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MG MGB GT V8 Factory Originals Technical - suitability of mgb 3.9 differential with T5

will the 3.9 differential on my 80 hold together for a year or so from torque loads of a v8/t5 conversion. i am still in the procurement process; the budget will probably be maxed with the engine/trans purchase. upgrades to the suspension/driveline may have to wait. i have read through a few threads regarding this which seem to leave the question open. thanks to any and all opinions.
todd wyatt

It's torque that kills components, so you should be fine with the 3.9 for a while as long as you are gentle with the throttle below 3500 revs. (IMHO)
David Smith

Todd,
The MG axle seems plenty robust. I ran one for several years with my Buick/T5 conversion. Changing axles is definitely something you can delay.
When I had time, money and motivation I switched to a Ford 8.8 axle. The gear ratio (3.27:1) made the car much, much more enjoyable to drive. If I ever feel motivated, I can simply and inexpensively try a different ratio. (Many are available). I'm running a Traction-Lok differential, but I could upgrade to a Torsen ("torque sensing") or inexpensively try a different limited slip. Someday I'll probably fit rear disc brakes. Also, I wanted to learn about narrowing axles because I thought it would be an interesting project. It was - and not a very expensive one either.
As you can see, there are plenty of reasons besides strength to change axles. To me, the main reasons were to improve the gear ratio and to open up opportunities for other improvements including especially the brake system.
Of course if MG never fit a Ford axle, they must have had a darn good reason. (I read something like that on another thread!)
-Curtis

Curtis

Todd:

Just make sure the t5 is a v8 version with the .65 overdrive.

Pete 67gt
peter plouf

Todd,
I did like Curtis and drive my V8 still with the "B" 3.9 rear end. Have covered now 30000miles and no problem. Still want eventually convert to Ford 8" rear end.
Good luck with your conversion.
Werner
Werner

Just to add a bit to Pete Plouf's comment on V8 vs V6 T5 boxes, the commonly found GM V6 unit has a 3.75 first gear (std NGB is 3.44), where the V8 has (AFAIK) a 2.95 ratio. The V6 has a .73 5th gear, and the V8 has .65, as Pete notes.

V6 first gear will be redundant with the std MGB rear axle of 3.9, and V8 torque. On the highway the V6 box in 5th will run about 2400RPM at 60MPH. None of these stats make the 3.9 axle unusable with the V6 T-5, but the V8 version will be better suited, and has somewhat more torque capacity. The better solution, albeit more costly, is a change of rear axle ratio, as Curtis points out.

Bear in mind that V6 boxes for many years now, have only had provision for an electronic speedometer (no cable drive).
bobwilson

Correction: should have read: "...T5 boxes for many years now...".

In regard to lack of speedo cable on modern gearboxes, I understand that VDO offer an electronic speedometer which can accept the signal from the gearbox VSS output, or at your option from magnets affixed to the driveshaft, and then be programmed to suit the axle ratio and tire size.

Have further heard that the guts of the VDO will fit in the Smiths case for an original look, but this sounds too good to be true!
bobwilson

i think I mentioned this in a recent post to the B technical list, but the VDO electronic speedos do work with the T-5 tranny directly, but only the 94 and later GM transmissions. The have an inductance sender that can be wired directly to terminals 7 and 8 in the speedo. I called VDO about this just last Wednesday, and their tech rep gave me the wiring color codes and connections. I don't think the ford tranny can be hooked up directly. You have to use the $50 optional sender, which has a bracket to bolt to the floorpan near the tailshaft and a stick on magnet to put on the driveshaft. That solution sounds a little Rube Goldberg.

The speedo is self calibrating. You set it to learn mode, punch a button at the beginning of a measured mile and again at the end. It calculates the pulses/mile and starts working. The 3 1/8" size fits in the stock dash opening. They have several styles. One matches very closely the Smith instruments. Unfortunately, it is the only style not made in 3 1/8"! I bought a speedo and tach with a similar design but black rim for my V6 conversion. I am thinking of trying to replace the trim ring or maybe refinish it. They use a big screw on mounting ring on the back, so they are easy to mount. The actual case containing the works is quite small, so it should be easy to modify them to fit into a Smiths case. It's not obvious that it would be a slip in operation though.
Arthur Shaffer

Arthur is correct that the VDO guage will fit in the Smiths case. It does require some cutouts at the rear of the case. I used both the VDO Series II tach and speedo in my V6 conversion.
Don Zeigler

The 4 cyl B tubed axle is effectively the same as the MGBGT V8's unit, just the CW&P differences. The one advantage of the 4 cyl's 3.9 ratio with V8 torque is that the lower gearing is more likely to see spirited driving result in wheel spin, which will not load up the transmission quite the same way as a higher ratio would.

Rog
Roger Parker

Auto Meter gauges also makes an electronic speedo, it's driven by a generator that is mounted on the transmission same place as the cable. It has 3 wires to the speedo, simple the same goes for SW gauges.
On the back of the instrument there is a place to calibrate the speedo. Instructions are clear.

