On Tue, 11 Dec 2007 03:40:42 +0000, Scott
wrote:

OK, this is where I probably made my wrong assumption (even though I
precluded my original post with the word "might" when referring to more
heat from 100 octane). So it's not heat produced strictly because of
higher octane, it's just that the mixture may still be burning as the
exhaust valves open on the exhaust stroke and "burns" the valves. So,
it may not burn hotter, but it seems to burn longer, thus heating the
valves more, so it's kind of semantics since the valve is still getting
hotter by burning 100LL rather than 80 (but for a different reason than
I originally posted and ****ed everyone off by regurgitating an old
wive's tale). Is this correct?

Scott

Not necessarily.
By definition high octane fuel does NOT burn slower than regular gas.
Burning FASTER would be more productive in reducing detonation.
Counterintuitive, but DEFINITELY true. The faster the burn, the less
heart absorbed by the "end gasses" in the chamber, and the less chance
they will "dissassociate" and destabilize - which IS what causes
detonation. Gasoline BURNS. Hydrogen radicals EXPLODE.
To prevent detonation you need to burn cooler, burn faster, or the
fuel needs tobe more resistant to thermal dissassosiation .

This is why detonation is GENERALLY only a problem at lower RPMs. You
can produce X HP at 2500 RPM and get detonation, and 1.5X at 3200 RPM,
for instance, with no detonation. - with the same or higher cyl
pressures at the higher speed.



Big John wrote:

On Tue, 11 Dec 2007 01:55:38 +0000, Scott
wrote:


FOUL!! 100LL has 4 times the lead content than the old 80 avgas and a
LOT more than unleaded auto! Therefore, the 100 octane valves should be
supper lubricated! But, in fact, the 80 octane valve tend to stick if
much 100LL is run through them...

So, somebody answer my question...what is special about 100 octane
valves? The ones I put in my A-65 were Stellite. What does Stellite
do? What about sodium filled valves? What's their claim to fame??

Scott


Matt Whiting wrote:

Scott wrote:


Like I said, MIGHT OK, if there is no difference in heat, what are
the 100 octane valves in my A-65 for? Or more generally, why do they
sell 100 octane valves for A-65s and C-85s, etc. that were certified
on 80 octane??? Like I said, I use them, I don't wrench on 'em


Because valves that were designed to be lubricated by the lead in leaded
fuel may not last long when using fuel with no or less lead.

Matt



Scott

High octane fuel burns slower in low compression engines. This lets
the flame front go out the exhaust valves and burn them if you run a
lot of hours.

We ran into this in the 'old' days when the fighters used 115-145
octane fuel. If we used that fuel in our cars we had to mix some oil
with it to prevent the burnt valves.Occasionally someone would burn a
tank full to clean their engine out (burn carbon off cylinders and
valves, etc)

This is probably the reason they put 100 octane valves in your low
compression engine so you could use 100 octane and not destroy the
valves?

Big John


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