Hello All,

This has probably been posted several times but am looking for some updated information. I currently have a 70 El Camino that I want to put in a IRS. I have a rear end out of an 83 Jag XJS with 2.88 gears and want to rebuild and upgrade the gears to 3.54. I can not find anybody that rebuilds these locally to me here in California. Do any of you know where I can get these rebuild for a reasonable price? I am also willing to swap it out but Coventry wants $2250 which seems a little hi to me but maybe not!! Any advise would be most welcome!!

 

The 1985 XJS with the 3.6 liter six had a 3.54:1 diff ratio POWER-LOCK. There were several that were imported but they are kinda rare.

The 4.0 XJS and the 6.0 XJS should also have the 3.54 ratio diffs with POWER-LOCK.

You should be able to swap out the diff assy BUT the later cars had OUTBOARD disc brakes. I don't remember if the calipers can be mounted on the inboard (modified) with the later diffs.

bob

 

when i rebuilt/changed rear diff, i went to a driveline shop in Daytona FL, brought with me the ring/pinion jag gears (3.07), guy looked at them went in back room came out with 3.73 ratio gears, siad they should be fine for acceleration!

he said he could help me if any probs. or bring whole pumpkin assembly in and do the complete job!

gears cost $150. dollars ,1995! they were for a JEEP!

22years , still no problems. LSD/Powerlock Jaguar, standard in 1978 Jag XJS.

 

XKs in San Luis Obisbo will rebuild and upgrade the unit.

 

I agree about finding a shop that will rebuild or change ratio's , I took the 91 diff with the speedo in the diff to old GTJ in Ohio years back for a rebuild and gear change for 3.54 gears (Dana 44's), long story it whined like hell after back in the car. Then it clunked/slop the second time. Just my experience with Jaguar diff's

 

It wasn't the design, it was the rebuilder.

Having never touched a diff, I have every intention of rebuilding
my own and do not anticipate any problems in the end ... it's
just going to take a really, really long time.

 

I never setup a diff either and I don't want to learn on a Jag taking the IRS, brakes, and exhaust apart then back together. Now if there would be a way to "bench test" it before I'd give it a try.

 

How do I identify what rear is in my 99 XK8? I'd like to put around a 3:73 posi in it. It feels like a Bonneville gear!

 

BigAlBre, normally there is some tag on the rear differential that will have a code on it that will tell you the gear. if you don't have the tag, then the next best thing would be to lift one rear tire off of the ground and then put the car into neutral (ensure it is blocked properly) and rotate the drive shaft as a second person watches the tire that is off of the ground. Let that wheel make 1 rotation as you count the turns that the driveshaft makes. Take the number of revolutions and multiple by 2. That will be your gear ratio.

On a separate issue, what are you attempting to accomplish by going to a new gear? Your car is most likely speed limited at the moment to 155 mph (factory setting). If you get rid of it, next comes how much power you got on hand. To hit 200 mph, you are going to need atleast 400ish hp. This is where gears in the tranny are going to start to play an affect. yes, cars like NASCAR run 3.73 and even gears into the 4.56 range, but their overdrive gears are very tall and they are turning the engines at very high RPMs. Looking around, you should have the 3.07 rear (the 3.27 was put in the supercharged versions, but it was not very clear what year this applied to). I would do a quick calculation of what the theoretical top speed of your car would be. This can be simply done by see what your car turns at say 70 mph (the faster you can do this, the more accurate it will be). From there you can take and use this equation: Max speed = Speed at test x redline RPM / RPM at test speed. So, to show you how this works. My 2012 turns 2000 RPM at 70 mph. My red line is 6500 RPM. So, theoretically, my car and run 70 mph x 6500 RPM / 2000 RPM = 227 mph. My car simply does not have enough power to push it at that speed. This is where I can help the car reach that speed by going with a different gear. If I was to put in say 3.07 gears, my new 70 mph RPM would be 2400 RPM (my car has a 2.56 rear end in stock form). So, redoing the equation, 70 mph x 2400 RPM / 6500 RPM = 190 mph, which would be essentially where I would become power limited (motor is rated for 385 hp). The benefit is I would get to that speed faster. Down side is my car would see a hit in the mileage as the motor would now be turning many more RPMs at highway speeds. this is a concern for some, not for others.

So, if I make a few assumptions about your car based on the year and what RPM it most likely turns at 60 mph (I am assuming 2000 RPM). This would result in a computer limited speed of 60 mph x 6500 RPM (assumed redline) / 2000 RPM = 195 mph. If you were to put in the 3.73 gear from the believed 3.07 gear, your top speed would drop to 160 mph. That would most likely be near where you would be motor limited unless modifications have been done. I would be looking at 3.55 more as that would put you closer to the point where your motor power and engine RPM are going to be both limiting at about the same time. Engine mods, then you are going back towards the 3.07 to have all things become limiting at the same time.

I have done some playing with gear ratios and how they affect the vehicles because of my truck. I can help you figure out what is best for what you are after. I will also point out the pros and cons of whatever gear you pick.

 

Thank you Chris for the input but I am more interested in acceleration not top speed.... I had a 1936 ford with a 3:73 posi and a small block 350 chevy....it would come off the line sideways with an automatic....I loved it and would like that kind of performance again. Don't know if I mentioned it but I am running an LS with an aftermarket cam and bigger torque converter...even at 330 RWHP the jag doesn't have the kind of quickness my 36 had....besides if I ever did achieve speeds like that and was caught I'm sure I would spend time in jail, pay a HUGE fine and possibly lose my license and my car....Cheers Mate!!!

 

BigAlBre, you may want to look into getting a different tranny then. I know my XJ will chirp the tires very easily. the first gear in the tranny is super low. This would give you the benefits you are looking for with the line launch, yet, still keep things streetable for the highway runs. Keep in mind that your case, I would start with a 60 mph run and see where the engine RPMs are. They should be in the 1900-2000 range. That is where most shops will recommend putting the gears for most street cars. Once you get the gearing in the ideal range, then you can look into what it would take to get a lower first/second gear to give you the launch that you are after. Not sure how easy it may be to get the 6hp28 tranny from out of a 2010-2012 XJ/XK/XF and put into your car. That would have all that you would want. Otherwise, some investigation into changing the gears inside your current transmission may be worthwhile.

 

Chris I have the GM 4l60E tranny which is a computer controlled 700R4...has a very low first gear

 

BigAlBre, then you should be set. If anything, I would be thinking that you can roast the rear tires too easy with what I am picturing. That would make for a slow take off. Getting the limited slip in there would help keep traction and help you scoot down the road.

 

Hi Big Al

I have an LS converted xk8 and would be interested in what engine mods you have made, mine dynoed at 280 rwp recently with a skimmed head and different headers. I don't need to travel at 190mph but like you would be interested in more low down acceleration. Any advice would be appreciated. San Antone TX

yours

garry
gvs1047@gmail.com