On Wed, 27 Jul 1994, Bob Hamilton wrote:
> A member of our British Automobile Touring Assn. would like
> information on lowering the differential gear ratio on his '51 TD. His manual
> states a ratio of 8/41 which sounds unusually high if it is in fact a ratio
> at all. He is presently turning 4000 rpm @ 60 mph.
>
> Has anyone in this group made this conversion? Moss Motors suggested
> he use an MGA rear axle :-&
That "8/41" is a ratio, but not expressed the way most of us are used to.
It means the pinion gear has 8 teeth and the crown wheel (ring gear) has
41 teeth. If you divide 41 by 8, you get 5.1255, which means the ratio
expressed the way we're used to is 5.125:1. That's the standard ratio for
the TD, and with it a TD does turn about 4000 rpm at 60 mph. That's
pretty high engine speed by modern standards, but it was chosen to get
sprightly (well, semi-sprightly) performance out of an engine that
produced only 54 hp.
The TD Mk II and TF, which had slightly more horsepower (all of 57!) used
an 8/39 (4.875:1) ratio. This is a good choice for the standard TD too,
if you can get the gears. It lowers rpms a bit without hurting
acceleration too much.
The 4.8755:1 ratio was available for the MGA as well, and although the
gears are not the same they can be installed with some fiddling (and the
services of a _good_ machine shop). Higher MGA ratios have been used as
well, to bring the revs down even more. The thing to remember, though,
is that the taller the gearing, the more acelleration will be hurt.
I've driven TDs with tall MGA gearing, and quite frankly they reminded me
of Model A Fords. They might be okay if liesurely long distance touring is
your thing, but in my totally unbiased opinion (grin) the result is not a
sports car. Personally, I wouldn't go any higher than 4.8755:1.
If your friend wants to go ahead with the MGA conversion, there are two
ways to go about it. He can install a complete MGA rear axle, or he can
install MGA gears in the TD casing. The complete axle is easier, but
looks obviously unoriginal (if that matters to him). The gears-only
conversion is harder, but leaves the axle looking original. Moss motors
used to sell a kit comprised of MGA gears (your choice of ratio),
miscellaneous special parts, and detailed instructions. I don't know if
they still have that kit, but they should at least be able to provide the
instructions.
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Chip Old 1948 M.G. TC TC6710 NEMGTR #2271
feold@umd5.umd.edu 1962 Triumph TR4 CT3154LO (daily transportation)
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