The 'E' Type Forum

Hi everyone...
I have just picked back up on my restoration of a S3 v12 manual coupe....... LHD currently!

The gearbox and diff are both stripped and on my bench awaiting reassembly, so my question is the diff is running a 3.54,
and i plan to change it to either a 3.07 or a 2.88 and keep the standard and original 4 speed box.

Any thoughts or advice on the choice would be very much appreciated. I am more of a leisurely type torque surfer than a fast and furious and the v12 is after all about torque isn't it?

I am sure this has been covered many times but not so much for the manual S3

Best regards Steve

Unless you plan on lots of high speed touring (over 70mph a lot), my choice would be the 3.07.

Hi Steve , I'm on the Same Path , maybe a Little Further Forward than you , i went for a Late Model XJS 2.88 Power lock because i just Cruise around Now Too . Russ

Hi Russ,

How you getting on with the 2.88? Spent much time with it yet.....

Steve

Hi guys
My car has a 2.88 and a Getrag 5 speed.
The reason a lot of cars get the 2.88 is that they sound too high revving with the original diff. I put the 2.88 in with the original 4 speed, but it still sounded too high revving to me, you always wished for another gear, hence the Getrag box.
Several years later I would say that it is a great match. The engine now runs 2000 rpm at 60 mph. Acceleration is still possible in 5th, but really if you want to overtake, then drop to 4th and you are good to go. Around town you will be in 4th or 3rd.
Cheers
Mark

Hi i fitted a 2.88 in my v12 roadster 3 yrs ago its just great we do not use our cars like young kids it will still go round a roundabout in top gear acceleration is still good and it cruises along lovely at 85mph and now with su carbs and vac advance with a PROPER ported vac returns 22mpg local and 27mpg on a run would never consider any other diff
Regards Rob

Not had any Time with it Yet Steve , ive been doing the left to Right Swap at the Same Time , thankfully coming to an End Now , I went to the Spares day to find out More about a 5 speed , Russ

So the question is........?
If i go 2.88 can i use the original carrier or do I have to change internals as well?

She is a manual from 1973.......

Russ we are on the same path me thinks, i am swapping to RHD too... what did you do steering rack wise ?

The Steering was the Easy Bit , A Modern Rack from The Guys at Vicarage , supplied exactly what they said they would and When they said they Would , Well made Mountings , Straight Forward Fit . Russ

Another rack offering from SNGB or CMC based on a Ford Focus rack. Going back to the diff discussion, I run a 2.88 with standard 4 speed box. Admittedly my car runs a 6.0 litre injected engine with AJ6 mods, but I find it just right. You don’t need to use the gearbox much! As I have posted many times before, be very careful what you wish for with the 5 speed options. Tremec T5 and Getrag do not have as clean a change as the Jag 4 speed.

StevenB wrote: ↑

Mon Oct 28, 2019 2:35 pm

So the question is........?
If i go 2.88 can i use the original carrier or do I have to change internals as well?

You need the next carrier in the series of three, which is for 2.88 and taller. Shouldn't be hard to find but if you're stuck I have one.

Barry wrote: ↑

Wed Oct 30, 2019 8:42 am

Tremec T5 and Getrag do not have as clean a change as the Jag 4 speed.

I can't comment on the Getrag but the change in the JT5 box is much more positive and precise than in any of my all synchro boxes.

So i had a Proper Test Drive today Steve , the 2;88 Diff Gives me 2,700 Rpm at 70 Mph However i Tried to Change up to 5th many times , in my Opinion The Gearing is Just Not Tall enough for today's Driving , and the Vicarage Steering was SOOOOO nice , Direct , Responsive , Nice Feedback , A No Brainer . Russ

Russ, did you recalibrate yr speedo? Yr rpm vs mph values seem a bit out, considering also that Smiths (most) speedos read slightly high ...
I have made me a calculator to address the perennial, which gearbox and diff question, this also predicts true rpm versus mph. Some examples:

sideways60 wrote: ↑

Wed Nov 13, 2019 6:11 pm

the 2;88 Diff Gives me 2,700 Rpm at 70 Mph

Actual maybe closer to 75 mph, ie:

42south wrote: ↑

Sun Oct 27, 2019 6:01 pm

My car has a 2.88 and a Getrag 5 speed. The engine now runs 2000 rpm at 60 mph.

Actual maybe closer to 70 mph, assuming the Getrag that was fitted to early xj6's?

Barry wrote: ↑

Mon Sep 09, 2019 7:05 am

fitted with rebuilt original 4 speed Jaguar. However, with a 2.88 lsd diff, it means at 90 mph, the engine is running at 3500 revs

HI Colin ,
Yes i had the Speedo Re Calibrated by Speedograph Richfield , took about 2 and a Half Weeks , My 70 Mph is 1 Mph High Against an All Singing Dancing GPS , so Very Happy with their Work . Russ

Steven
I have an EFi HE V12 with 4 Sp manual plus overdrive and a 2.88 sitting at the back.
Unless you want racetrack takeoff and high revs in each gear. sit back and enjoy the lazy smooth quiet cruising of your S3 bliss! Its well up to the job,.don't think its going to be slow its not!!
Peter.

PS
don’t forget to get the speedo redone
P

Here is the maths:
Engine speed, ES = 2700 rpm
Gearbox ratio, GB = 1 (4th gear)
Diff (final drive) ratio, FD = 2.88
Wheel speed, WS = ES / GB / FD = 937.5 rpm
Tyre perimeter (distance travelled per wheel rev) can be calculated from the designation on the tyre, e.g. for 215/70R15:
Nominal Tyre height, NTH = 15 inches + 215 mm x 70% x 2 = 682 mm
Tyre perimeter, TP = PI * NTH = 2.143 m
Distance traveled per time = TP x WS = 120.52 kph = 74.9 mph
Doesn’t include corners and wheelslip, which would cause actual distance traveled to be less than calculated. Still, yr GPS 69 mph is a lot less than 75 mph.

Reason for my pedantics, I’m trying to justify decisions already made when the benefits are marginal. I have an LdN overdrive reco’d ready to install, diff is currently 3.54. I have updated my calculator to display comparisons, some observations:

2.88 diff without OD versus 3.54 diff with OD: Cruising speed/rpm is the same. Below ~30 mph the 3.54 has 20% more torque available so could accelerate harder except for the extra gear change. It could also crawl slower. So maybe better for < 1% of the time (drag-racing and when stuck in traffic jams) but very annoying for the remaining 99% of the time.

Adding OD to a 2.88 diff: Cruising at 70 mph the OD car could rev 500 rpm slower, 2000 rpm instead of 2500 rpm. I suspect higher throttling losses might limit any mpg benefit. At this operation the OD car would have relatively weak acceleration (30% less torque available), corrected by flicking the switch; imo this is the big advantage of an LdA overdrive compared to a true 5 speed, you don’t have to change down to get back on song.

Adding taller diff to an OD: Combination of the above, ie:

Hi Colin...if you want to be pedantic then use the rolling radius of your wheel/tyre combination to calcucate your distance traveled.....Steve

I’ll see your pedantic and raise you one… that is sometimes done but imo is wrong, distance travelled is the tyre perimeter. We only use the radius to calculate the perimeter, which doesn’t change no matter how much the tyre is squashed. So the correct calculation is actually the easier one ...