Best Diff Ratio for V12 roadster / BW Tremec T5 gearbox

Technical advice Q&A

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Meonstoke
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#1 Best Diff Ratio for V12 roadster / BW Tremec T5 gearbox

Post by Meonstoke » Sat Nov 07, 2015 10:54 am

Hi,

I'm considering changing my current diff 3.31 to a lower one (2.88 or 3.07) as I find the revs too high on my manual S3 V12 roadster. I may in the future - if I'm a keeper for the car - place a 5 speed gearbox in place. I'm advised by the chaps at M&C Wilkinson that a BW T5 gearbox would be a good choice (instead of a reconditioned Getrag long version) together with a 2.88 diff ratio - so as to make full use of the 0.8 top ratio. Keeping to my 3.31 diff would mean choosing the 0.63 top 5th gear ratio which would be fine but not optimal apparently.

Hence, my draft cunning plan is to change the diff to 2.88 (but keep the original) ; and if I still feel the need for a 5th gear then I still can of course later in the future.

I'm sure there are folks out there who has gone down this path before and can advise on their experience. I'm particularly interested in those who have the BW T5 gearbox and who can give their thoughts on the diff ratio options.

Thanks.

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Alty Ian
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#2

Post by Alty Ian » Sat Nov 07, 2015 11:09 am

If you are going to fit a 5 speed box then why sacrifice all that torque (from a 3.31 diff) when you can simply fit a lower ratio 5th gear and have the best of both worlds ? The earlier S3's had 3.31 diffs but they changed in Jan 73 to 3.07 diffs as standard.

The S3 is a really quick car off the line with a a 3.31 diff, its only downside is high speed cruising but even then its quite acceptable with a 4 speed manual box on the 2+2, but I imagine a bit noisier in a roadster.

I would go with the low 5th gear option and leave the diff as is.
64 S1 4.2 OTS 1E10012 73 S3 2+2 manual 2013 V6 F type OTS

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Meonstoke
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#3

Post by Meonstoke » Sun Nov 08, 2015 10:25 am

Alty Ian wrote:If you are going to fit a 5 speed box then why sacrifice all that torque (from a 3.31 diff) when you can simply fit a lower ratio 5th gear and have the best of both worlds ? The earlier S3's had 3.31 diffs but they changed in Jan 73 to 3.07 diffs as standard.

The S3 is a really quick car off the line with a a 3.31 diff, its only downside is high speed cruising but even then its quite acceptable with a 4 speed manual box on the 2+2, but I imagine a bit noisier in a roadster.

I would go with the low 5th gear option and leave the diff as is.
Thanks Ian for your thoughts. Right now I certainly don't need the hassle of an engine out for a T5 box. I'll ponder over it a while longer. At speeds above 50 mph the car is too noisy / busy with the 3.31 diff. The S3 is really a boulevard cruiser - off the lights acceleration is irrelevant for me. Plus with the low floor you can't really play with any Etype without damaging the underside and exhaust. Unfortunately where l live there are too many sleeping policemen; so unfortunately the V12 is limited mostly to motorway driving.

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Alty Ian
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#4

Post by Alty Ian » Sun Nov 08, 2015 10:47 am

Fit spacers to your rear shocks/springs and increase ride height and you wont need to worry about sleeping policemen. Easy and cheap job to do.

The S3 is designed to rev to 7000 so even at 70mph you are only doing about 3300 rpm which isnt high at all. I love the acceleration the car has and dont do many long journeys in mine.
64 S1 4.2 OTS 1E10012 73 S3 2+2 manual 2013 V6 F type OTS

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PeterCrespin
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#5

Post by PeterCrespin » Sun Nov 08, 2015 4:18 pm

If your V12 is too busy/noisy over 50 mph with a 3.31, then you either have a non-standard exhaust, lots of missing jute mat or a badly-fitting top. Or perhaps unrealistic expectations of how refined a 40 year old car is ever going to be?

You car isn't going to sound radically quieter with the revs dropped by few hundred. Nor will you recoup the conversion cost any time soon by a marginal fuel saving. P More V12s die of black sludge and under-use than ever died of revving too hard. A hard top might be a better solution, or ear plugs for multi-hour high speed trips. I toured Europe in an S3 OTS and even my wife didn't complain. Maybe optimise the cant rail and top rail seals or check the B post velcro and press stud?

Still, if you're determined, then I agree with your plan to raise the diff gearing but keep the old one for reinstatement if you ever fit a 5-speed or manual overdrive box.
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 2001 Vanden Plas

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Meonstoke
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#6

Post by Meonstoke » Sun Nov 08, 2015 9:24 pm

Thanks guys for all your input. Much appreciated. Indeed, l intend to raise the height via the shocks.

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Paul Buckley
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#7 Re:

Post by Paul Buckley » Sat Nov 23, 2019 9:58 am

Meonstoke wrote:
Sun Nov 08, 2015 10:25 am
Alty Ian wrote:If you are going to fit a 5 speed box then why sacrifice all that torque (from a 3.31 diff) when you can simply fit a lower ratio 5th gear and have the best of both worlds ? The earlier S3's had 3.31 diffs but they changed in Jan 73 to 3.07 diffs as standard.

The S3 is a really quick car off the line with a a 3.31 diff, its only downside is high speed cruising but even then its quite acceptable with a 4 speed manual box on the 2+2, but I imagine a bit noisier in a roadster.

I would go with the low 5th gear option and leave the diff as is.

I have a 3.31 diff and fitted a T5 gearbox from Elite, specifying a .63 fifth gear.

Unfortunately Elite fitted the wrong box which had a .80 fifth gear.

I found that there was little difference in rpm from fourth to fifth, not enough to satisfy me anyway, so Elite had to bear the cost of exchanging and refitting the correct box which they did without any hassle.

The .63 box gives me exactly what I was trying to achieve, an 'overdrive' top gear but leaving the rest of the gears ratios as per original. It is a delight to drive on the motorway, and off the motorway I have the original performance if I require it.
Paul

1972 S3 OTS, 32k mls, a survivor.

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