Andy,
I agree with your thought that these cars were designed for high revs
per mile, but think that was not very important at the time they were
built.
When they were built, less than half the roads in England were paved,
and a fast day's motoring meant travelling at an average of about 40
mph. Autombiles were generally geared so that the most common highway
hills could be climbed in top gear, in the USA, on the Continent and the
UK. This hill climbing ability, and the engineering practice that called
for the horsepower peak to occur at the point where the very great wind
resistance to the car limited the speed to the developed horsepower and
lead to this gearing that we find uncomfortable today.
As the MkII cars and the TFs had 3 or 6 more hp (depending on the
source of the information) they had the 4.875 gears instead of the 5.125
gear ratio that was carried over from the TC with its larger wheels. The
TDs, then, suffer from this high engine speed per mile/lower cruising
speed problem more than TCs.
What I am trying to get around to noting is that the high revs/mile
were a result of road conditions and existing tooling rather than an
intention to have high revs. When the cars were built, multi grade
detergent oils were just being developed, grease intervals were 500
miles, and it was expected that the dampers would be removed and tested
every 12,000 miles. It didn't really make for a problem, though, because
no one drove MGs that many miles in the course of daily life, and they
didn't drive long distances at high speeds. While the engines will
happily rev up and run at 4200 all day long, they just won't do it for as
many days as one would like.
Cedarstrand notes the tendancy of the engines to crack their bearings
at an early age when run at US speeds of the early 70s. Our 55 mph speed
limit was a good thing for our TDs.
I changed the gears in my MkII to 4.1, running on 5.60x15 tires. Next
time I buy tires, it will be 165x15 radials, with a smaller rolling
circumference, as I think the bias plys give an overall gearing that's
just a touch too high. There is bound to be some degradation of
acceleration, but I haven't had a chance to drag race a standard car so
don't really know how great it is. Second and third gears are much more
useable than before.
I thought about the Datsun gearbox, but figured that the gear change
was more easily reversible. We live in coastal CT where it seems that
any place we want to go almost always requires turnpike driving. Were we
in another area, I might not have changed the gearing. On the back roads,
I do find it necessary to use 3rd gear to climb some hills that could
formerly be done in 4th, but I think (without doing the math to be
certain) that the car is running up the hills at about the same road
speed.
My concern was, actually, not so much the wear on the engine from high
rev operation but the fear of being run over by drivers who think that
the current 65 limit is the speed at which one still has a gear to shift
up into, and that more than five feet between them and the MG ahead is
too much space.
I had not heard about the OD idea. Do you think there is enough space
at the rear axle? I don't think there is any extra room at the gearbox
for it, judging from the size of the unit in my MGB. If you see any more
information on it, please forward.
Bob
On Fri, 08 Jan 1999 14:10:23 -0800 Andrew Moyce <amoyce@pol.net> writes:
>I have followed the discussions about overdrives and 5-speeds with
>interest.
>My TD has a TF (1:4.875) rear end which matches nicely with the
>rebuilt
>and ported engine. I feel the car is at peace with itself until I get
>up
>above 50 mph, where I find myself reaching for a 5th gear.
>The standard solution is to "upgrade" to a 4.55 or 4.3 MGA rear end .
>.
>. not an uncomplicated or inexpensive proposition and with some
>compromise in low gear performance (the exent of the compromise varies
>in description from mild to significant.)
>Another solution is to change to a Datsun B-210 5-speed transmission,
>which I assume has an overdrive 5th gear. Again not inexpensive (about
>$1800) and with some philosophical drawbacks as to originality and
>honoring the Mark and all that.
>There is an outfit that makes overdrive units that are designed for
>interposing into a split driveshaft in any vehicle . . . cost about
>$1500 and concerns over whether there is room along the T car's drive
>train to accomodate the unit.
>There were some postings on the UK web site a few months back about an
>Austin Healy overdrive unit modified to fit a TD, but it hadn't been
>installed yet.
>Somewhere out there there has to be an add-on overdrive unit that we
>could adapt to the T cars . . . possibly mounted at the rear of the
>drive shaft under the storage area.
>Until I find such a thing, I am inclined to stay where I am, enjoying
>the low gear performance. I take comfort in the DuBois' experience
>with
>high rev motoring. I think that's how these cars were designed.
>
>Andy Moyce
>Oakland, CA
>52 TD
>
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