Several members with first generation XKRs (v8) changed their rear axle ratio to achieve more low-end performance/pull. It came at the expense of highway mileage but was cited as even more effective than supercharger pulley upgrades for acceleration from a dead stop. For the pre 2003 XKR models with the Mercedes transmission, additional re-programming was required at $2,000 by itself so only a handful of owners went that far. I think everyone who did rear axle ratio changes also did supercharger upgrades as well as limited slip differentials so quite the expensive package of upgrades.

Is X300 4.0 litre inline six sedan amenable to a simple rear axle ratio change or are their enough complicating factors that it's simply too much money considering the value of the car itself?

 

No problems. Just change the ratio. I went from the standard 3.27 to 3.58 without changing anything else. Even the speedometer remains accurate.

Cheers
DD

 

I should mention that your car should already have the 3.58 so you'll need to find a 3.77 or 4.09 gearset or differential

Still, though, you shouldn't have any problems

Cheers
DD

 

To clarify, Doug is saying that the X300 XJR came standard with the 3.27, where the non-XJR X300 came with the 3.58.

 

Thanks fellas! Two additional questions

1. Sources for a "3.77 or 4.09 gearset or differential?"

2. Labor cost to be expected?

 

A fair estimate is around 9 hours labor to replace the final drive unit.

bob gauff

 

This should be an interesting cost proposition compared to nitrous via the Wizards of NOS supplier in U.K. and the V8 conversion from JaguarSpecialties.com in CA USA.

So, I change the ratio and retain the existing differential without adding the XJR's LSD "Posi" unit as well? Local Jag mechanics here in Denver CO are very conservative so I have to present this well up front, despite being a 45 year old guy.

 

Gonna be hard to find in the USA as Jaguar used them in the smaller (3.2 litre) engine cars. But perhaps a dealer or Jag parts specialist can order/find a set for you.

Cheers
DD

 

Not really sure what you're asking, sorry.

Cheers
DD

 

Your answer is helpful as to what may be realistic for Coventry West or Jaguar Heaven to be able to provide/find. Now I realize neither may have access to what I am seeking since both are in the USA here. Guessing overseas source is going to be far more expensive. Thanks.

Andy's bracket just arrived today so that may be it for a while if additional options all come to 1/2 if not the full value of the car itself.

 

Doesn't take a lot to squeeze a bit more out of these engines without expensive mods. Depends how much fire you really need to create. The 3.2 is effectively the same engine as the 4.0 n/a, but with a shorter stoke. Fitting Andy's bracket plus a direct air intake has pumped my super-smooth 3.2 up appreciably on acceleration and I can't really tell much difference between it and a 4.0 that I occasionally get to drive. Since you are used to a n/a engine, you might find that these 2 simple mods will keep you satisfied without the big $$.

 

I think it would be fun to change the rear end gearing IF one already had a need to repair the rear end. ...or one has a bunch of $$ lying around and the XJ is their hobby car

Mine is a daily driver, so I'll likely never change the rear end, but if I have to and it is still worth it... I'll definitely consider the XJ6 3.58 gearing.

 

If I where to get a new rear in my jag I'd probably go with a 3.08 but mainly because i would have a 3 speed no need to go with shorter 4+ ratio the 3.58 with od is perfect for that engine so just squeeze more power out of it rather than hitting 3000rpm at 65 haha.

 

Bryan Barnett at Coventry West has confirmed they can provide the parts.

A 4.09 "power-lock" (LSD) differential is $995 with no core charge or need to ship the existing 3.58 core back to them. So I could achieve the stronger off the line performance and get the LSD that would make cornering more fun. There are slightly less expensive alternatives that involve shipping the 3.58 core back to them although shipping of $100 (approx.) back to them makes that a debatable use of time and effort.

Changing the ratio only (no LSD) is not a cost saving effort.

It appears doing this is a $2,000 project. While that is half the price of the car, it is still cheaper than buying a different car. Gives me a nice goal to shoot for by the Spring!

 

Is that the cost of an entire rebuilt differential, or just the gear set?

I think this would be a fun modification to put the 3.58 LSD in my XJR and get better off-the-line performance, but in my case I can't really justify the cost since my rear end is still good and I do 90% highway driving so MPG impacts do add up.

Keep us up to date on the project, especially with regard to cost, difficulty, lessons learned, etc

.

 

That is the entire rebuilt differential. Coventry West's Bryan Barnett said it is not complicated "but that it does take a good bit of work to get the old one out and the new one in".

Now that I know this option (probably the best for my driving priorities), time to figure out nitrous oxide versus V8 from JaguarSpecialties (Andrew).

 

If I understand this correctly I can buy a rebuilt rear for $900 with 3.77 gears / correct? If so that will be my next mod after the suspension project I have going now.

 

They don't have the 3.77 in stock. You'd have to find those.