Mogas: what happens to EGTs?

"Newps" wrote in message
news:1UQ2c.82422$PR3.1197113@attbi_s03...


Jim Weir wrote:

-
-1 What is it going to show happened to the EGTs when you switched
-tanks?

The EGT should show a very slight increase. The higher the octane, the
smoother
and cooler the burn.

I don't think I agree with this last sentence. First of all the burn may
be just as smooth with a lower octane with a low compression engine. High
octane fuel like 100LL has no benefits for an A-65 Continental engine with
6.3:1 compression ratio and loves the 80 octane gasoline it was designed
for.

And if the burn is cooler when 100LL (rather than 80) is run through the
carb, why is it the engine must have special 100LL exhaust valves, which are
designed with high-temperature alloys?


You have gone from 100 octane to (arguably) 87 or 93
octane and the car gas should burn hotter.

Are not the auto and aviation octane scales different?


I don't know the answer but I tell you what I suspect. I suspect the egt
of 100LL will be higher. Think about it and correct me if I'm wrong. The
low compression Continentals when they are rebuilt -- the A-65's, O-200's,
and O-300's --- have optional 100-octane exhaust valves available from the
parts supply houses like Fresno Airparts. Now WHY is that, other than
because the mix is still burning when the exhaust valve opens? My
suspicion. You (anyone) tell me, with an intelligent explanation, if I'm
wrong.

Higher octane gasoline like 100LL, furthermore, is a little less volatile
than mogas and burns more slowly because of the higher RON numbers, so that
when the exhaust valve opens combustion is less complete than with the same
charge of mogas -- in the low-compression O-300. So if you're running
100LL in an O-300-powered 172, why do you need 100-octane exhaust valves,
whose faces are something like inconel or another exotic alloy designed for
temperatures much higher than the stock exhaust valves?