In a message dated 97-12-26 10:40:08 EST, william.eastman@medtronic.com
writes:
> A while back there was a discussion about when our LBC engines deliver the
> best fuel economy and whether OD improves said economy. I believe that
> this discussion was triggered by Dan Master's continued attempts to make a
> decent driver out of a Tri**ph. Its a good thing the MG's are fun to drive
> as is!
Bill:
There is nothing wrong with any of our LBCs that a little more horsepower
can't improve, not even in a Tri**ph, which has a leg up on an MG in this area
already! (I can say this without bias, as I own both MGBs and Tri**phs, and
have lusted after MGAs on more than one occasion)
> As most of you know, I was trained "a few" years back in internal
> combustion engine theory (the text was written in WWII back when an engine
> was an engine). You probably also remember that I have not looked at these
> text books for "a few" years so, if this info is less than perfect, blame
> the lapses on the unreliable firing of my Lucas OF synapses.
You got me there - my OF synapses will have to fire without benefit of
training.
>> A lot of good stuff snipped for the sake of brevity <<
> Bsfc is, in general, less sensitive to
> throttle position than bmep so bsfc peaks at a lower rpm than bmep since
> this reduces parasitic losses- friction in particular. So reducing rpm at
> cruise is usually a good idea for fuel economy.
If I were to rewrite that last sentence, I would add "all else being equal."
But, as we know, all else seldom is equal. Going back to my original position,
the horsepower required to be delivered to the rear wheels under any given
condition remains constant, regardless of anything going on with the engine or
the drivetrain. The only way economy can be improved is for the engine to
provide that constant horsepower TO THE REAR WHEELS while consuming smaller
quantities of fuel in the process.
For the sake of argument, I will concede (although I am not yet convinced)
that reducing rpm will enable the engine to consume less fuel while delivering
a CONSTANT horsepower AT THE FLYWHEEL (my point of contention is contained in
the term "CONSTANT horsepower" We can discuss this later.)! There is, however,
a lot going on between the flywheel and the rear wheels. The world reknowned
authorities at Currie Enterprises in Anaheim, CA, stated recently that the 9"
Ford axle requires at least 5 horsepower MORE just to turn over than the newer
design 8.8" Ford rear axle. Given that the differences in these two
differential assemblies is in the details, and not in the design concepts, one
wonders just how much horespower is lost all together, and how much more
horespower is lost to spin the whily-gigs and spinner-ma-bobs in an OD unit,
to say nothing of the horsepower consumed in driving the hydraulic pump
required to keep it engaged!
Using just 5 horsepower as a conservative estimate, when the engine rpm drops
from the engagement of the OD unit, the engine will have to produce 5 MORE
flywheel horsepower with the SAME FUEL CONSUMPTION just to break even! So, in
order to improve economy, the rpm must drop by 500 rpm or more, flywheel
horsepower must INCREASE by at least 5 HP, and smaller quantities of fuel must
be consumed in the process! Neat trick!
While admitting that I am in unfamiliar territory (I'm more at home with
electrons and wires), I still have a hard time accepting the claims for better
fuel economy from an OD unit. Even if I'm wrong (and it wouldn't be the first
time - not by a long shot), I still don't think the gains will justify the
cost and effort involved in making the swap. I will also admit that I wouldn't
mind having an OD, but I would be happier with a 5-speed.
In a car with only 95 horsepower to begin with, the usability of an OD unit
would be much less here in East Tennessee than somewhere in the flatlands
anyway. If I had, say, 300 horsepower (wink, wink, nudge, nudge), I might be
able to get more use from an OD or a fifth speed while careening up and down
the hills and through the curves in our mountains here.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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