Re: race fuel question

SimRacer <NOsimracer68@xxxxxxxxxxxxx> wrote in
news:mnpd63178650alvph25mfptm0kprmkp944@xxxxxxx:

I admit to being a dummy when it comes to octane and its environmental
impact or its bearing on engine durability. I can put a basic
carburated engine together from scratch, but never got into the
science of octane. Engine oil and LocTite? Yes, but not the gas. lol I
just put the gas in that they say is legal to run, and go with that...

Short form: you burn the fuel in a specially designed single cylinder
engine whose compression can be varied, and see how high you can run
the compression without detonation. Compare that with the compression
you get with 100% octane fuel, and the ratio gives you the octane
rating of the fuel (there's several slightly different ways to do the
test, the "motor" method (MON) and the "research" method (RON) being
the most common. The average of those two gives the "pump octane",
the number you see on the pump.)

Raw gas (which is mostly pentane and heptane) has an octane rating
around 70, I think. Adding various goop (tetraethyl lead, MBTE,
ethanol, etc) raises the octane rating. Add enough of the right
stuff, and the fuel can support higher compression than pure octane
- hence things like 110 octane race fuel, or 120 octane av-gas.
Some of these additives are toxic, and some are absorbed by the skin.
(Ethanol, by comparision, is relatively un-toxic, hence the push to
replace MBTE with it a couple of years back).

In general a higher octane fuel has _less_ energy content and burns
more slowly than a lower octane, neither of which are particularly
desirable characteristics. The higher compression you can run with
a higher octane fuel more than compensates, however, resulting in
more power. (note that if you have an engine designed for lower
octane fuel, putting in high octane at best wastes money and at
worst reduces, not increases, the power output).

John
.

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