Rear posi trac / Gear ratio change
 

Hello Everyone,

I have 2 jags. A 99 XJR and a 2000XK8 convertible.

I am interested in making both of them faster. I have read a few posts on the
rear end mods and don't see where there was any solution. Apparently there are some Limited slip options for the Jag diff, but they are crazy expensive and the diff seems to be weak. I have read that in the supercharged cars with the merc trans the TCU will need to be reprogrammed?

So I will outline what I am trying to do with my XJR. When I first drove one I was impressed but thought this would be an awesome car with more gear. I am cheap so as much as I would like a 4.70:1 rear gear I don't want to be stuck with it. Since the jag has independent rear it would be easy to fit a 2 speed gearbox just ahead of the diff ( think powerglide with bellhousing and tail housing removed. ) From what I have read the stock jag rear gear is roughly 3:1 add 1.7 from the powerglide and BAM 4.70:1 flip a switch and back to 3:1 and the ability to cruise with 20+ MPG.

Obstacles to overcome:

!) Method to trick the Trans computer when in low range

2) ?

I have read that there is a transmission input speed sensor and an output sensor via ABS that allow the transmission to select the proper gear.

I posted a thread about removing the ABS because I just don't like it. Call me selfish but I like to drive my car and don't need the car trying to modify my inputs. I was told that perhaps a Jag wasn't a good fit for me...lol

It is my impression from what I have read that if the ABS fails the car will still drive but it will display a red light on the dash? I belive this to be true...mine has the red light on but still drives fine.

I don't know how the transmission output speed is acquired by the TCM....Canbus?

Any thoughts would be appreciated!

Thanks

I have looked at pictures of the input shaft sensor and it looks like it might be a hall effect sensor? It might be Viable to intercept and scale the output of this sensor using a micro processor while the low gear is selected?

PS I am also contemplating switching the diff to a Ford 8.8 as a new posi unit is only 250.00 USD. There will be some work to fit it but it will be stronger and still less expensive than working with the Jag diff.

The car has a gated shifter which has manual shift mode. I wonder if the speed sensors are ignored in manual mode?

You can perform a gear ratio check bypass procedure in manual mode every time you start the car included in the thread below, that is if your car is not an early '98 model XJR with the Vid Block incorporated into the ECU and not solitaire like newer years.....as far as I know anyhow.

https://www.jaguarforums.com/forum/x...change-108750/

Yes it operates on a canbus system and sadly it will cost you at least $2500 to have your tcm reprogrammed accordingly. I had custom rear tone/reluctor rings made to compensate for my gear ratio change, but you are talking a whole different dream. I don't think you could physically fit enough teeth on an x308/100 tone ring to read and compensate the gear ratio you speak of to trick the speed sensor/abs sensor communication.

I have discontinued my abs/traction control system and you are correct, you will need to physically delete the abs light but will always have the abs/stability control failure message on your message center.

Kick in the butt is you can throw whatever diff you want in your XK8 and drive into the sunset, only the R cars with the 722.6 will give you nightmares. Good luck!

I posted a thread about removing the ABS because I just don't like it. Call me selfish but I like to drive my car and don't need the car trying to modify my inputs. I was told that perhaps a Jag wasn't a good fit for me...lol

I have no problem with anyone doing anything they want to their cars, but, here in the UK, insurance would be invalid if such modifications were made and not authorised, and I can't see any insurance company agreeing to the removal of ABS on any vehicle to which it was originally fitted. I do have a problem with anyone who takes a car on to a public road without valid insurance, which would be the situation in this particular case.

This won't work. The 8.8 doesn't use the half shafts as upper control arms and therefore isn't designed to take the side loads.


With a cutting torch and welder and lots of steel plate you can cut out the entire rear end and then somehow add in a T-bird or Cobra IRS subframe but that would take a ton of fabrication.


I'd love to see a vendor modify an existing type of torsen to work in the XK8 rear dif (like an 8.8 in a s-type Jag/Lincoln 8.0) but Nobody has figured this out, yet.

