Do I understand correctly, that it's not possible to swap gears in 03-06 XKRs? I conducted a few searches and found such comments as:

"The only other thing I would do is rear posi and steeper gears. But the 6 speed gear box won't let me. Something to do with the computer and shift points."

"One of the best mods I've done is the rear axel ratio change, which is not possible on the 4.2L XK8/Rs because of their ZF transmissions."


Being a long time Thunderbird guy (currently with a supercharged 4.6), when I swapped to 3.73s it was the single best bang-for-the-buck performance improvement that I made.

As a new 04 XKR owner, I would like to know if a gear swap is possible. I don't need to go 150mph. I understand (and fully accept) the reduction in fuel economy. My primary goal is to increase my 0-60 for those instances when I pull up next to a vette or a hopped up mustang...or even my own supercharged 4.6 Thunderbird.

If steeper gears truly are an impossibility, what is commonly done to increase 0-60 and 1/4 mile performance? I'm not trying to build an all-out dragster...just want more oomph down low...and I know from experience that gears would be the solution. What else can be done on an XKR that's reasonably economical ($$ wise)? I've seen the Mina Performance Kit (pulley, intake & exhaust) and it seems like a good start.

 

Do a Twin Screw conversion that is all you need.

 

As hot rods go, these cars are not the most economical way to get your speed fix, but they definitely look good doing it.

The transmission monitors the input and output shaft speeds and if they are different from the expected ratios by more than a certain percentage then it will throw an error.

You have already hit on the best bang for your buck improvement on an XKR. There is plenty more potential but it all comes down to the time (and money) you re willing to spend.

 

Thanks for the feedback. Twin screw is out of my budget as of now...maybe down the road. I'm willing to drop $2k in bolt ons right now. Other than a pulley and intake mods, what else can you guys suggest?

 

Hey Majik Lasers,
Which 2004 did you end up getting? Congratulations, enjoy!

 

There are some STR guys running nitrous. Not my cup of tea, but it gets the job done.

There is also the killer chiller, which JgaXKR can comment on.

 

Hey tberg! Thanks! I found a great one in Springfield, MO. 45K miles, had another forum member check it out and a shop performed a formal inspection for me. Its really a nice car. I was just surprised with the off the line performance. Can't keep up with my Thunderbird with barely 340hp and 3.73s. Other than that, it's perfect!

Ha...no nitrous for me.

 

Hey Majik--congrats on the new toy.

I always heard as well that the best bang-for-the-buck in terms of getting red light to red light performance was going for a gear swap. I thought Quaife had some rear end available that would accomplish this but it could be that it was the same ratio but limited slip.

When I went from my "lowly" 300hp Corvette to the impressive-sounding 400hp XKR I figured that the drive would be just as thrilling. I never did any scientific measurements other than the butt-o-meter but the Corvette just felt light-years more responsive off the line than the XKR. Of course that might just have been the "feel" of the vehicles since you never really feel the speed in the Jaguar but you look at the speedometer and suddenly it is racing up.

Still the biggest obstacle to any real performance is the huge weight of the Jaguar--basically pushing 4000 pounds or thereabouts. That would be like loading 4 more adults into the Corvette.

Doug

 

If you want to experiment with lower gears without actually swapping them, borrow some smaller tires and try that. You'd just have to be careful about having the back end too low, and it'd look kind of stupid unless you raised the rear end with some kind of doodad. Of course your speedometer would be off, but it'd be the same SOTP feeling as with the gear change.
ccfulton said "The transmission monitors the input and output shaft speeds and if they are different from the expected ratios by more than a certain percentage then it will throw an error." That's true, but the rear end wouldn't change anything in the transmission.
Beyond all that, how willing (or able) would you be to make something fit? A body saw, grinder and welder can make about anything fit into anything.................

 

Quite right, sorry about that.

Meant to say that the TCM checks those shaft speeds against against the vehicle speed.

 

I'm not an expert in this but as I understand it here is a transducer giving a pulse train and the frequency is the "speed". It would be fairly simple to put a small Arduino computer in-between, "fixing" the frequency to match the one it should be by just counting the input pulses and skip some of them on the output connected o the ECU.

But again, I could be totally wrong.

 

TCM gets the speed over canbus not directly from the wheel sensor. Speedometer and stability control are also using same data so modifying the sensor input months work but is going to affect those systems as well.

But I suspect you are on the right track. If you could use the arduino to intercept the can messages, do the math and then rebroadcast them locally to the TCM, then the transmission would not know the difference.

I've recently made something similar to this that stands in as the linear switch module to see if I could make a paddle shifter for the 6spd trans.

 

If you are successful, or even if your not, please let us know what you do. I am thinking of doing something similar to my F-type.

 

You should probably know that my Thunderbird is also a 4000 pound monster, especially considering the amount of audio gear in the trunk.

Even with less hp, the Thunderbird is much faster than the XKR off the line. My XKR won't even break the tires loose from a stop. My 4000 pound Thunderbird, with an automatic trans, will break the tires loose for nearly a full second when shifting to second gear. Stomp on it from a dead stop and you'll have traction issues for 100 feet or more.

I'm so very disappointed in the performance of the XKR. I've owned it for less than a week and am considering selling it already. Such a shame. Its one of the most beautiful cars I've ever seen, and certainly "looks" fast. Boasting 400hp, I assumed it would be pretty fast and certainly faster than my "lowly" Thunderbird. I'd venture to say that the XKR is no faster than my wife's 2013 Hyundai Genesis V6! I've looking forward to owning this car for years...searched for about a month...pulled the trigger...and now I'm facing a disappointment. Its really a heavy feeling in my gut.

