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Jaguar XJR

Published: 16th Mar 2015 - 0 Comments - Be the first, contribute now!

Jaguar XJR
Jaguar XJR
Jaguar XJR
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Jaguar’s XJR has sharp enough claws for many but this classic cat still has a lot more to give. We show how!

If anything tangible came out of the underrated and ridiculed XJ40 saloon, then it’s the exhilarating XJR which was the flagship of the revised and much improved X300 range. It’s no exaggeration to say that this super supercharged sports saloon is the modern alternative to the classic Coombs Mk2 yet is much faster as well as cheaper to buy and own. Also, there’s an enjoyable, affordable and competitive racing championship for such Jags where you can even use a road-going car and have the best of both worlds.


The XJ40 was a fantastic design sullied by reliability issues which the X300 revamp only partly alleviated. Rust can be terminal although XJRs seem to be better looked after. Major rust can break at the A-pillar rot spreading to the sills and front inner wings. Front and rear shock absorber top mounts can wear, as can front A-frame bushes. These will ‘clonk’ by way of warning, and should be replaced. Headlamp modules and heaters have been known to give up the ghost, and electric window, dash and central locking issues are par for the course.

Mechanically, so long as the service history has been kept up (but many aren’t) things fare better. Early V8 suffered from abnormal bore wear (does it start up from cold okay?) and timing gear breakages mean a £1000 bill (fit beefier 4.2 items while you are at it). The six is robust but head gasket failure isn’t unknown yet on all, the supercharger is reliable. Brakes and suspensions take a pounding mind so check for worn dampers, bushes, discs etc, all of which will probably be replaced during the uprating process anyway.


Simple mods first. Bin your aircon system if you are going track day or racing. The compressor is driven by the engine, so by ditching this and you’ll see a small increase in power and economy.

Another cheap and easy mod is to have the engine thoroughly checked on a rolling road to ensure it is in good health and you’re getting all the supercharged horses. A good cheap power increase can be had by having the engine ‘super decoked’ by TerraClean – the process carried out at centres or mobile really works for just over £100.

The first tweak on a tight budget should be a better air filter system from the likes of K&N or Pipercross; noise may go up but so should power, especially if this is complemented by a performance engine chip for the ECU from the likes of Superchips.

AJ6 Engineering knows this straight six engine well and is a leading expert on the unit. It offers modified induction systems and ECU reprogramming; 10-15 per cent performance gains are claimed with this mod. At around a grand, the company’s Plus Torque kit, is said to offer 30bhp over standard and 13 per cent extra torque.

Having got more in you need to get it out and the likes of Elite and Performance Jags and AJ6 Engineering market a variety of performance exhausts, although the latter adds that cylinder head tuning should only be done if track days or racing is in mind as the benefits for pure road use aren’t that great for the costs involved.

A much easier ploy is ‘fooling’ the supercharger to provide more boost by way of a smaller pulley making it spin faster, although speak to a specialist first as some don’t like this particular tweak on both the six and especially V8 due to housing mods.

Ah yes the V8! Kicking out 370bhp in standard tune, do you need extra grunt if it’s been set up properly? Do the basics first! After similar induction and exhaust mods to the ‘six’, you can then start to tweak the supercharged unit. Paramount Performance offers a special induction kit at around £300 which is said to up the ante by more than 10bhp for starters. Spend ten times that the company promises in excess of 400bhp by way of better supercharger cooling and a sportier exhaust. This is superior to the 4.2 unit fitted to the XKR (400bhp and 408lbft of torque). Before you ask, while fitting the brawnier 4.2 sounds logical enough, it’s not as easy as it sounds due to ECU and transmission issues you’ll encounter.

Making a supercharger spin faster for added poke works exceptionally well on the Jaguar V8. Racing Green Cars says that, along with better breathing and its sports exhaust, an additional 50bhp and 60bhp is unearthed, with another 20bhp on offer if the engine’s brain, the ECU, is modified to suit.

The company uses a larger crankshaft pulley rather than a smaller supercharger pulley to reduce the chance of belt slippage at high speed plus – unlike some of the other performance pulleys you see for this engine – fits, works and is reliable, it adds…

From America, a Kenny Dell supercharger upgrade can see as much as 650bhp from the V8, and the block can be stretched to as much as 4.8-litres!

What about engine swaps? There’s some scope. For example, you can upgrade a normal XJ40 with the XJR engine reasonably easily enough as it uses the same engine crossmember but the ECU and its gubbins need fitting but you can use an aftermarket alternative. Substituting a V8 is much more involved however and probably not worth the effort given the prices XJRs still sell for.


Let’s talk transmissions first. If you want a manual gearbox then the earlier straight six XJR is the car to have although technically you can adapt an Aston self-shifter for the V8. Elite and Performance Jags is known for such a swap, but admits interest has virtually vanished due to the sheer cost – now at over £10,000 – more than what a top XJR sells for! What you can have instead, from Racing Green Cars, is a sequential ‘Speedshift’ fitted, which operates the J-gate selector manual override via steering wheel paddles. At £1554 fitted it’s a much more viable proposition although still a luxury for most XJR drivers.

You’ll probably welcome a limited slip diff to complement the standard traction control to harness the added horses. Produced by Quaife to Racing Green Cars’ specification, the ATB (Automatic Torque Biasing) differential is a good solution.

When it comes to improving the handling then you’re on a sticky wicket because while you can make an XJR handle better than Jaguar did, you’ll also find the factory made a darn good job of it initially and without spoiling the serenity Coventry cats are famed for. Unless you are specifically making a track car your first step is to make sure everything is A1 – dampers, bushes geometry, the lot – before embarking on improvements where good quality tyres recommended for the XJR has to be the first step.

Uprated shock absorbers (AVO come recommended by West Riding Jaguars who prepares XJR race cars – 01924 494400) and springs are next; you can lower the car by 25mm but the ride suffers for road use and screws up the geometry settings, which West Riding Jaguars says has to be addressed or it will eat road tyres quickly and hinder handling.

Polybushing the many bushes is doable and certainly effective but be careful here warns Elite and Performance Jags because if you over do it the car will be terrible on the road tram-lining everywhere although West Riding counters this by saying budget tyres are usually the main culprit for this trait.

Ultimately, it depends on just how much track work you intend to do. The XJR can be made into a great racer or road burner but you can’t have it both ways!

When it comes to brakes, it’s worth finding a donor XJ40 V12 (also known as XJ81) because the front hubs and brakes are an invaluable asset for uprated cars due to their twin-piston brake calipers – but only on the early six-cylinder models. Fast road pads such as EBC ones along with braided brake lines for a firmer middle pedal are good, low cost mods. Further uprating can be done with Brembo or AP hardware but such rapid, reliable retardation doesn’t come cheap.

That’s not quite the end of the story however because there’s lots of little tweaks and tips that the racing teams use to make their cars quicker than the rest. Extracting the info out of them is another matter entirely so you’ll have to find your own – the hard way!



* AJ6 Engineering’s High Flow or High Torque kit

* TerraClean treatment

* Aftermarket pulley for supercharger on all engines


* Paramount Performance upgrades

* 4.2 engine alternative

* Hotter cylinder head and camshafts

Handling The Power...


* Dampers and springs

* Harder compliance bushes

* Anti-roll bars and wheel spacers

* Quality tyres


* Manual gearbox (V8)

* Speedshift fitment

* Limited slip diff (Quaife spec)

* V12/XKR brakes (Six cyl only)

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