I only got my XJ-Sport 4litre today and am impressed with it's performance but a little puzzled. From about 50mph it's thrust is fantastic all the way past 100mph.


However getting it to 50mph it's still good but it feels like it's holding back. With all that power it ought to burn rubber if given too much gas. Other powerful autos I have owned would do.


Is it possible the car is out of tune?
Is it more likely that traction control is just being careful?
I do feel it's holding back but I also know the previous owner would not have minded this. I am being quite demanding.

 

It's gearing is what's holding it back, zero to 60 in these things is pretty sad. Even with Andys bracket. It's a HEAVY car with highway rear gearing. Quick Zero to 60 and tire spin can be had in the XJR but not in the normally aspirated version.

 

It's interesting what you say about gearing. I suspect that the ZF keeps it in too high a gea. 1st must be low enough if it was used. When I watch the evs they can got pretty high once going at high speed and that's whee the power seems to be, at high speed. I find this very interesting as the gearbox has electronic control which I am sure can be told to work differently.

If more power is needed then it's at the lower end of the rev range. Everyone always does things to increase power up the top end but maybe this is backwards?

 

The Sport switch makes a huge difference. It allows the revs to climb to the max before changing up. Clearly this is the best car I have had for motorway acceleration. To accelerate fast when you're already going 60 is very impressive. I am pretty sure the car is accelerating faster at 60 than it is at 50. I am still not bored of losing Audi A8s when they are up my bumper. I dunno how the previous owner drove but he only got 19mpg according to the dashboard computer. I am getting 22mpg on the computer.

 

The sport switch also let's the XJR start in first gear versus second from what I understand, which also makes a difference.

I too find that the XJR rolls on highway speed with what seems to be more power than a standing start. Again, gearing is the key I'm sure.

My average MPG is a solid 18.5. I see very low double digits, sometimes singles instantaneous around town, and mid 20s on the highway.


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I am a little confused that the car would simply ignore first gear in normal mode. I find it very interesting that you get the impression that your XJR accelerates faster from rolling. With that Supercharger you are making more power at 2000rpm than I am and this is where the revs are when you launch from standstill.

Having more power at 2000rpm is one way to make this car accelerate faster from standstill. The other way would be to get the revs up to where the power is which seems to be 4000rpm.

I just love the way when in sport mode the revs top out in one gear then it changes up and the revs drop to 4000rpm and you get this amazing push! Seems to be around 70mph this happens I think. Then it blasts to 90 and does it again going past 100. By this stage anyone who thought they could keep up is half a mile behind. Gotta love these cars.

 

Keep in mind we are also talking about two different transmissions, and likely rear differential gearing too.

The XJR uses the GM 4L80E and has taller rear gears, so the experience will be different than an XJ6 Sport.

Regarding the first versus second gear start, this has been discussed a few times in this forum and in the X305 (XJ12) which also uses the same transmission and has the S/N switch.
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FWIW my '95 XJR started off in first gear in both S and N modes. And, under WOT, the trans upshifted at exactly 5500 RPM regardless of which mode was selected.

The difference between modes was limited to upshift point at low-and-medium throttle openings. It was quite a noticeable difference, however.

Not sure how the ZF is calibrated and what is/isn't to be expected.


Cheers
DD

 

You'll unquestionably know when the traction control is in play. The throttle is mechanically over-ridden. You can actually feel the pedal pushing back against your foot.

Cheers
DD

 

How can I tell by looking under the bonnet if I have traction control. Looking at the throttle cable pully I can see a spare pulley with nothing connected.

The quote about when the 1995 XJR changes gear is interesting. In normal mode mine changes up at about 2000rpm when driving normally, perhaps higher when peddle to the carpet. However in Sport mode full throttle the revs got to 6000+ before changing. My car has less power before it changes up to after it changes up. It goes something like, 6200rpm then 4000rpm to 6200 then 4000. It really pulls at 4000.

Increasing power at the top and making sure it selects 1st on launch would improve 0 to 60 massively. Currently 8 seconds but I am sure it already has power to do 7 if only it would shift perfectly.

 

I see the XJ-Sport 4litre 244HP has a 3.58 diff and the 3.2 litre 211HP has a 4.0 diff.
Book acceleration in the 4litre is 8 seconds and in the 3.2 litre it's 9 seconds.
They have mostly overcome the difference in power with the diff.

So if I fit the 3.2 Litre diff then my car will accelerate faster. What do you think? Has anyone done this?

 

Yes. Owners have swapped the 4.0 diff gears for 3.2. I personally replaced the diff gears in my XJR to the lower 3.58 from a non-supercharged XJ6 4.0. Off the line acceleration is noticeably quicker, but the downside is the slightly less economical higher rpm top gear cruise.

 

Some my XJ6-Sport 4.0 has got the Diff that an XJR6 owner would want and a XJ_Sport 3.2 has the diff that I want.

Did you measure the difference in acceleration and MPG?

Mine is 8 seconds to 60 and 20 to 22MPG

 

Apples and oranges comparison. I have a supercharged XJR and you are looking to improve a NA XJ6.
0-60 in my car was about 6.5 seconds when I purchased it. Since then I have replaced the diff gear set, fabricated a cold air intake system, replaced the ignition coils, installed Pulstar Plasma plugs, replaced crank position sensor bracket with Andy's 5 degree advance bracket, and removed the secondary catalyst replacing with dual 2.25" including an X pipe. 0-60 was down to 5.7 seconds average and average mpg dropped to 17.
I recently replaced the 255/40ZR17 Goodyear Eagle F1 tires with larger diameter 245/50R17 Continental touring tires. Now 0-60 is up to about 6.2 and average mpg is hovering around 20.
That said, the 5.7 seconds was probably truer to 6 flat because the smaller than stock diameter tires and lower gear set were throwing off the speedometer. Meaning when it indicated 60 I was probably only moving along at 50 something.

 

You've done a number of things to your car that could work on mine. On further tests I see in sport mode my gear shifts are at 5500 rpm not 6000 which I had thought I had seen. Getting good at the J shift in normal mode is satisfying because I can down shift better than the auto because I can see the corner coming and be in the right gear to power out *before* I open the throttle.

However I can tell by the time the engine hits 5500 it's lost a lot of power compared to the power it then gets at 4000rpm when it up shifts. Sorting out spark plugs, coild and the Andy bracket would improve that.

I just love how sure footed this car is.

PS: The speed sensor is on the wheel not the transmission I thing so only the tyre diameter affects the speedo. I don't expect your speedo is that far out. Maybe find one of those speed signs that warns you how fast you are going and drive past it and compare.