At 02:07 PM 10/11/2000 -0700, Bullwinkle wrote:
>....
>Did you look at the power curve yet at
http://www.cdsorkin.com/spridgetintro.htm or
http://www.mgb-stuff.org.uk/carbs.htm#power ?
Yes?
>Do you know what those graphs represent and how the data was gathered?
Okay, I'll bite. Of course the data represents wide open throttle
performance at all points on the graph.
>....
Now, here's the teacher coming into play. Write down what you think is
going on. .... Nobody's going to know but you. Don't hide ....
Hide hell, I'm standing up waving. What's the point?
>Many of you own modern FI cars with automatics equipped with OD. These
cars have a fuel management system, .... etc.
>
>One day .... You've got your automatic transmissioned vehicle with OD so
loaded up that you've got the throttle practically to the floor to keep it
going at your cruising speed. Under what conditions will the the automatic
transmission shift out of OD?
Well, from practical experience, I was doing exactly this a few days ago
while using the Chevy (small V6 and automatic) to tow a car on a tow dolly.
There would never be any downshift unless I mashed the throttle nearly
flat to the floor, or unless the engine speed dropped to about a fast idle,
at about the same speed where it would shift out of overdrive with or
without the heavy load. As it was, the engine was only doing 2000 RPM at
70 MPH, so this example has nothing to do with the peak of the torque or
power curve.
Now to answer your question, it should happen under either of two conditions:
a.) When the load exceeds the engine torque capability at the current
engine speed (losing speed at full throttle) and the downshift engine speed
would not exceed the upper shift point for the lower gear. This is
downshift by power demand. According to these MG power curves this should
be done at about 4500 rpm, assuming the power requirement is such that you
have the throttle mashed to the floor at the time. This also applies at
any slower engine speed, where if you mash the throttle you should get (or
want) instant downshift.
b.) When the fuel consumption curve starts to rise significantly as the
engine speed drops. This is downshift by fuel demand, but it ain't going
to happen at full throttle, as condition "a" would occur first. With a
light throttle setting and decreasing engine speed this should happen
around 2000-2500 RPM for the MGA (maybe) and about 2500-3000 RPM for the
MGB (also maybe), but you can't really tell from these curves, because they
don't show the fuel consumption at light throttle setings.
>From experience, my gut feeling is that at light throttle settings the fuel
curve is not only much lower, but also a little farther to the left.
During the Alaska trip I was on a couple of occasions driving my MGA on a
gravel road at about 45 MPH, about 2600 RPM (and occasionally a little
slower) in steady state for hundreds of miles at a time. Fuel economy was
right at 30 to 31 MPG, very close to the maximum economy achievable with
this vehicle under any conditions (about the same as steady 59 MPH on a
smooth road), and I'm sure that a downshift to increase engine speed would
only increase fuel consumption through increased "pumping loss". A change
of gear ratio to reduce engine speed would almost assureadly reduce fuel
consumption (but maybe not a large difference).
For light load conditions I believe you will always get the best fuel
economy at the slowest possible engine speed consistent with not lugging
the engine. Lugging the engine at slow speed is in effect operating at
near full throttle condition, which puts the circumstances back into
condition "a". The reason for this effect is that the engine works like a
big air pump and dumps a substantial amount of hot compressed gas out the
exhaust (wasted energy), and this pumping effect (energy loss) will
increase with increasing engine speed.
Did I get it right, teach?
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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