Hi Art, In my opinion improving the gearing on any pre- 1962 MG is
worthwhile. These cars were built to perform well on English secondary
roads where acceleration was more important than a high cruising speed.
The introduction of superhighways and our much longer driving distances
has made a higher ratio rear axle gearing very desireable. However, keep
in mind that a stock TC overall gearing is much better than a TD or TF due
to the larger wheel/tire diameter of the TC. Speed in MPH is 15.64 per
1000 engine RPM in top gear, so 4,000 RPM gives you 62.56 MPH, not bad.
Changing to a 4.875 ratio gives 16.43/1000 RPM or 65.72 MPH at 4,000 RPM.
Changing to a 4.55 ratio gives 17.65/1,000 RPM or 70.6 MPH, possibly too
much for a stock engine.
You can calculate your own figures using the formula: MPH = RPM x Tire OD
in inches divided by Gear ratio x 336.
Of course any change in axle gear ratio will affect the speedometer
reading,
for a single ratio change down, I don't think its worthwhile to alter the
speedo
as it was probably optimist anyway.
Would I change a a stock TC axle ratio just for the sake of better gearing
when driving 2,000-4,000 miles a year? Probably not, unless I had to buy
new gears anyway.
So here are my comments on the question of how to improve the gear ratio:
The MGA gears and Carl C. book are applicable ONLY to the TD/TF rear axle .
No use for TC except for general discussion unless you are thinking of
changing the whole axle unit ( see below ). I have listed some of your
options below, of course any reference to a TC also includes the TA and TB
as they all use the same axle design. I have driven a TC for 60,000 miles
with a 4,875 ratio and found it very satisfactory. When I rebuild the car
again,
I will use a 4.55 ratio as the more relaxed highway cruising speed for our
roads is justified. I also have a TD with 4.3 ratio MGA gears and a TF
XPEG
1500 engine, this also works well but again, I would install a 4.1 ratio
next time.
To inprove the gearing on a TC you need to either buy a TA gear set ( 4.875
ratio), a Roger F. gear set ( 4.625 RATIO ), or change the whole rear axle
to something else which has better ratios. Sorry, one more option is to put
a Morris 1000 differential ( 4.55 ratio ) using the TC housing. Also,
apparently some Ford units fit, but these require special axle shafts.
I chose changing the whole rear axle on my TC to an MGA unit with the
track reduced to TC dimensions and now have a choice of 5.125, 4.875.
4.55, 4.3, 4.1 or 3.9 ratios which will all fit the MGA housing. Of
course
these last three ratios are totally wrong for a TC. I used the TC brake
back
plates, brakes, Alfin drums, wheel bearing carrier and wire wheel hubs so
the installation looks entirely stock. The housing, gear unit, axle shafts
and
wheel bearings are MGA. The MGA axle unit requires a slightly shorter
driveshaft which I made from an MGB unit. The MGA axle unit gives a
stronger unit than stock TC, better choice of gear ratios, is less
expensive for me, and all the components are MG.
Changing the gear ratio to 4.875 or 4.625 TC gears is a fairly straight
forward bolt-in job, untill you come to setting up the gears. I have not
much experience in setting up a gear set from scratch and don't know if the
Roger F. 4.625 set has any detailed instructions. The TC manual, as well
as other MG books, give some details on how to set the correct mesh and
backlash of the gears.
Installation of a Morris 1000 4.55 diff unit requires removing the TC
spider gears and installing them in the Morris diff in order for the TC
axle shafts to fit the splines. My analysis indicated that about 1/8" has
to be machined off the face of the Morris housing in order to get the axle
centerline the same as the TC. However, some installations have been made
without this machining and apparently work OK. You have to drill the TC
bolt hole pattern into the Morris diff and then bolt it in.
I have no details or experience fitting a Ford diff.
This reply is far from a complete "how to " instruction and is intended
only to explore some of the options available. Cost and necessity are
important considerations when making your desision. No doubt other options
are
possible and I would like to hear of any other alternatives or experiences
from owners.
Regards, Bob Grunau
> To: cgrunau@pathcom.com
> Subject: TC rear axle gearing
> Date: April 16, 1998 06:49
>
> Dear Bob,
> Have you had any experience in changing the TC rear axle ratio? I'm
> thinking about getting one of Roger Furneaux's new ring and pinion sets
> while I'm in Europe this summer. I think it's about a 4.8 (but I'm
> working from memory at the moment, it might be a 4.6). I've also got
> Carl Cederstrand's book on converting the TD/F which has some
> interesting comments on using a Stewart Warner speedo gear modification
> to get the speedo back on track.
> Apparently from Carl's comments the main effect is to slow down engine
> wear at "normal" highway speeds (65 mph or so) at the cost of some hill
> climbing ability that might require a bit more frequent changing down to
> third. Acceleration is much less affected, I guess. Since over the
> next decade or so I anticipate moderate distance driving to GOF's etc, I
> thought it would be a worthwhile modification. I realize that MGA ring
> and pinion sets can be used, but suspect that the setup and machining
> required might be more than what seems to be a "drop in" for Roger's
> gears.
> Any thoughts on this? Would you say its worth the bother for a TC
> driven 2-4,000 miles a year (maybe more as I get closer to retirement in
> 10-15 years)?
>
> Best regards, Art
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