Turns out Eurotoys is local to me (about a 30 min drive). Since the Twin screw is out of my budget, I'll have to try upgrading the pulleys (upper and lower). Any thoughts on these EUROTOYS PULLEYS? I talked to someone over there and he also suggested Blue Top Solenoids to firm up and speed up my shifts. Are these available for my trans? Feedback?

It seems like a gear swap is out of the question. On my Thunderbird I completed the 3.73 swap, changed the speedo gear and a tune for about $1k. I assumed it would be similar with the Jag. Nope....but why can't the Jag's computer just be re-flashed for the steeper gears? Seems easy enough. If it can be done to Fords, then why not Jags?

Absolutely! One of the best looking cars I've ever laid eyes on! I fell in love with the styling of this car more than 10 years ago! I just assumed that boasting 400hp, it would have the performance to match.

So what exactly do you mean by plenty of potential? What else (other than pulleys, intake and exhaust) is suggested to increase the low-end performance for these cars?

I still think 3.73-3.90 gears would be a PERFECT match...if only there was a way........

 

Just a thought here since the XKR performs better than what you are finding--have you checked the throttle cable? With this car (as well as others I have owned) sometimes the cable that opens the throttle plate, or the cable that pulls the sensor that opens the throttle plate, gets slack. Consequently, when you step on the pedal it could be that the throttle plate is not opening all the way.

You might want to check that.

Otherwise for these cars you can get a bit of extra performance relatively cheaply by doing the supercharger pulley swap--that would certainly be in the $500 neighborhood I would think. At the other extreme is the twin-screw type of stuff that is more in the $5K to $10K neighborhood which is way too rich for me.

Doug

 

No, I haven't checked the throttle cable, though I don't suspect there is an issue with the vehicle. I threw my back out over the weekend and I'm in no position to even lift the weight of the hood. I won't discount your suggestion though, and will check it as soon as I can. However, I think I may have been simply over-expecting. Regarding the Hyundai Genesis comparison, for those who aren't familiar, the 3.8l V6 engine produces 333 hp and is a very peppy car for an entry level 4 door luxury sedan. Still though, I expected the XKR to literally blow it away...and I prematurely conveyed that notion to my wife. She's also unimpressed. :/

I've since looked up the 0-60 and 1/4 mile times of the XKR and my car is likely within these ranges. Nearly a 14 second quarter mile! Thats not what I expected. I went to the track with my 340hp Tbird once and managed to break into the 12s. Perhaps that's the one thing (other that gear swaps) that I should have researched a bit more. I assumed that the original $90k price-tag combined with the 400 hp supercharged V8 and the sleek, exotic styling would equate to a much faster car. Ie; corvette status...at least a mustang! I know...I know...this is a "GT" and not a sports car. I still think Jaguar should have made it faster!

I've seen the upper pulleys for well under (half actually) $500. Are you referring to the lower pulley as well? The only place that I've seen the lower pulley kit is at Eurotoys, and it's $1200...or $1500 for both upper and lower.

 

Plenty of potential meaning ported heads, twin screw, stroker crank, there is a ton of power that can be found, but like Doug says, it ain't cheap.

There simply isn't enough market to have a cheap aftermarket for these cars, so it all ends up being custom work.

For example: I have a twin screw and now can't even give the car full throttle in first gear, else it just goes up in smoke. If I am gentle enough that it doesn't make a one tire fire, then she short shifts into second. LSD pumpkin is built and waiting to go in the car... One thing leads to another.

It all can be done, but will cost you many times more than if it were a Ford.

 

Sounds like you jumped into the water before checking the depth. I mean, there are multitudes of road test reports that put the XKR at a mid five second 0-60 and a nearly fourteen second quarter mile.

Cheshire Blower
But who wants to play golf when this fast car is around to drive? The XKR coupe is seriously swift, bounding to 60 mph in 5.2 seconds and through the quarter-mile in 13.8 seconds at 105 mph. That's not as quick as the car we tested in England (it hit 60 in just 4.9 seconds), but it's as quick as the 0-to-60 performance Jaguar claims.

Clearly, the folks at Jaguar did not have the zero to sixty and quarter mile fixation that us folks "across the pond" have.

I too, question why they made a car with such high gearing, as if people can actually drive around at the 155 mph limited speed. Clearly, there was a different mentality at work than the muscle car concept in the states.

If you were looking for a stoplight warrior, you bought the wrong car.

Edit: As stated by others, that is not to say that your XKR cannot be turned into a formidable zero to sixty and quarter mile car, but it won't be some cheap fix.

 

Your XKR was marked as a Luxury Grand Touring, which it certainly lives up to. I never read anything in the literature about tire smoking and Drag Racing.

 

Also recall that back when the car was introduced in the mid-1990s, 300 horsepower was fairly rarefied territory and 400 was unheard of except for super exotics. The real horsepower wars came a bit later when year by year horsepower kept going up so that by 2000 or so, 300 hp was no big deal. Jaguar did not keep pace and kept the specs essentially the same for the entire 10 year model run (other than introducing the XKR). My impression is that the focus was more on cruise-ability at high speeds rather then off-the-line red light to red light performance that is the sole focus here in the states.

Doug