XJS ( X27 ) 1975 - 1996 3.6 4.0 5.3 6.0

Re-gearing the XJS V12

 
  #21  
Old 04-20-2019, 03:53 PM
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Originally Posted by Daim View Post
Out of personal experience, a larger tire size or better said larger wheel circumference will result in a slower acceleration again...

I used to go from a 215/45R18 summer wheel to a 195/65R15 winter wheel and would notice a HUGE change in acceleration between both. The 15" being at ~1.99m circumference where as the 18" was at ~2.05m circumference. The car would feel a lot more lively on smaller wheels.

It may just be easier to fit a size or two smaller tire walls and gain more that way - with a speedo correction to adjust for the difference in relative wheel speed. The effort of removing the diff and going through all the hassles of alignments and co would really be annoying if it doesn't work out...
Hi Daim,

That is an option however small tyres on an XJS just look wrong!

Small tyres definitely make a noticeable improvement but wouldn't be enough to get what I want without looking really silly.

I would need to drop to a 23.5 inch tyre to achieve the same performance! (25-50R15 would do this)

But the size difference is mammoth... (Standard XJS wheel is on the left)






Either way I need a speedo adapter.

Craig
 

Last edited by Crackerbuzz; 04-20-2019 at 03:59 PM.
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  #22  
Old 04-20-2019, 04:04 PM
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Originally Posted by JigJag View Post
Regardless of ratios and acceleration, those tires are waaaay too big for my tastes. Like large pizzas delivered in a medium box. Where will these go when you turn or go deep into the suspension?
Yeah I tend to agree... I might need to drop down to a 225-75R15 or a 245-70R15.

I will post pics withe 235-75's when they are mounted to see how it looks.

its only on the rear that I need to increase, so won't affect the front arches.
 
  #23  
Old 04-20-2019, 04:07 PM
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Originally Posted by Mac Allan View Post
Given your original insight that the car can't legally be driven at Autobahn speeds, and that most of your driving is going to be from 0 to 60mph, why muck about with the tires at all?

Either you want it significantly more lively, or you want it quiet at speed. As long as you stick to the Turbo 400, I think you'll need to pick your poison.

My guess is that if you run oversize tires, your gut will tell you that it doesn't look right and feels too boy racer for a Jaguar.

I am sure I will come to a mid point that works somewhere in between after a few trials.

It needs to balance looks, body feel, top end RPM and launch.
 
  #24  
Old 04-20-2019, 04:16 PM
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Originally Posted by Crackerbuzz View Post
I am sure I will come to a mid point that works somewhere in between after a few trials.

It needs to balance looks, body feel, top end RPM and launch.
I'm looking forward to hearing from you how the 3.54 works out. I think for a lot of 'real world' driving it could be a big upgrade in sportiness. When I used the XJ-S to drive down to Southern California, the 2.88 was awesome, but I don't use it for such long road trips at speed anymore so the 3.54 sounds kind of fun to consider.
 

Last edited by Mac Allan; 04-20-2019 at 04:23 PM.
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  #25  
Old 04-21-2019, 03:19 PM
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crackerbuzz, you got me thinking(DUH), about tires size diameter, measured my 275-35-18s and they are 25.75" inches diameter , of course wider for more contact patch area!

more contact gives better grip that relates to better initial acceleration! something to take into consideration , is a large tire weighs more and is harder to turn at a faster rate, so some time performance could be a effected !
ron
 

Last edited by ronbros; 04-21-2019 at 03:22 PM.
  #26  
Old 04-21-2019, 03:50 PM
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Originally Posted by ronbros View Post
crackerbuzz, you got me thinking(DUH), about tires size diameter, measured my 275-35-18s and they are 25.75" inches diameter , of course wider for more contact patch area!

more contact gives better grip that relates to better initial acceleration! something to take into consideration , is a large tire weighs more and is harder to turn at a faster rate, so some time performance could be a effected !
ron
ronbros...

running 3.73 gears and 25 inch tyres.

that must be brutal but awesome at the same time!!!

 
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  #27  
Old 04-21-2019, 07:55 PM
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yes crackerbuzz, its a fun car to drive , yet in 4th overdrive, it cruises great relaxed and smooth,, 2.60 final ratio, better than 2.88!
ron
 
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  #28  
Old 04-21-2019, 09:30 PM
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Originally Posted by Doug View Post
FWIW I have a 3.31 LSD in my car (Series III V12) with the old TH400 transmission. I love it.

