Russell Kent wrote:

Steve Webre wrote:

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

Russell Kent replied:

Chevron disagrees, but the difference is only 1.75%.  By contrast, oxygenating
gasoline reduces the energy per gallon by 3.5%.

Steve replied:

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... :)

Sorry, guess I wasn't clear enough: I was trying to connote exactly that ("we're
... splitting hairs") with my comment about the difference being 1.75% and that
oxygenation has a *greater* (laboratory) effect and we don't even notice it.

Russell also wrote:

So for peak performance of your airplane when running mogas, get the
non-oxygenated premium gasoline if you can.

Steve replied:

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.

I don't think so.  I believe the STCs prohibit mogas oxygenated with *ethanol*.
Mogas oxygenated with MTBE or ETBE isn't a problem; see:
http://autofuelstc.com/pa/faq.html

Steve continues:

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.

Hey, I did preface my claim with "for peak performance".  Translation: "if you're
Bruce Bohanan-like". :-)

I think we are agreeing on the principals and just differ on some of the technical
nitty-gritty.

Russell Kent

 
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