Assuming that the transmission is in 4th gear (1.0 ratio I believe) and you have the stock 3.06 rear axle ratio, the rear wheels turn 3.06 times for every revolution of the drive shaft. So if you increase the rear axle ratio, the rear wheels will turn more per rpm. This makes you quicker off the line (i.e. at a stop light). It essentially shifts the torque and power curves down to where you hit the peak at a lower speed. However, it reduces your top end. This has never been a big deal to me since my 2002 XKR’s top end is 175 mph (according to Jaguar) but is limited by the engine control module (ECM) to 155 mph. In addition, I run out of b*lls far before I hit even 155 mph. The higher rear axle ratio makes you quicker 0 to 30 mph, 30 mph to 100 mph, etc. I think this is what you are looking for.

Supercharger (s/c) mods increase power in the mid-range, say 30 mph to 120 mph. Because a s/c is belt driven by the engine, there is no lag as there is with a turbo charger (t/c), which use exhaust gases to spin the turbine. Both s/c and t/c work by forcing more air into the combustion chamber, thereby allowing more gas to be burned, which in turn produces more power. Because a s/c is driven off the engine, it reduces the top end because power that would otherwise be available to the rear wheels is used by the s/c. A t/c increases your top end, but is not as effective as a s/c in the 0 to 30 mph or 0 to 60 mph in increasing the quickness or acceleration of the car.

A LSD basically causes both wheels to be turned by the drive shaft instead of just one. This gives you more traction and hence less potential to, as we used to say when I was a kid, “burn rubber”. More of the power goes into actual acceleration.

Because I knew I would upgrade the s/c, I opted to first add an LSD (and there are several available for our cars with various advantages and disadvantages). Since installing the LSD required disassemble of the rear differential anyway, I choose to increase the rear axle ratio at the same time. I went with a 3.27 rear axle ratio because I found one another forum member was selling and, our cars are GTs, and I didn’t want to reduce my gas mileage too much. As it is, I went from about 22 to 24 mpg to about 19 or 20 mpg.

I am very happy with my results. I can basically smoke any car I want (except an exotic) at a red light and the car still has reasonable gas mileage.

Mark

 

Sorry ... but no.

With a 3.06 rear axle ratio, the driveshaft will turn 3.06 times for each revolution of the rear wheels in any gear, not the other way around.

Changing to a 3.27 has the effect of lowering the torque peak in relation to wheel revolutions, but it is really about torque multiplication and leverage. A gear set is as much a lever as a bar on a fulcrum. It just works differently.

The torque multiplication/leverage is compounded by the transmission. Thus, multiply the transmission gear ratio by the final drive gear ratio to calculate the overall gear ratio. Of course, tire diameter also comes into play. The smaller the tire diameter, the better the leverage.

Horse power is simply a derivative calculation of torque, but torque is king when it comes to acceleration.

 

I stand corrected. Mechanical Engineering was never my strong suite; I'm a Nuclear Engineer and am more comfortable with the neutron diffusion equation. I knew that the torque curve was lowered because I can feel it every time I step on the gas.


Mark

 

And now we know why 1/4 mile drag race guys love 4.88 gears!

 

Jaguar Quaife Differential Install – Jacks Transmissions LLC

 

THANK YOU Jack's Tranmissions. I'm 1.25 hours away in Denver so I may be visiting.

Three questions I hope someone can answer.

1. If you want a different rear ratio for stronger pull down low, can you keep the existing differential to save $?

2. What is the difference between what the Quaife LSD would provide vs the rebuilt XJR 6cyl diff that some have obtained from Coventry West? Quaife cost for parts appears to be $1500 (mjlaris) vs. Coventry $1300 (incl mount plate and shipping).

3. What do other LSD suppliers provide by comparison such as Aisen and OS Giken (?spelling?)?

 

Very interesting but remember, if you have a Mercedes transmission (2002 or before) then changing rear axle ratio requires reprogramming the transmission control module. There is a place in Germany (with offices in New York City) that can do this reprogramming (do a search of this forum for a link, I can't remember it). However, it is my understanding the reprogramming is very expensive (on the order of $2k).


Mark

 

Here is the link to get the Mercedes TCM reprogrammed.

Automotive Tuning and Electronic Engineering. ECU programming and tuning

 

So I have a 2004 XKR and will not need a TCM reprogram - what is the cheapest way to move to a 3.27? Can I buy the rear end from a Mexican XKR?

 

You have to stay with your current setup as you have a ZF 6 speed. I know of someone in the UK working on a solution. I will find out how he is doing with it.

 

So what going on. Still no solution for us 6 speed guys?

 

+1, you know this is killing me....

 

Cambo, did you find out what the problem in your diff was and get it sorted?

 

He was working on modified ABS rings, the problem was it effected the speedo. I am not sure where he left off on this but will ask.

 

Yes Charlie, it was the pinion bearing, chewed to bits. Turns out when the workshop in Switzerland put in the Quaife they over-torqued the pinion nut which loaded up the bearing. All sorted now.

It's worth asking.

 

Bump.
Please tell me he is making progress. This is one mod I really want to do.

 

Ford Racing Products now has a 2nd gen box that does all they lying for you.

The first one would do this too, but the 2nd gen is better.

Still won't fix the speedo problem unless you can split the signal to the
speedo, or run a separate circuit to drive only the stepper moter on
the speedo needle. I suspect the remainder of the ICM still needs the
"corrected" speed.

It's in the FRP catalog under differential accessories.

Looking at the catalog, you can only dream that Jaguar would ever offer
that level of aftermarket motor sports support.

If your ABS sensors are compatible, and they ought to be, you are
good to go.

 

In the meantime there might be a solution for the X350 owners.

https://www.jaguarforums.com/forum/x...ng-sdd-130103/

Seems that the diff ratio is indeed managed by the ECU, not the TCM, in the X350 at least...

 

A way to fix the speedo problem is an aftermarket GPS speedo. Since I have a 2004 I'm waiting for someone else to try the newer model diff change so I can learn from their experience.


I'm still looking to try to convert to a full digital dash.




http://www.mguk.org/users/1913/20/60/23/album/n1a10.jpg

 

Aren't the ABS rings on the wheels? Changing the axle ratio should have no impact on the relation of the wheels to the road just the engine to the wheels. Is there something I'm missing?