The acceleration is naturally much better compared to the original 2.88 ratio.

We've all become very accustomed to modern cars with 4-5-6 speed transmission with overdrive ratios. Engine RPM at highway speed with this combination is a bit busy at highway/freeway speeds, yes, but the engine is at a sweet spot, just beginning to boil a bit. The throttle response is very satisfying.

Addendum

Had a reason to make a quick 100 mile jaunt on the freeway this weekend.

The 3.31 gears make this car a real blast. (And the AJ6 Super-enhanced ECU helps a lot, too !) Particularly, 60-to-85 mph is especially satisfying. Just squeeze the throttle and the car really takes off! Especially useful for overtaking slow pokes. A V12 Jag belongs at the head of the pack, after all. Choking on the exhaust of those snotty Bimmers and Audis just ain't right . If I push a bit harder and the trans kicks-down, well, it's almost like a muscle car.

Also, with my low-restriction exhaust, there's a subdued but very satisfying growl. No droning; just a slight snarl. I found myself turning the radio down, even on my favorite songs, just to hear it.

Had to share. I'm a bit chuiffed as you can tell!

Carry on!

Cheers
DD
 
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  #29  
Old 04-21-2019, 09:36 PM
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Originally Posted by Doug View Post
Addendum

Had a reason to make a quick 100 mile jaunt on the freeway this weekend.

The 3.31 gears make this car a real blast. (And the AJ6 Super-enhanced ECU helps a lot, too !) Particularly, 60-to-85 mph is especially satisfying. Just squeeze the throttle and the car really takes off! Especially useful for overtaking slow pokes. A V12 Jag belongs at the head of the pack, after all. Choking on the exhaust of those snotty Bimmers and Audis just ain't right . If I push a bit harder and the trans kicks-down, well, it's almost like a muscle car.

Also, with my low-restriction exhaust, there's a subdued but very satisfying growl. No droning; just a slight snarl. I found myself turning the radio down, even on my favorite songs, just to hear it.

Had to share. I'm a bit chuiffed as you can tell!

Carry on!

Cheers
DD
Haha... Nice Doug.

Is that the XJS or the XJ12?

That's exactly what I'm trying to replicate. These cars really need to be geared shorter as 300HP back in the 80's was huge but too subdued for fun driving.

I have done the exhaust and already paid the deposit for the ECU and the throttle bodies from AJ6.

The ol' girl is getting a birthday!

 

Last edited by Crackerbuzz; 04-21-2019 at 09:38 PM.
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  #30  
Old 04-21-2019, 10:16 PM
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Originally Posted by Crackerbuzz View Post
Haha... Nice Doug.

Is that the XJS or the XJ12?
XJ12

An XJS is a couple hundred pounds lighter (in coupe form, at least) so the results should be even better

Cheers
DD
 
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  #31  
Old 04-26-2019, 02:53 AM
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.

Can't say I have an issue with my choice. Now I just need to install the diff.

I know - White lettering...

But man I really like the stance on this. After all let's get real. 300HP 5.3L V12.

It's not a limousine in all reality, its the closest thing the UK has got to a muscle car and they handle far better than a muscle car.

I know some of you will think otherwise, but I think the XJS has so much natural potential without installing american junk onto them to make them go fast.


 

Last edited by Crackerbuzz; 04-26-2019 at 03:49 AM.
  #32  
Old 04-27-2019, 03:39 AM
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Well, ypu rounded that up quite well... From 268 to 300 hp... I think my little Fiat Spider now does 320 hp. Just rounded it up from 140 hp...

Btw, there are loads of UK cars with muscle... And I mean V8 muscle. Vauxhall Monaro, Jensen Interceptor, Bentley Arnage T, Rover 3500 Twin Plenum Vitesse, of course the XJR models with supercharged V8s, the AC Ace, Morgan 8 models, etc. The XJ-S is just 'Sophisticated Muscle'
 

Last edited by Daim; 04-27-2019 at 03:41 AM.
  #33  
Old 04-27-2019, 04:04 AM
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Originally Posted by Daim View Post
Well, ypu rounded that up quite well... From 268 to 300 hp... I think my little Fiat Spider now does 320 hp. Just rounded it up from 140 hp...

