What/how does compression ratio affect an engine?

Only if they are Ivory and it is Snowing

{;-)

Jim

--
"If you think you can, or think you can't, you're right."
--Henry Ford

wrote in message
...
On Dec 10, 2:32 pm, "RST Engineering" wrote:
Oh, not this old wive's tale again. THERE IS NO ADDITIONAL HEAT
GENERATED
BY A HIGHER OCTANE FUEL.

Jim
----------------------------------------------------------------------------------------------

Gee, Jim... that can't be right. Because if it IS, it means 99 &
44/100% of the 'experts' out there are WRONG...

:-)

I do, and I do.

100 octane valves have a different seat angle and a smaller stem diameter
than 80 octane valves.

Lead is the big problem with 100 octane in a low compression engine. While
the gasoline itself and the combustion process makes not a whit of
difference in the temperature of combustion, the compression ratio does.
Gasoline in a high compression engine burns hotter, be it 80 or 100 octane.
Tetraethyl lead requires a relatively high temperature to be completely
vaporized and exhausted in the combustion process. A high compression
engine does this, while a low compression engine does not. Bromine is
introduced to the mixture in an attempt to "help" the lead "burn", but even
bromine doesn't do much in a low temperature environment.

The different seat angle is an attempt to "get the lead out" of unburned
tetraethyl lead. Even so, the low temperatures encountered in a low
compression engine lets the lead coagulate on the first cool surface it
finds. That happens to be the valve stem. Therefore, the valve stems are
ever so slightly reduced (.005" comes to mind) so that the lead can plate
out on the valve stem and still not cause the stem to stick on the guide so
often. "So often" is the operative term here. 100 octane will cause valve
sticking, but with the 100 octane valves, just not quite as soon.

Jim

--
"If you think you can, or think you can't, you're right."
--Henry Ford

"Scott" wrote in message
.. .

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

Scott

On Dec 11, 6:57 am, "RST Engineering" wrote:

The different seat angle is an attempt to "get the lead out" of unburned
tetraethyl lead. Even so, the low temperatures encountered in a low
compression engine lets the lead coagulate on the first cool surface it
finds. That happens to be the valve stem. Therefore, the valve stems are
ever so slightly reduced (.005" comes to mind) so that the lead can plate
out on the valve stem and still not cause the stem to stick on the guide so
often. "So often" is the operative term here. 100 octane will cause valve
sticking, but with the 100 octane valves, just not quite as soon.

Jim

That lines up with what we used to experience with the small
Continentals. The valves tended to stick if the stem/guide clearances
were set at the minimum spec. The other problem with many small
engines is their mixtu the Stromberg carb either had no mixture
control, or it was safety-wired at full rich. So the engine gets too
much fuel, which was OK when 80/87 was available, but it's not OK with
100LL which has four times the TEL spec. My A-65 runs much leaner and
the valves don't get fouled up.

Dan

The actual truth of the matter is that 80/87 was allowed a MAXIMUM of 0.5
grams of TEL in a gallon, but in practice after the 1940s, no TEL was needed
to make spec. Since TEL is a hell of a lot more expensive than gasoline,
there was virtually no TEL in 80 from the 1950s on.

100LL on the other hand needs every bit of the allowable 2 grams/gallon to
meet spec, so in fact there is an huge increase in the lead content.

Jim

much fuel, which was OK when 80/87 was available, but it's not OK with
100LL which has four times the TEL spec.

"RST Engineering" wrote:
Oh, not this old wive's tale again. THERE IS NO ADDITIONAL HEAT
GENERATED BY A HIGHER OCTANE FUEL.

My understanding is that you are correct, yet not fully correct. At the end
of the combustion phase of the cycle there is no significant difference in
the amount of heat released between a high octane and low octane fuel.

BUT - one is more likely to burn unevenly and/or more quickly (i.e.
"knocking" or "pinging") which leads either to unplanned overpressures or
localized hot-spots (i.e. _high heat concentrations_). So sure - no _final_
difference in released energy, but time and space concentrations can result
in metal melting or breaking in one case and not the other.

I suppose one analogy would be to consider the difference between what a
bullet does to the human body versus what eating a large meal does. The
bullet may have about as much energy as the large meal (probably even less
than the meal), but the bullet will do a lot more damage to you than the
meal. ;-) It's all in how quickly the energy is released and the manner of
the release.

"Jim Logajan" wrote

BUT - one is more likely to burn unevenly and/or more quickly (i.e.
"knocking" or "pinging") which leads either to unplanned overpressures or
localized hot-spots (i.e. _high heat concentrations_). So sure - no
_final_
difference in released energy, but time and space concentrations can
result
in metal melting or breaking in one case and not the other.

So, in your example, the high octane would be the cooler burning fuel,
because it burns more evenly, and slowly, so it prevents knocking.

Other scientific data showes that the higher octane fuel, the lower the /btu
content. That's fact. Not a lot of difference, but a difference, none the
less.

I suppose one analogy would be to consider the difference between what a
bullet does to the human body versus what eating a large meal does. The
bullet may have about as much energy as the large meal (probably even less
than the meal), but the bullet will do a lot more damage to you than the
meal. ;-) It's all in how quickly the energy is released and the manner of
the release.

I don't see your analogy having anything to do with the discussion of octane
and valve differences.
--
Jim in NC

"Morgans" wrote:
"Jim Logajan" wrote
I suppose one analogy would be to consider the difference between
what a bullet does to the human body versus what eating a large meal
does. The bullet may have about as much energy as the large meal
(probably even less than the meal), but the bullet will do a lot more
damage to you than the meal. ;-) It's all in how quickly the energy
is released and the manner of the release.

I don't see your analogy having anything to do with the discussion of
octane and valve differences.

It wasn't intended to address that specific aspect. It was an analogy,
maybe a lousy one, but it's not like I get paid to do them!

:-)

"Matt Whiting" wrote in message
...
... Some believe that higher octane gasoline burns more slowly and thus
can
put more fire past the exhaust valve. However, most of my reading on the
subject suggests that this is a myth and there is no substantial
difference in burn rate as a function of octane.

Matt

Burn rates are essentially the same.

There are problems (in automobiles) with some premium auto fuels since the
changes in the chemistry to get higher octane can result in slower
evaporation which can cause an engine to stumble when you first step on the
gas.

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

Jim Logajan wrote:
"Morgans" wrote:
"Jim Logajan" wrote
I suppose one analogy would be to consider the difference between
what a bullet does to the human body versus what eating a large meal
does. The bullet may have about as much energy as the large meal
(probably even less than the meal), but the bullet will do a lot
more damage to you than the meal. ;-) It's all in how quickly the
energy is released and the manner of the release.

I don't see your analogy having anything to do with the discussion of
octane and valve differences.

It wasn't intended to address that specific aspect. It was an analogy,
maybe a lousy one, but it's not like I get paid to do them!

:-)

Here's one, maybe it depends on which end the meal made gas depart the
body?

"Morgans" wrote:
"Jim Logajan" wrote

BUT - one is more likely to burn unevenly and/or more quickly (i.e.
"knocking" or "pinging") which leads either to unplanned
overpressures or localized hot-spots (i.e. _high heat
concentrations_). So sure - no _final_
difference in released energy, but time and space concentrations can
result
in metal melting or breaking in one case and not the other.

So, in your example, the high octane would be the cooler burning fuel,
because it burns more evenly, and slowly, so it prevents knocking.

Except that it appears what I wrote is not correct. :-(

The octane rating appears to be a measure of the activation energy of
combustion, not a measure of the combustion rate.