In a message dated 97-11-28 , I wrote:
>>As for improved gas mileage, in my simple mind I can't see how putting the
transmission into OD will reduce the energy required to move the car down the
road, and, since an engine is most efficient at its torque peak RPM, the
mechanical energy required to move the car can most efficiently be converted
from the energy content of the gasoline if the engine is operated near that
torque peak, not at a relatively ineffecient 2500 RPM.<<
To which Jerry Causey replied:
>>If you're running down the freeway in an MGB at 2500 RPM in 4th OD, you
probably should be more concerned with how you're going to remove the semi /
lorry which is about to implant itself in the rear end of your 'B. My OD only
drops the revs a few hundred RPM, not a couple of thousand. I'm doing at
least
3000 RPM (60 MPH) at freeway speeds.<<
And to which Christopher G. Moog also replied:
>>Thats at full throttle (highest efficiency at torque peak). At the power
level
needed to cruise down the highway (probably 15 - 20 hp) the most efficient is
usually at lower engine speeds and greater throttle openings as compared to
higher engine speeds and less throttle opening. At the lower engine speeds
there
are less friction losses and more importantly less pumping losses (energy
required to pull the mixture in against the intake manifold vacuum). Also
with
lower engine speeds the small amount of unburnt fuel is a smaller percent of
the
total fuel used.<<
Jerry and Christopher:
By my calculations, 75 mph in a B, with 185 70 R14 tires, is 4100 rpm. In OD,
75 is 3280, a reduction of 820 rpm. (the 2500 rpm I used was the number given
by the original poster of the question).
Given that Jerry is correct, and assuming that Christopher is also correct
(and I believe he is - thanks for the clarification, Chris), Just how much
more efficient can an engine be just by dropping the rpm by 820? How much
difference in throttle position is there? If it takes 15 - 20 hp to move the
car at freeway speeds in fourth, it still takes 15 -20 hp to move the same
car at the same speed in OD. Is there enough difference in efficiency to make
a real difference? How long will it take to pay for the cost of converting to
an OD if you don't already have one, particularly since most of the driving
we do in these cars is off the freeway?
There is no question that an OD makes for a somewhat quieter ride, but other
than that, how much is it realy worth? Isn't it also possible that the
inefficiency of the OD unit itself nearly offsets any small gains?
Inquiring minds want to know. Heck, even my simple mind wants to know!
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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