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I'm looking at options for reducing the cruising rpm's on my S2 XJ. Would a lower ratio Power-Lok differential from a late 80's / early 90's XJS fit in the cage? Will basically any in-board rear brake differential swap with another?
It would, but a V12, 2.88:1 ratio might be a bit tall depending on what your V8 is like, so a 6 cylinder version's diff might be better? Also make sure you get a GKN/Salisbury one, not a DANA. All 6 cylinder XJSs had GKNs and most V12s. If a V12 and it has a drain plug it is a GKN, no drain plug = DANA.
DANA output shaft parts are just about unobtainable, also the spline count is different from the GKN so even second hand parts are very rare.
I'm looking at options for reducing the cruising rpm's on my S2 XJ. Would a lower ratio Power-Lok differential from a late 80's / early 90's XJS fit in the cage? Will basically any in-board rear brake differential swap with another?
I agree with Greg. Should fit.
What ratio do you have now? Probably a 3.31.
What ratio are you after?
The most readily available is the 2.88 ratio. As mentioned it was used in all the V12 5.3 HE cars. These will have limited slip.
The 2.88 was also used in the Series III 6 cylinder cars after 1981 or so. Should be very easy to find but almost none were fitted with limited slip. The early Series III 6 cylinder cars used a 3.07. Some pre-HE XJSs had the 3.07 as well. Most had the 3.31
Series II 6 cylinder cars all had the 3.31 as far as I know
In all of this I speak of automatic transmission, USA-market cars.
If you do presently have a 3.31 diff and make a change there are people who might be interested in taking the 3.31 diff of your hands.
Thomas, what transmission do you have? I hope it's not a th350-400. The 700r4/ 4l60 has an overdrive and should play well with the 3:31, good grunt from a dead stop and good rpms at cruising speeds. Lots easier to change a tranny than a pumpkin, and the OD transmissions are miles better in more ways than one.
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Current transmission is a 4L60e with overdrive. My normal highway cruise speed is 75-80 mph and the revs are just over 2,000 rpm. I'd like to get that down to around 1,700 rpm for better economy.
I believe the current diff is a 3.31 based on the tag bolted to the rear cover (which I never trust) and by counting the number of turns of the drive shaft for one revolution of the wheel.
The diff I have now is open so if I could find a 3.07 or 2.88 PowerLok that would be a nice touch. The other option is to change the 4L60e to a T56 with the .50 6th gear overdrive but that would be more costly.
Ah I see. And that sounds about right for a 3:31. The t56, while it's a blast to drive it would be a whole other project and pricey. I do have a 2:88 open diff with less than 10k miles from new that I got from a dealer in atlanta, the story was they replaced it in a customer car for a noise but it turned out the noise was still there in the car and it wasn't the diff. I'll never use it. If you wanted limited slip my advice would be to forget the pwr-lok, get a new Yukon or Auburn carrier and Motive gears of your choice and use your pumpkin. I spent more money on the pwr lok and waaay more time than if I had just gone with new. It's a deep rabbit hole. With a yukon carrier and Motive gears though it can be fairly cost effective. At any rate it won't be a weekend job.
The only ring gear I could find that was dual drilled was the Motive. Prices for the same part number ranged from $399 - $149 (3:73 ratio). Found the $149 on ebay and it was perfect. Motive tech support was personal and impeccable. Run out on the pwr lok face was .003, I'd get a yukon.
I'm looking at options for reducing the cruising rpm's on my S2 XJ. Would a lower ratio Power-Lok differential from a late 80's / early 90's XJS fit in the cage? Will basically any in-board rear brake differential swap with another?
If your diff is solid, upgrade to a 700R4 / 4l60e. That would get you lockup and a lower ratio 1st gear too. 3:31 is the perfect ratio for a 700R4 / 4l60E.
