In a message dated 96-10-03 20:32:23 EDT, bleckstein@monmouth.com writes:
<< I always used to use regular in my TF and TC on the same theory that 87
octane is jet fuel compared to pool petrol in Britain in the late forties.
However on advise from many, I switched to premium, not for the octane but
for something to do with the volatility. The cars seem to run with less
problems in heat etc, but maybe its mental. Anyone with facts on this choice?
>>
Yes.
Higher octane fuel is more resistant to preignition, one reason being that it
has a lower vapor pressure, meaning it doesn't eveporate as easily. First
understand that liquid gasoline doesn't burn, it has to vaporize first, then
the vapor and air mixture can burn. At about -50F gasoline has such a low
vapor pressure that you can douse a burning match in a can of gasoline
without starting a fire (don't try this at home). Gasoline with lower
volitility is less likely to cause vapor lock in warm weather.
These days much gasoline contains 10% alcohol (remember the term "gasohol"?).
Many excuses for this prevail, but the real reason is that some years ago in
the midst of a fuel crisis the government found it in their infinite wisdom
to drop the motor fuel tax from alcohol to encourage its use in place of
gasoline. Alcohol is more expensive than gasoline, but without the added tax
burden it's cheaper, so you see a lot of it these days.
In some high polution areas of the US, the sale of oxygenated gasoline is
mandated. That stuff is more costly to produce at the refinery, but one
cheap and easy way to get there is to add alcohol, which is an oxygenator.
Also, alcohol seems to be a knock inhibitor, so adding 10% alcohol to
gasoline raises the octane rating about 2 points, another cheap and easy way
to make fuel for which you pay a higher price.
Now the kickers. Alcohol evaporates much easier than gasoline, so gasohol
has a higher vapor pressure which may cause vapor lock and/or lean running
conditions in many of our LBC's in warm weather. Personally I avoid the
stuff with a passion, as it gives my MGA fits when it's hot out. Another
thing to consider is that alcohol has only half the energy per unit volume as
gasoline, so if you get 30.0 mpg on gasoline (wishful thinking), you may
expect to get only 28.5 mpg on gasohol. For my money straignt gasoline is
worth 5% more than gasohol just for the better mileage, and I never see
gasohol sell for 5% less, so I can't afford to buy the cheap stuff anyway.
Gasohol is always identified by a sticker on the pump, not by name, but the
sticker says something like "Contains 10% Ethanol". If it says "Contains 10%
Alcohol" they may be trying to hide tha fact that it's really methanol which
is considerably harder on the rubber bits in your fuel system. My SU H4
carbs have the float chamber fastened to the carb body with a shoulder bolt
and a pair of rubber step washers. These rubber washers need replacing every
couple of years or so, presumably because of attack by alcohol in the gas.
The alcohol stickers on the gas pumps can be hard to spot, they're sometimes
pretty well hidden or camouflaged, so check the pumps over carefully. So far
as I know Amoco and Shell still sell the genuine stuff everywhere, but Mobil
has definitely switched to gasohol in the Chicago area. For a long time I
thought Phillips 66 was straight too, but I have recently spotted the 10%
stickers on some of their pumps as well.
Barney Gaylord -- 1958 MGA
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