Don't have much of a problem with modifications here in NH. You would not like the fact that insurance is not necessary in NH, although I insure all of my cars, I also add uninsured motorist insurance. Live free or die, gotta love NH

https://cimg8.ibsrv.net/gimg/www.jag...29750d565b.jpg

insurance and such
 

Hi,

The US hasn't become as restrictive as the UK yet, There are many cars running around without abs. I reckon that there are still some classics in the UK which have managed to survive without ABS...Sprites, Healys, MGA...etc
Here in Florida insurance is required, vehicle inspections are not so no worries about having a car rather than an atoma-ton on 4 wheels. I hold a racing license from a sanctioning body here in the US. I am not trying to be snipey but MY opinion is ABS is made for socer moms and people who panic in stressful situations. So ABS is fine for the masses. There are some of us who have the talent and desire to drive our cars and enjoy them in a different way than most.
Think Jeremy VS Captain slow One is not more correct than the other, Just different.

Yo Greverrr,

Thanks for the links. When you removed your ABS did you just disable it? or did you remove the hardware and re-plumb it or both? And what did you do with the traction control?

How does hello became Yo,? Mysteries of the universe.

RE: 8.8 wont work

I biuld CNC machines for a living. So I have experience programming microchips and doing signal manipulations. I have a CNC machine shop to modify and create things with. I have an extra XJR rear suspension unit which I will remove from the car and build on the bench.
I also have a hobby foundry which can pour 200# of aluminum.

So what is the plan to make an 8.8 a reality?

The 8.8 in lincolns had ally cases which can be welded to and remachined potentially to
function as the stock jag does.

second method:

fab up bits to attach to the stock hubs for upper control arms to remove all load from the driveshafts and use whatever driveshafts are handy or easy. Tubular is easy.
If the wheel hub carriers are not up to snuff I will cut some 9 guage plate on the CNC plasma and weld up new ones.

Anyway it is very doable

Hi Greverrr,

Your car sounds AWESOME! Do you have any pics posted ? Or would you mind posting a few here?

Thanks for sharing !!!

Greverrr,

I think it is also doable to make a board which will send a message via canbus to the computer that all is well with ABS / Trac thereby eliminating the warning message.

https://www.jaguarforums.com/forum/p...-ratio-167206/


If anyone wants to come into the group buy let me know soon. My spare TCU is already in the process. The job is done by an US based engineer who is proved to be about the only one who has succesfully made ratio changes changes to EGS51 - at least for Jaguars. It's not a simple process and costs $$. There are some spare TCUs available if you do not have one. So, this is just for XJR/XKR 1998-2003 with MB 722.6 transmission.

Agreed on both points. Bit OT, but I can't think of any driving situation where ABS would be a disadvantage?

Mike

ABS driving disadvantage
 

There are times when You want to abruptly chane direction, maybe 90 or 180 degrees. Jaguar placed the Ebrake in a position where it is easily used for this purpose. I haven't completely thought it through but it seems that if you lock the rear wheels at speed some bit of computer gadgetry will become unhappy.

8.8 Independent rear
 

Here is a picture similar to the mods I envision for the Jag rear

This should be able to be fitted within the confines of the original chassis without modification.

Hey Fred,
Thanks for the compliment, clearly you and I fall from the same tree. As much of an uproar as it arises every time it is mentioned, I also agree with your stand on ABS and you are correct about older cars and lack of abs. I grew up rallying and if I had not disabled ABS on any of my cars I can only imagine I would have plowed right into a tree or off a cliff. Its not for everyone, but its my comfort zone, especially for racing. ABS wasn't really a standard item until the late 80's/early 90's as far as I can recall. And I can assure you my MG Midget (Spridget) doesn't have abs and no classic MG (or any other classic) ever did, except the more modern MG's.

My ABS is disabled by a "desoldered" or otherwise broken ABS module. I am just running a module with the common solder fracture points not fixed. And that is that, no functioning ABS with secondary traction control failure because of non operable ABS. My traction control is my LSD and my brakes work with no complications other than no ABS. Not running ABS also allowed me to save some $ and run only rear custom reluctor rings, fronts are still the originals that are not read by the sensors.

If you look in my albums under my profile there will be plenty of pics for you to check out or my instagram is #grevemotorsports, i'm new to it but there are some recent projects, my Midget, and plenty of glamour shots of the Jag :) oh ya here too https://www.jaguarforums.com/forum/x...nt-lip-159191/
and by the way, don't know where "Yo" derives from either, but I commonly use it as well
https://cimg5.ibsrv.net/gimg/www.jag...ce62d4b1d.jpeg

https://cimg0.ibsrv.net/gimg/www.jag...a08d0a16f.jpeg

Hi Greverrr,

Since making the gear change would you say that your car has more or less of a tendency to spin the rear wheels when accelerating from a stopped position?

