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MG TD TF 1500 - Is the 3.9 or 4.1 or 4.3 Best for TF1500?

Hey Guys - I am interested to know if any TF1500 owners have gone beyond the 4.3 and fitted a 4.1 or 3.9 ring and pinion when converting from the standard ratio.

I know the 4.1 or 3.9 will be more difficult to locate, but was just wondering if anyone has that fitted in a TF1500 - considering the improvement in power that you have with the XPEG engine.

I am not interested at this time in going the 5 speed route but am interested in the above conversion. I already have a spare 4.3 that I can use - but am tempted to go to 4.1 or 3.9 as an alternate.

Any actual conversions out there like this on a TF1500?

Thanks in advance,

Jeff
J. W. Delk

The XPAG certainly can push the 4.3...(and maybe 4.1)...would think the 1500 would have no problem with either of the others. ( I would be tempted to go 3.9)
gblawson(gordon)

Hi Jeff -- I have had a 3.9 in my TF1500 for about 4 years now and am enjoying it very much. The engine is about stage 3 in the XPAG/E tuning manual. It does not require as much down shifting as you would think and of course keeps up with interstate traffic very nicely. The MPG did not increase as much as I thought it would but the carbs are set a bit rich. 3.9's are easier to get than 4.3's because they come from MGB's and more MGB's are in the breakers yard than MGA's. I didn't personally do the changeover but my understanding is that it works out the same as described in Cederstrands writeup.

I think you will be very pleased with the results if you make this change.
Cheers,

Bob
R. K. Jeffers

I think it depends on what you're looking for and where you live. If you just want higher cruise speed then yes the 4.1 or 3.9 would be fine on a 1500. Be aware of the trade off in acceleration and hill climbing ability. For me I live in the high country of AZ and have some hills to negotiate, never need the speeds you would want to keep up with fast freeway driving, and am totally pleased with the change to the 4.3 from the 4.78 my 1500 had stock. Perfect for my situation.

Dave
Dave Clark

Jeff,

I have a 4.1 in my TF 1500, It does very well even in the North Georgia Mountains. I can out pull most MGB's in the mountains. However my engine has been stage III tuned, rebuilt by Bob Wagner, dyno tuned, etc.
(75 HP mrasured @ 5000 RPM, (est. max. of 85HP @ 6000 RPM)

I am doing 65 MPH at 4000 RPM using Dunlop 165R15 tires instead of 55 MPH with the stock 4.85/1 rear end

It would easily handle a 3.9. (would give 65 MPH at 3800 RPM) but with some loss in power

The key is you have little power below about 2200 RPM so hve to downshift if going too slow in the mountains,

Note that both the 4.3 and the 4.1 give a comparable final drive ratio than the overdrive 5th gear with the stock 4.85 TF rear end
Don Harmer

The other issue, is finding a 4.1 cwp. They originnaly came from the MGA Mark II, and only 8700 were made.

Gord Clark
Rockburn, Qué.
Gordon A. Clark

Wow - what a quick show of responses from the group. Thank you so much for your thoughts.

The idea of 65 at 3,800 rpm is very tempting to me as here in GA we have long gentle hills in 80% of the state - the N. Ga mountains would be different and I recently enjoyed a club tour to W. VA with 4.875 ratio that is currently fitted - I did slow the group sown though on the flat drives and this prompts the change. On balance though, I think I would rather have the longer legs on the daily driving and shift a little more when in the mountains or in hill country.

My engine is a standard XPEG with no modifications - and it will most likely remain so. I know that the 4.3 is a safe bet with no real worries - the lure of the 3.9 (early MGB) is a chance I MAY take- Bob's and Don's comments certainly make this sound ok - I assume a stock XPEG wouldn't be to weak for that ratio.

I will think about this a little - I may chicken out and just go with the 4.3 spare diff. that I have. Gord makes a good point on the difficulty in finding a 4.1 off of the MGA MKII - was that the only application?

I'll also look around for that 3.9 from an early MGB.

Thanks again guys - very helpful to read about actual experiences.

Jeff

J. W. Delk

Jeff,

I thought I would add my comments to the above. I have an XPEG block with a Laystal head for my TD with the 4.3 diff. I live in the mountains and do not see a lot of flat roads. This combination is great for my driving conditions. If I lived in flat country, I would consider a 3.9. It sounds like you could get along fine with a 3.9 or a 4.1. The drop in RPM makes any of these ratios worth the effort, when you have the benefit of the extra torque from the XPEG block.

George Raham
TD 4224
G. L. Raham

Gentlemen;
What fabulous information! Thank you. I have a question. Is there a way to determine what gear ratio I have in my diff besides computing the information given by my 55 year old speedo and tach? Is there any sort of marking on the differential housing to tell what was installed at the factory?

Tom
tn Norby

The ring /pinion gear teeth numbers are stamped on the differential ( e.g. 42/8 = 5.125 std TD, 39/8 = 4.875 Std TF, 41/9 = 4.55 Optional TD/TF.

MGA was 43/10= 4.3 or 41/10 = 4.1. MGB was 39/10 = 3.9
Don Harmer

"I may chicken out and just go with the 4.3 spare diff. that I have."

If you have it why wouldn't you just give it a try? You could change it out in about 3-4 hours max.

BTW, I still have my 4.875 in storage and will not convert it because it should go with the car and a core rear end can be found for $150 or less if you decide to go with a 4.1 after the 4.3 experiment.

I currently have two 4.3 gearsets, three core rear ends and all the tooling to do the conversion but have decided to get out of the business due to lack of time and interest. If anyone is interested in purchasing this package email av8tor**()#theriver**()# replace the junk figures with the normal at and com.

Dave
Dave Clark

Jeff, I know that this is a little off-topic. I have been running a 4.3 in my stock TD 1250 for several years. I can run 60 mph at 3600 rpms all day. I am not comfortable going any faster, because I think that the car is a bit of a death trap! I drive the car very conseratively and defensively at 60. The car has decent acceleration and at highway speeds, the car has sufficient power for hills. Even if the car had a 1500 cc engine and a bit more power, I would stay with the 4.3. There are no negatives, as far as I am concerned.
Steven Tobias

To tnNorby Oregon - Tom if you go to the following website there is outlined here an easy way to calculate which rear end you have:
http://www.mgaguru.com/mgtech/rearaxle/ra105.htm
Regards
Rod M
Rod Murray

Dave,

I am interested in the gearing but for the life of me I cannot figure out your e-mail address. Could you clarify it for me.

Thanks,
Mark
Mark Strang

Mark

It's "av8tor" and then at "theriver" dot com

Just trying to avoid any configuration posted publicly because I had troubles from that on this site a few years ago.

Dave
Dave Clark

This thread was discussed between 11/07/2009 and 14/07/2009

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.