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MG TD TF 1500 - improving highway performance

I have a 52 TD with the original engine, transmission, etc. I'd like to improve its highway performance, say to cruise comfortably at 60 mph. What's the best way to go about it: change the transmission, differential or a combination of both, and to what? Thank you.
REG Bob

Every change affects another component.

A 4.3 differential change (or 4.55) is the least expensive mod, then a 5-speed to take advantage of the gear change - 5th is an overdrive. But then you'll be down on power, so get a supercharger and/or a Fanelli roller cam. Then you will have performance similar to a Miata.

Tom Lange
MGT Repair
t lange

I put 4.55:1 in my TD to give me better Hwy and top end speed. I figure it that is not enough then I would go the five spd route as well. With the T9 and the 4.55s it would give about the same as a 4.30 final. The reason I would prefer to do this instead of directly changing to the 430 is that I stay closer to the capabilities of the engines abilities to pull hills in higher gears and have the OD to give me that long legged 430 end gear. Which doesn't do as well when hill climbing. At least that was my thought on the subject. I actually like the 4.55 by themselves and may not even add the 5. Spd but I have more miles to add before I make that leap.

Bill Chasser
TD-4834
W. A. Chasser

Above remarks pretty much sum it up.
The cheap way I changed gearing was buy a junk MGB with wire wheels for $95... installed the 3.92 rear end, disc brakes up front and added new wire wheels onto the MGB adapters. I sold off many B goodies so ours parts were pretty much free; the seats alone for more than the price of the car.
This gearing is pretty drastic (about the same as 4.3 + 5 speed), so a supercharger is essential. Cruising is approx 3000rpm @ 60mph and 4000rpm @ 80mph. It works, but bear in mind, it is running in midrange whereas peak power isn't until around 5500.
I acquired a roller lifter cam with street/ supercharged grind for the next engine- pretty conservative grind but supposedly very strong midrange.
JRN JIM

The Ford T9 fifth gear is 0.82:1, so with a 4.55 diff, the overall ratio becomes 3.73:1, which is too high for either an XPAG or an XPEG. A 4.3 + T9 fifth gear gives 3.53:1 - probably impossible.
A 4.875 + T9 fifth gear gives 4:1**, which might be just right.
** actually 3.9975:1.
R A WILSON

I have a Ford T9 and a 4.55:1 diff and the combination is great. The engine is 1330cc (+0.080") and has a Stage 2 conversion. Remember that 4th gear still gives an overall ratio of 4.55:1, which is very driveable, with 5th gear coming in as an overdrive at 40mph plus.
Dave H
Dave Hill

My TF 1500 has a new 4.1 rear from Dave Clark and runs like a rabbit. I have a 3.9 on the shelf, but a blower and other upgrades would be needed for more power to use it. Stock transmission. PJ
Paul161

Rear end gear change is the least expensive. Doing just one and cost isn't the determining factor....probably the 5 speed. It will give you higher cruising speed at the same engine rpm and the stock gearbox isn't particularly robust. You also gain a synchro 1st gear. If money isn't an issue, do both. The car will not accelerate any faster, in fact if you do a rear end gear change the acceleration will be slower. With the 5 speed available, I believe the first 4 gears are comparable to the stock unit so your acceleration would be the same as you have now if you keep the stock rear end.
L E D LaVerne

For interest. A 4:1 overall ratio gives 18.48 mph/1000rpm. A 4.1:1 overall ratio gives 18 mph/1000rpm. 3.73:1 gives 19.81 mph/1000rpm.
(There will be some slight variation according to tyre size).
R A WILSON

There are more than suttle performance differences between a 1250 and a 1500 engine. Ensure you take this into account during your decision making process.

Providing you can source the parts it is in the not too hard basket to do a diff conversion yourself, process is well documented.
G Evans

Bob, see http://www.ttalk.info/RearEndConversion.htm for more on rear end gearing changes. You may have to do a copy/paste. I enjoy a lot of highway driving with my TD. Latest was Plymouth, MA to Marietta, GA. The conversion to a 4.3 rear end is the best thing that I've done to the car, IMHO. I've also added a bit of extra displacement to 1308cc by going .060" in the cylinders. That's been tied in with going to a 9.1:1 compression ratio by taking .100" off of the head. These internal modifications have given me the added oomph to motor comfortably on interstates and hill country. Bud
Bud Krueger

This thread was discussed between 11/09/2016 and 13/09/2016

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