I used a set(Kit 5 gauges + Tach $355) on my 62 MK II V6 jag and will be using a set on my V6 GT. A very simple insatallation.
Contact PAW in Ca.805-776-1694 or any Speed Shop.

I have raced my GT on SCCA EM Class with the stock rear Axle with a 2.8 Ford V6 215 Hp It took all the punishment of racing for 4 years. The same axle is back on the street.
I agree with everyone, it should be your last investment, 8.8 or 8.0 Ford These are call poor man's Quick Change Rear Ends. Easy to change from 3.0 to 4.11 etc. 8.8 is cheaper to narrow than the 8 due to the axles taper, the axle has to be cut more than 4" in each side, if not the axles can not be re-spline, and new axles are needed $300 and up. Must rears are 631/2" wide and wider. You need 52" for a MG. If you are going to cut one, have it narrow to 50" and used the wider wheels and tires with the V8/V6 That would be the great advantage of a narrow Ford Rear. Also look at import rear axles etc...Ford is just the must popular.
William Guzman

Just a quick note on T5's. I looked at several late model wrecks this week while "shopping" for parts, and noticed that the latest V6 Firebirds and Camaros appear to have V8 trannys. They are easy to tell apart, since the speedo plug is on the drivers side in V6 trannys and on the passenger side in the V8 tranny. I don't have any documentation, but I wonder if the V6 tranny was dropped with the narrow angle V6? The 3.8's are more powerful engines and the decision may have been made to go with one size fits all. Bottom line is, don't just use 6 vs 8 as a criteria when shopping for transmissions.
Arthur Shaffer

Not al v8 tranies have the tall overdrive either. Buyer beware.

Pete
peter plouf

There seems to be too much confuson about the VDO gauges and the GM T5 transmission. Here's what works right out of the box: the VDO electronic speedometer fits in the dash of my 74-1/2 MGBGT like it was built for that purpose. The matching tach does the same thing. Next, the optional VDO remote speedo sender screws on the speedometer fitting on the T-5 without trauma and the wires run to the speedometer. My T5 is from an '88 Camaro, so this business about the VDO only fitting 94 or later just in correct.

The VDO programmable electronic speedometer is a joy...it can be precisely calibrated in just minutes and doesn't care about tire size or anything else. It is perhaps the most satisfying and easiest to accomplish part of the entire MGB V8 conversion!

Mike

Mike in Western Colorado

Hello to the list,

I have a T5 tranny from a V8 with the .63 overdrive that I am installing, initially, with the stock MGB rear end. My intent is to one day go to a Ford 8 inch and was thinking of a 3.23 ratio.

My question: Would the 3.23 rear end and the .63 overdrive give me too high a final ratio for my fairly stock ( .030 overbore, 9.3:1 cr, Offy 360 dual port intake and AFB 500 cmf carb) Buick 215 to handle. I have to consider the altitude here in the Denver area. I would like to have all 5 gears usable (not a redundant 1st ar an under powered 5th).

Thanks in advance,

Larry

P.S. Mike in W. C., what is your email address, I lost it.
Larry Diede

One more piece of VDO info.

I wasn't clear in my post maybe, but the 94 and later trannies will work with the electronic speedo with no need for a new sender or adapter. The electronic sender outpur from the transmission is hardwired directly to the instrument without the expense of the optional sender (or converter). For pre 94, they require the transmitter/adapter.
Arthur Shaffer

I have the VDO catalog in front of me, and I am making my Christmas list. I measure the tach and speedo in my 79 B to be about 4" diameter. I was going to buy the Vision series gages part # 333 160,
and 437 135. Are these the ones the fit like they were made for the car? I am using a Rover 5 spd tranny any info on the sending unit I would need? I am also using the stock rear end. The rover has 3.32 1st, and .83 5th ratios. I find that 1st is a little short and reaches red line rather quickly but is not unusable. At 70 mph the engine turns about 3400 rpm in 5th again quite usable but not ideal. The car feels quick in all gears and I am usally in 4th by about 40mph during normal driving. I would think a t-5 with V8 ratios would make a good combination with the 3.9 rear end ratio, but then again I would prefer the car to have quicker acceleration than for the engine to loaf in high gear.
Bill Yobi

Maybe someone with a rubber bumper car can comment on the size of the instruments. I haven't owned one in years. (about 22) The tach and speedo in my 73 are 3 1/8" in size. Four inches seems awfully big for a stock dash, but I may be wrong.

As far as sending units for different transmissions, the add on magnetic impulse sender which mounts beside the tailshaft can always be used. Call JEG's in Columbus,OH and talk to them about the instruments. They can custom order any of the VDO equipment and get it in a few days. They can also give you the VDO tech number so you can speak to their applications people directly.
Arthur Shaffer

This thread was discussed between 27/10/1999 and 29/11/1999

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