just a couple of comments on this thread (gasoline 101):

When the fuel mixture explodes all hell brakes loose between the piston and
the cylinder head. Pressure waves expand quickly and resonate in different
directions. If you graph this process then you have a curve which goes up
steeply and then declines with a couple of bumps (the different cross
waves). If these cross waves become stronger, then you have what is called
knocking. It rattles the engine, produces more wear and makes the whole
process less efficient. The higher the compression in gasoline engines, the
more pronounced it can be. That's why higher octane fuels have inhibitors to
actually slow down the combustion process somewhat and therefore be more
gentle on the engine. To make things more confusing there are different ways
of measuring octane. The regular gasoline grade in the USA has less Octane
than in Canada and its quality seems to vary more. Whenever I cross the
border I upgrade. The fuel quality need also depends on your load and
driving habits. The more you go full load the higher the need for more
Octane because the pressures in the combustion chambers increase.

The detergents are supposed to prevent carbon deposits in the engine. This
is particularly important for fuel injected engines since carbon deposits
can change the spray pattern and clog up injectors. This is  a major problem
for people who drive mainly short distances since uneven temperatures and a
richer mixture cause more condensation. The down side is that everything we
burn ends up in the air with who knows what effects.

Some companies add alcohol. Mohawk advertises "mother nature's fuel" in
Canada because they add ethanol. In Brazil there is a whole network of
alcohol propelled cars. Some racing cars like CART series use alcohol for
mainly safety reasons. Alcohol is water soluble (it actually attracts water)
and therefore fires can also be extinguished with water. Alcohol burns
cleaner but does not quite have the energy of gasoline. Ethanol (one
particular type of alcohol) has been associated with breakdown of Bosch
injection components. So be cautious.

My rule of thumb:
When I am doing daily driving with little load I use regular from a company
using detergents. I also make sure that the engine gets a good long run once
in a while. When I load up the car or go into the mountains or off road, I
upgrade the gasoline. You can actually fill half a tank regular and the
other half premium to upgrade your octane rating somewhat.

Bj�rn Ratjen

Mill Bay, B.C. 

1987 Syncro Est Alia (self camperized)
1995 Passat TD
(1982 Vanagon Diesel)
1979 VW Bus (self- camperized)