I took a step back and opened up Spicer's gear ratio calculator to put some quantitative data around this. My current combination with 215/75-15 tires, the 3.31 diff, and the .69 4th gear ratio of the 4L60e comes out to 2,125 rpms @ approx 77mph which is my normal highway cruising speed. 2,125 rpms is about what I see when I am driving.
If I changed from the 3.31 to a 2.88 rear diff, at the same 77 mph (ish) the engine would be turning 1,850 rpms. This would be at the expense of acceleration which is about 6 seconds 0-60 on the 3.31 diff.
If I kept the 3.31 diff and changed the transmission to a T56 with the .63 6th gear, 77 mph (ish) would be at 1,950 rpms.
Keeping the 3.31 and going to a T56 with a .5 ratio 6th gear gives me 1,545 rpms at 77 mph (ish)
Furthermore, let's assume that I drive the car 5,000 miles a year and any of the above options improve fuel economy from 22mpg to 27 mpg. I think 22mpg is what I was averaging last year. We'll also assume $4.09 for a gallon of premium gas which is what I paid last night.
Continuing the financial analysis, assume changing the diff would cost $600 and a T56 swap would cost $4,000 to $6,000. The $200/year in fuel I would be saving IF these changes netted me a 5mpg increase in small and would result in a pay-off period that ranged from 3 years to when I retired.
In conclusion, there is no sound financial basis to seek an increase in mpg and if I were to attempt anything it should be the one that would be the most fun.
I went from a 2.88 to a 3.31 in my V12 Series III. I'm still running the TH400 transmission.
The highway mileage dropped 2-3 mpg. No significant change in city driving fuel economy. Maybe 1 mpg or less?
I don't do much long distance/highway driving these days so the increased highway speed RPM and increased fuel consumption isn't really an issue for me.
As you might imagine the difference in acceleration and response is significant. For me, it's a good trade-off.
I changed the original 2.88/open to Motive 3.08/Auburn pozi and like it. Short tires (25") and the 4L60e trans give me right at 2000rpm at 70mph. I'm like Doug in that gas mileage isn't an overriding factor, but this setup gives me enough passing options on the road to keep me happy. For example: at 70mph I can add just a little gas and disconnect the torque converter lockup for a little more pep or shift into 3rd gear (about 2900rpm from converter chart) and get with the program. Anything more is wasted on me.
I went from a 2.88 to 3.54 in my 6.0 LS/turbo swapped S3. This was only due to the original 2.88 eating itself from oil starvation. In reality the 2.88 was actually a fantastic gear which gave low cruising revs when paired with a lockup torque converter. The torque of the large displacement combined with a somewhat loose 3200rpm torque converter (which is really key) means that you get the best of both worlds. I do miss the 2.88's relaxed nature on the highway, but 3.54s are very lively around town which is where I do the majority of my driving.
In my humble opinion - 22 mpg, in a stylish, comfortable vintage car that has plenty of power, is actually remarkable. For context, I averaged 20mpg in my 2009 BMW M3 on a road trip last fall cruising around 80-85mph with a few spirited blasts. If you want MPGs - take the Prius!
^^^
We don't drive Jags for their economy.
We drive Jags for their Jaguarness, whatever that may be for each individual.
Incorrigible and unrepentant Tom Boy that I am, driving Nix makes me feel like a Lady!
(';')
It would, but a V12, 2.88:1 ratio might be a bit tall depending on what your V8 is like, so a 6 cylinder version's diff might be better? Also make sure you get a GKN/Salisbury one, not a DANA. All 6 cylinder XJSs had GKNs and most V12s. If a V12 and it has a drain plug it is a GKN, no drain plug = DANA.
DANA output shaft parts are just about unobtainable, also the spline count is different from the GKN so even second hand parts are very rare.
pre HE Xj12/XJS had 3.07 rear diff, which might be a good compromise. You should be able to find one in the UK and have it shipped to France. There should be a tab on one of the cover bolts that has the crown and pinion ratio stamped on.
Rear Differential Compatability