I am looking for maximum stoplight to stoplight performance. Currently Jags setup is "I believe" to limit rpm to 6500 and use the supercharger to make torque. I have always felt that a 7500 redline is way more versatile. So what I am thinking about doing is installing a larger supercharger pulley (Under drive instead of the usual overdrive), run the 2 speed rear with a very low gear, increase redline to 7500 and run a programmable throttle stop. So I would limit the throttle opening from a 0mph situation (When in manual gated mode) so that the maximum amount of torque will be applied to the tires without wheel spin. The throttle stop will be stepper activated and programmed to increase the throttle opening as MPH increases thereby providing maximum constant torque to the rear wheels. I think this may also provide better mileage when back in the stock final drive ratio as the supercharger will consume less power and make less heat and belt life will be improved as well.
I don't see 7500 being a problem as long as the chains/guides are in good shape. Any thoughts based on your experience?

Thanks

PS where did you get the Hood Vents?

The XK8/R exhaust is pretty restrictive for a 7500 RPM engine, from the manifolds to the tiny pipes snaking through the rear suspension.


Pretty much all of it would have to go.

Much less tendency to spin the tires, but that is due to LSD as opposed to the normal one wheel fire open diff from the factory. I also run 285s in the rear which helps hook up. If the gears were changed to 3.58 w/o LSD obviously it would spin the one tire even easier, but I have created a great balance imo. I can brake stand it, but when I let off the brake it just hooks up almost immediately and goes. I made a custom rear sway bar so I can "play" on the track more and take advantage of the LSD at the limits and have some fun. As you can tell I usually don't just throw parts at a job just to say I did, its usually for a purpose.

Personally I don't think increasing rev limit with an m112 is going to help you. Efficiency and output runs out much earlier than 7500. If you came to me and I was doing the work, I would recommend throwing on a Kenne Bell and just throw in an LSD with 3.58s, pretty much a proven set up and I bet you would be happy. But since i'm not doing the work, its all you buddy ;). Plus with your background I could see why you would want to lean toward your own engineering.

Hood vents are actually a donation from an '87 trans am, saw them at a junk yard one day and had to have them! They purpose dually to let some extra air flow over the intercoolers and I have it cut so it flows air directly into the air box.
https://cimg9.ibsrv.net/gimg/www.jag...846ba6918.jpeg
https://cimg2.ibsrv.net/gimg/www.jag...3c6b3daa5.jpeg

https://cimg1.ibsrv.net/gimg/www.jag...3d9fccdd9.jpeg

May I ask how do you change the 6200rpm rev limiter in stock Denso ECU or remove other limiters in this box?? How do you modify gear change rpm from max 6000rpm in EGS51? I guess you do not have plan to change all the engine and transmission control units to aftermarket ones. Do you know how soft the stock valve springs are if you try 7500rpm? When you change upper or lower pulley you just make the same power at lower rpm. Stock M112 almost dies over 5500rpm if well overspinned.

RE valve springs and ecu
 

Hi,

I did figure that I would need to change the valve springs although they dont need to be super strong due to the relatively low valve train mass. Since these cars have dual cams one can alter the cam timing to shift the power peaks around a little or enhance cylinder pressure. For 7500 I would definitely be wanting to run a ms3-pro ecu which looks to be awesome ! It has sooo much tunability and it speaks canbus and has outputs for methanol/water injection etc. so it would really be slick. I also don't think it will freak out and put you in limp mode at in opportune times. Hearing that wheel spin is just not a problem with a LSD is going to keep me restrained. I will do the rear end first and then add torque until it won't take it.

I have decided to put a 6 speed and the 7500 mod in my XK8 with probably a 3.25:1 gear

PS for 7500 I said that I would slow the blower so the blower speed was the same (at 7500) as a 3# pulley is at with stock ecu at 6000 The blower speed is a limiting factor just like a propeller diameter is the limiting factor in an airplane. All hell breaks loose when any part of it goes supersonic