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Which is the Toughest Differential for a 2.5L Vitesse?


Wagger
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Hi yet again folks.

Checking my Diff carefully, it is a 3.63:1 ratio version with a round prop flange and rectangular drive shaft flanges. All of the bolts are 3/8 UNF wih a spanner size of 9/16 A/F. I know that the 3.27:1 version is best avoided as it is fragile.

3.63:1 is quite a desirable ratio for the 2500TC engine fitted to my car, but just how much torque can it cope with? I am prepared to stop the throttle from attaining full power if it is an issue. However, I know from my experiences in the 1970's that my 1600 Vitesse was kinder to its diff than a tuned 12/50 Herald. More even torque gave less 'Shocks' per rev of the engine. Three per rev instead of two.

What is the most vulnerable part of the transmission? I have just done the drive shaft uj's. I will never race the car or do any hills climbs etc. However, it would be nice if it could surprise a few boy racers at the traffic lights, so long as nothing breaks.

 

 

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I broke a few 3.27 diffs with my 2.5 vitesse. Fitted a 3.63 and it survived for many years. Then did service in my 155bhp zetec spitfire, and now lives an a mk3 spit in Belgium. When I sold it I had it stripped and checked over, nothing required except new seals as a precaution.

My vitesse had a saloon box with a low first gear, te vitesse box has a longer first so is less likely to break it. In fact the box is probably the weakest part of the transmission.

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Thanks Clive, that is very re-assuring. The Gearbox is the late 306468 three rail Vitesse, GT6 and early Dolomite version. I have no idea just how tough that is. I will take all unnecessary weight off the car when I use it. Minimal toolkit instaed of carrying all of the spares in the boot.

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The way to make the gearbox last as long as possible is to do as much of the hard acceleration as possible in 4th because then its a direct drive and the layshaft is not in use. With the torque available from a 2.5 engine this should be easy to do so let the boy racers get off the line first and then steam past them😁 

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Hi,

 "I know that the 3.27:1 version is best avoided as it is fragile." 

 I have a 3:27 & it's lasted longer than all the 3:63's that broke.

Use the 3:63 then. Enjoy motorway cruising at higher RPM's. I hope you have an OD?

Cheers,

Iain.

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7 hours ago, SpitFire6 said:

Hi,

 "I know that the 3.27:1 version is best avoided as it is fragile." 

 I have a 3:27 & it's lasted longer than all the 3:63's that broke.

Use the 3:63 then. Enjoy motorway cruising at higher RPM's. I hope you have an OD?

Cheers,

Iain.

The quaife you use overcome much of the 3.27 weakness. ie the carrier. But the quaife is not a direct fit, the 3.27 is a slightly oddball size compared to the rest of the diffs. 

 

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3 minutes ago, johny said:

A question: does the ratio itself, regardless of the diff design, affect the durability of the unit?

Alot of bmw rear diffs are 2.9 ect and have more power through them, so i would guess its the build and design not the ratio

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So, the answer is that the part quality and strength depends upon who made it and how (as per usual). I had many BMW's and never broke a diff, drive-shaft or gearbox. Crankshafts were 'Bomb proof' but cylinder heads were the weak points after a few years and poor coolant usage.

The original Getrag five speed (with the dog leg 1st gear) was a bit weak, but that was superceded.

British manufacturers just 'Used up' old stuff before moving on. R&D done by the customer. What a shame nothing has changed! The 'Bean counters' ruined it all. Off soap box now.

Where would I find a Quaife 3.27:1 diff on a pensioner's budget?

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3 hours ago, johny said:

A question: does the ratio itself, regardless of the diff design, affect the durability of the unit?

With Triumph diffs, most of them are essentially the same internal design and will be equally durable regardless of the ratio used.

- except the 3.27:1, where they had to reduce the thickness of some of the metal of the carrier in order to get the crown wheel to fit, and that reduction in thickness makes the design inherently weaker.

 Fitting a different ratio to a 3.27:1 casing will not make it any stronger. 