Btw, there are loads of UK cars with muscle... And I mean V8 muscle. Vauxhall Monaro, Jensen Interceptor, Bentley Arnage T, Rover 3500 Twin Plenum Vitesse, of course the XJR models with supercharged V8s, the AC Ace, Morgan 8 models, etc. The XJ-S is just 'Sophisticated Muscle'

Agreed Daim, but the NZ/Australian version is actually 299Hp as far as I am aware. No cats here down under.

The Vauxhall Monaro is also actually an Australian bult Holden Monaro. I have also owned a 450HP V8 LS1 Holden HSV Senator 6 speed manual so know exactly what you mean.

I have had the new stuff but just isn't the same as the XJS and I am a bit smitten at the moment. Hope it lasts!

The XJS is so different . Raw, 60's/70's DNA and somehow home-made and hand stitched. V12 old school muscle in my eye.

Craig

 

Last edited by Crackerbuzz; 04-27-2019 at 04:19 AM.
  #34  
Old 04-27-2019, 07:11 PM
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Aussie version was rated at 213kw (284HP) but this was rated with 91RON fuel for a Marelli car. According to my owners manual all cars were delivered with the strategy link not installed so in the 91RON position and you had to see the dealer to have it changed to 95RON. I know my car did not have the strategy link installed when I got it. I am the 3rd owner and have all the books and receipts. It is reported that with 95RON power was 220kw (293HP).
 
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  #35  
Old 04-27-2019, 07:28 PM
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Originally Posted by Crackerbuzz View Post
.

but I think the XJS has so much natural potential without installing american junk onto them to make them go fast.


I love this XJS in this color and the white letter tires looks very vintage in a good way. Have you ever driven a Muscle car or an XJS with a well done modern Chevy V8?. I think I know the answer...I suspect if you did you would feel different. Last week I had the occasion to drive a XJS with a 2015 Silverado engine with a 6 speed auto. It was a great driving experience and far smoother and more refined than a 90's V12 by far. The latest crop of pedestrian GM truck engines can be purchased for a very reasonable price and offer world class power and refinement. I presently have 2 V12s in my possession to use as a direct comparison. By contrast my 1994 6.0 XJS with the 4 speed auto felt lethargic and anemic... and thats fine, its how it was built and it represents the technology and choices Jag engineers made at the time.

As an american I take offence to the "American Junk" line. I understand that when you diminish others in the presence clear greatness you're actually only diminishing yourself. Simple and reliable American push rod motors have proven their prowess and competitiveness for decades. Soon a new generation of American V8s will become the envy of the world as GM gets ready to make their next aluminum V8 with 4 valves, high HP and low weight. Im sure history will repeat itself and it will be shipped all over the world and find its way into the cars and garages of guys of modest means offering up great satisfaction along the way.
 

Last edited by icsamerica; 04-27-2019 at 11:25 PM.
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  #36  
Old 04-27-2019, 09:25 PM
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Sorry Icsa my bad... I didn't mean anything bad by saying American Junk. This was a poor use of words. It was meant to be more of a metaphor. (Maybe I should edit my post)

My other favourite classc car is the 70 Chevelle 396 Big Block and I owned (and loved) a 2004 Holden HSV with a GM 350 Chevy engine, so yes very familiar with American V8s

I purely meant not adding American stuff to a Jaguar.

I am not normally a fan of engine swaps from one manufacturer to another as it seems a bit mongrel to me. I would rather own one of each!

Cheers for you comments though.
Craig
 

Last edited by Crackerbuzz; 04-27-2019 at 09:49 PM.
  #37  
Old 06-12-2019, 09:32 PM
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Finally found a gap in my mechanics schedule!

Needed a hoist for this one.

IRS removed,

New pumpkin going in...


 
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  #38  
Old 06-12-2019, 09:36 PM
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Changed rear tyres to a 235-70R15 tyre as the 235-75R15 looked a bit stupid. (Like a chick with big botox lips!)

The rear tyres are now 30mm (1.1'') Taller than standard.
 

Last edited by Crackerbuzz; 06-12-2019 at 09:42 PM.
  #39  
Old 06-13-2019, 12:57 AM
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Craig - looking forward to the update.
Love to do the same... one day, and will be asking you questions when the time comes.
Great work!!!
 
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  #40  
Old 06-13-2019, 01:05 AM
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Originally Posted by Dukejag View Post
Craig - looking forward to the update.
Love to do the same... one day, and will be asking you questions when the time comes.
Great work!!!
Duke, I do the have the option to get my hands on another 3.54 but I think its an open diff.

Cheers
Craig
 
 
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