Fuel Prices For The July Pilgrimage

"Jay Honeck" wrote

I know you probably realize this, but your observation makes no
sense. There is no difference in "power" between avgas and mogas.


Actually, there are slightly more BTU's in a gallon of mogas than in a
gallon of avgas.

If an engine was to have its fixed advanced spark too far forward, the lower
octane mogas could produce less power. That is the only way I could think
of, other than a high compression engine that needed the higher octane to
keep from knocking.
--
Jim in NC

Morgans wrote:
"Jay Honeck" wrote

I know you probably realize this, but your observation makes no
sense. There is no difference in "power" between avgas and mogas.


Actually, there are slightly more BTU's in a gallon of mogas than in a
gallon of avgas.

If an engine was to have its fixed advanced spark too far forward, the lower
octane mogas could produce less power.

How so?

Matt

"Matt Whiting" wrote

How so?

Whem I was much younger, an all cars had points and condensers, I would do
my own tune-ups.

Sometimes the timing would need to be adjusted. How to do it, if you did
not have a timing light handy?

I would loosen the distributor bolt, until it was tight enough to move by
hand, but not so loose that it would not vibrate around. After that, I
would get the engine warmed up, and take it out on the road, and punch it.
It was not so obvious if the timing was retarded a bit, until it was really
retarded. It was _quite_ obvious when it was too far advanced, as would not
have much power, and would knock like crazy.

That a good enough answer?
--
Jim in NC

Morgans wrote:
"Matt Whiting" wrote

How so?

Whem I was much younger, an all cars had points and condensers, I would do
my own tune-ups.

Sometimes the timing would need to be adjusted. How to do it, if you did
not have a timing light handy?

I would loosen the distributor bolt, until it was tight enough to move by
hand, but not so loose that it would not vibrate around. After that, I
would get the engine warmed up, and take it out on the road, and punch it.
It was not so obvious if the timing was retarded a bit, until it was really
retarded. It was _quite_ obvious when it was too far advanced, as would not
have much power, and would knock like crazy.

That a good enough answer?

Maybe to another question. You didn't address why having the timing
advanced too far would cause a greater power loss with mogas than with
avgas. And noise from igniting the mixture too soon and detonation are
two different phenomenon.

Matt

"Matt Whiting" wrote

Maybe to another question. You didn't address why having the timing
advanced too far would cause a greater power loss with mogas than with
avgas. And noise from igniting the mixture too soon and detonation are
two different phenomenon.

Sorry, I'll try again.

The higher octane of 100LL will allow more advanced spark settings without
knock and detonation, than will the lower octane mogas.

End result would be that the mogas could show a loss of power, when avgas is
still allowing the engine to function normally.

I don't disagree with the fact that in normal conditions, and a normal
engine, the power of an engine running 100LL or mogas should be virtually
the same.
--
Jim in NC

Morgans wrote:
"Matt Whiting" wrote

Maybe to another question. You didn't address why having the timing
advanced too far would cause a greater power loss with mogas than with
avgas. And noise from igniting the mixture too soon and detonation are
two different phenomenon.

Sorry, I'll try again.

The higher octane of 100LL will allow more advanced spark settings without
knock and detonation, than will the lower octane mogas.

End result would be that the mogas could show a loss of power, when avgas is
still allowing the engine to function normally.


This is only true for engines that have very advanced ignition timing,
which isn't true for most GA piston engines, or engines with very high
compression ratios, again, not true for most GA piston engines. Keep in
mind that most were designed to run on 80 octane avgas...


I don't disagree with the fact that in normal conditions, and a normal
engine, the power of an engine running 100LL or mogas should be virtually
the same.

Yes, I can't imagine many GA piston engines where this wouldn't be the
case. Maybe a turbocharged engine running very hot.

Matt

Matt Whiting wrote:



This is only true for engines that have very advanced ignition timing,
which isn't true for most GA piston engines, or engines with very high
compression ratios, again, not true for most GA piston engines. Keep in
mind that most were designed to run on 80 octane avgas...


I don't disagree with the fact that in normal conditions, and a normal
engine, the power of an engine running 100LL or mogas should be
virtually the same.


Yes, I can't imagine many GA piston engines where this wouldn't be the
case. Maybe a turbocharged engine running very hot.

Matt

There are plenty of high compression GA piston engines that can't use
mogas because the octane requirement is too high. The majority of GA
engines are lower octane (something like 78% of the engines IIRC). The
majority of the hours flown in piston aircraft however is done by the
21% of aircraft that have higher octane requirements than mogas can
offer, my 260 HP O-540-E4B5 included. It is a normally aspirated 6
cylinder engine, but with high enough compression that there is not
sufficient detonation margin with less than 96 octane fuel.