other way depending on the weather.