Bob and fellow listers,
Bob's first point of fuel use being directly proportional to air use is
true especially with our SU equipped LBC's which have no way to lean out
the mixture at low loads. However, air use is more a function of power
ouput than rpm. At high rpm and low power, there is very little pressure
in the intake manifold (high vacuum) so not much air is forced into the
piston on the downstroke. However, because the pressure is lower, it also
takes more work to suck that little bit of air into the engine and that
increases pumping losses. In an "ideal" engine (adiabatic, perfect mixing,
perfect valve timing, no friction and no pumping losses) rpm would be
independant of air use and air use would be the only variable in
calculating output.
On his next point, Bob caught me on a brain fart. I meant to say that peak
power decreases as rpm decreases so running an engine at lower rpm for the
same power output means running it closer to 100% power which means a
higher intake pressure (lower vacuum), and thus lower pumping losses.
Also, the SU constant depression action only effects the pressure between
the jet bridge and the butterfly, not manifold pressure (vacuum).
>>> Robert Allen <boballen@sky.net> 12/29/97 01:19pm >>>
Bill Eastman wrote:
>
> <snip> Still, for a constant power need, an engine turning at a lower
> rpm will usually use less fuel than one at a higher rpm.
I've thought this was true on the basis that an engine is an air pump.
Some gas is mixed with the air. The more air (and gas) going through it,
the more gas is going to be burned. All of this assumes that the current
load on the engine at a given rpm is somewhat less than what the engine
is producing -- that is, not near full throttle.
Thus, all things being equal, lower RPMs ingests less air and gasoline
-- better fuel mileage.
> If you assume that you are below the peak torque rpm for the engine,
> then maximum power output decreases with rpm so, for a given power need,
> you would be closer to the maximum power potential at a lower rpm than at
a
> higher rpm.
Bill has been in the egg nog again. "When below peak torque ... power
output decreases with RPM." I've seen lots of 'A's driven with this
attitude but I think power will continue to build until peak torque is
attained and then it will taper off.
> This === vacuum ===
> means that the manifold pressure would be higher at the low rpm power
> setting.
I think the beloved SUs screw this all up with their "constant
depression" preference. In conventional venturi carbs, vacuum has more
to do with throttle plate position, cam, and RPM.
> Someone asked about tuning an engine to maximize the vacuum signal at the
> PVC port. If my memory serves me correctly, this port opens between the
> throttle butterfly and the dashpot slide. If your carb is working
> correctly, this area should always be at the same vacuum reading since
the
> job of the slide is to maintain a constant depression across the metering
> jet. You would see some fluctuation during throttle position changes due
> to the dashpot oil but at constant load and speed this signal wouldn't
tell
> you much.
Actually, printed right on the face of my vacuum gauge, is a chart that
says if vacuum is this low, you probably don't have enough advance. So I
tried this out on the boat (it would be sorta tough in a car). I had the
boat running at a constant speed in the upward 2/3's of it's power band
and adjusted the timing for maximum vacuum. Worked great: good power,
better throttle response, but the boat wouldn't idle. Oh well, not
enough mechanical advance and no vacuum advance on the boat's dizzy.
Interesting test on a sunny afternoon, however.
The gauge says:
0 - 10 Inches Hg: Late valve timing
14 - 18 Inches Hg: Late Ignition timing
18 - 22 Inches Hg: Normal
I don't imagine a rocket scientist calibrated the gauge but it is food
for thought.
Bob Allen, Kansas City, 69CGT, 75TR6, 60Elva
"The state of Texas leads the nation in executions with 37 this year.
Gotta love them Cowboys."
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