"Webre, Steven P." wrote:

Hey Russell,

So I'd say that Steve's 123,299 BTU/gal figure for gasoline is a bit on

the high

side, while the 118,950 BTU/gal for Diesel is too low.

Likely guilty as charged.  I pulled the figures out of just one source (an
older engineering handbook).

Steve continues:

Higher octane numbers have NOTHING to do with the energy content.

Chevron disagrees, but the difference is only 1.75%.  By contrast,

oxygenating

gasoline reduces the energy per gallon by 3.5%.

Yep.  If I put my engineer's hat back on, you're completely right.  In fact,
if you talk with the refiners, they will point to minute differences between
fuel batches depending on the base stock and how their system is running
that day.  Thus the ranges you found.  However, outside of the engineer's
world, these differences are insignificant except in the most demanding
applications.  I think we're starting to split hairs here... :)

So for peak performance of your airplane when running mogas, get the

non-oxygenated premium gasoline if you can.

For certified aircraft, even with mogas STC, oxygenated fuel is usually a
"no-no".   Modern oxygenated fuels can play havoc w/ older fuel lines and
some carb internals.  Most mogas STCs specifically forbid it.

If  you need premium or not goes back to the engine and set-up.   My
Continental 145 hp (C-145 w/ 7:1 compression) is quite happy on 87 octane
mogas.  It was originally designed when 80 octane avgas was common.

I sincerely contend that for our use (outside of a lab), we can't tell the
difference in BTU content between gasolines of the same octane.  Differences
are too small and other variables too many to make this any more than a good
talking point over "adult" beverages.

Fortunately for the gas station owners, most folks don't believe that

and run

the higher priced gas thinking they are doing good things for their car.

Um, some car's owner's manuals actually call out mid-grade or premium

grade gasoline.

You're right, a few do.  I wouldn't fill up a Viper or a Turbo 911 w/ 87
octane from the corner store if I was going to run hard (who wouldn't?).
However, my Suburban and wife's minivan don't care as long as it's "fresh"
(another story) 87 octane or higher.

The comment was a observation about the majority of our driving public who
mistakenly believe that they will get real,  tangible, performance benefits
by feeding their car premium gas.  The marketers have done well in that
respect.  However, for the average American car, there is no difference.
Too many other variables.

Steve Webre
Broussard, LA

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