Hi Dan,
Your arguments all make sense to me, especially the ones about spinning
additional gears in the OD. I think that the pump pumps constantly, so
would not think that it alone would amount to much resistance when OD is
engaged.
However, it's been my experience in the B-GT that the OD is worth 2-3
mpg improvement during highway driving. Is that worth the bother of
making the OD swap? Well, @ 3mpg @ 1.39 per gallon, maybe in a hundred
years or so, but the OD does make for a more tranquil ride. Since our
highway driving in CT is on Rts 95, 91 and 84, anything that can increase
peace of mind is worthwhile.
Around town, I rarely use it. It's neat to flick on 3OD instead of
shifting to 4th, but forgetting to flick back to 3N before a stopsign
results in an unexpected lurch when the OD engages itself again in 3rd at
about 25 mph. I have an indicator light, but it's on the console in a
hole that was there, so it's just outside one's peripheral vision.
Bob
On Sat, 27 Dec 1997 22:42:42 EST DANMAS <DANMAS@aol.com> writes:
>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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