If I understand correctly, a 3.27:1 crownwheel cannot be made to fit the other casings without machining work. 

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1 hour ago, KevinR said:

With Triumph diffs, most of them are essentially the same internal design and will be equally durable regardless of the ratio used.

- except the 3.27:1, where they had to reduce the thickness of some of the metal of the carrier in order to get the crown wheel to fit, and that reduction in thickness makes the design inherently weaker.

 Fitting a different ratio to a 3.27:1 casing will not make it any stronger. 

If I understand correctly, a 3.27:1 crownwheel cannot be made to fit the other casings without machining work. 

Its just that I would have thought, due to the torque multiplier effect, dumping the clutch on a 3.27 diff would be more stressful for the gears (and other parts of the drive train) than doing the same on, for example, a 3.89....

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Well i have a 3:27 in a Vitesse with a saloon box and tuned 2.5 engine,i had the diff rebuilt by Mike Papworth with a stronger carrier(TR7)?

Whilst i don`t dump the clutch,it gets driven fairly hard and has tons of torque so high revs not really needed,it gets scary to drive before you get anywhere near top end.

The only thing that broke in mine was a driveshaft snapping at the hub end whilst driving,bye bye back wheel and brakes.Drum still attached to the wheel as it departed.I changed to CV shafts.

There is a company that will build an LSD into a 3:27 ,but the name escapes me.

Steve

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2 hours ago, johny said:

Its just that I would have thought, due to the torque multiplier effect, dumping the clutch on a 3.27 diff would be more stressful for the gears (and other parts of the drive train) than doing the same on, for example, a 3.89....

Why? The torque available at the flywheel, through the gearbox and down the propshaft is unaffected by diff ratio. The torque on the diff's internals and the drive shafts will be less with a 3.27 than any of the other diffs.

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I think what people are getting at is getting the car to move forward and how much torque is needed to do that.

On a bicycle trying to take off in the highest gear is harder so the same should be true here.

Still, that doesn't necessarily mean things are under more stress... right?

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If you're anywhere near at risk of stalling then you're using minimal torque for your pull-away. It's WAYYY less than you're subjecting it to in a traffic light grand prix. If you are trying to spin the wheels then the torque down the drive line is, as I said, not affected by the diff ratio until the diff itself, where it's less on a 3.27

The only possible caveat is that you might, due to the reduced torque at the wheels, be able to put more torque through without breaking traction, if you're being a hooligan.

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yes the stalling was just an example.

Perhaps leverage is a better way to explain it: by changing ratio your changing the length of one side of a lever in relation to its length the other side of the pivot point. For lifting a set weight you have to apply more force the shorter the lever and, if the characteristics of the lever are kept the same, isnt it more likely to break? 

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But you're only changing the torque required to achieve a certain outcome, not the torque that's available. So you will be accelerating slower with a 3.27 at the point where you break your gearbox, but if your driving style involves that much acceleration then you'll break the gearbox just as quickly on a 3.89 - you'll just be launching yourself into a ditch rather faster when you do so. People don't drive to a very specific acceleration when using all the power they've got, they only do that when constrained (e.g. by the car in front), and that's not the condition where you break things.

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Thanks for all of the discussion. Tried the car out today. Really enjoyed it but, even with a 3.63:1 diff, it is very difficult to achieve that 'U' turn between the kerbs that the car is so famous for. On returning home an all round check revealed o/s caliper leaking fluid. There is always something else to do on this car! I thought I'd finished??

 

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34 minutes ago, Wagger said:

Thanks for all of the discussion. Tried the car out today. Really enjoyed it but, even with a 3.63:1 diff, it is very difficult to achieve that 'U' turn between the kerbs that the car is so famous for. 

 

The diff doesn't do that, the steering does. 😕

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I think , IF I wanted to be a "Hooligan" at the traffic lights. I wouldn`t naturally choose a small chassis Triumph?. Now. a tuned 2L Pinto, in a stripped out Mk1 Escourt?. Maybe. So long as you can avoid the trees!.

Pete

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