Thursday, 8 April 2010

XJ40 Tuning And Modification Guide

I thought I'd write a piece on XJ40 tuning and modification as I'm carrying out some of this development on my own car and have done some researched on the subject. I'm by no means an expert, just a DIY enthusiast, but hopefully some of this info is useful to those considering similar modifications.

The '40 isn't a performance or sports car as such, it's a large and heavy barge of a car so there obvious limitations to the acceleration and handling capability. It is however a powerful, large engine car with good handling, perhaps like a more refined British equivalent of an American muscle car in some ways. They can be bought very cheaply here in the UK, and can be relatively fast and a lot of fun to drive. With the XJ40's appealing aggressive, retro looks and the Jaguar “grace, pace and space” experience there are few cars that can rival it for value in my opinion, and it can be further improved with some choice performance mod's if your so inclined.

Most of the parts I mention are OEM items fitted to different XJ40 models so can be picked up cheaply second hand these days, my own project car has a tight budget so used parts are important. This piece is a bit of a work in progress, I'll keep updating it as I remember and find out new information...


Obviously the aim of engine tuning is to produce more power and ideally torque. Requirements will depend on what the power goals are and whether you are trying to put together a fast road or race engine. The aim for my car is a cheap, mildly tuned fast road engine so this is primarily what I will be discussing.

The American's have a saying, “there ain't no replacement for displacement”. Obviously with all things being equal a larger capacity engine will be more powerful than a smaller one. The straight six AJ6 engine comes in four flavours, 2.9, 3.2, 3.6, and 4.0 litre. The 2.9 is pretty much a boat anchor, it has the 12V SOHC head from the V12 and is weak in a big car like the '40. The 3.2 replaced it and was a big improvement having the 24V DOHC head, and it pulls the car capably, though it's still a bit weak as a performance proposition. The 3.6 and 4.0 are bigger and better in power terms, the 4.0 having a small increase in power but a decent increase in torque due to a longer stroke crank. The 3.6 is perhaps more revvy in nature and the 4.0 is possibly more prone to head gasket failure. I went for a 4.0 car for my project as I felt this was the best starting point, though the larger AJ6 engines are bolt in swaps for the smaller variety.

The AJ6 engine is a very tough unit with no real fundermental weaknesses. The block was orginally designed with a diesel in mind so is very stiff, the whole bottom end stands up well to intergalactic mileages (with appropriate servicing) so doesn't really need improvement. If extra strength is called for then the steel crank from the AJ16 (X300 engine) could be used in a rebuild, or the complete AJ16 engine perhaps. In addition to the AJ6 engine Jaguar also produced a 6.0 V12 for the XJ40 from 1993, a mighty and complex engine with famously smooth power delivery. There are various ways to tune this engine including alternative cranks and/or pistons to increase capacity, most of which are expensive and beyond the scope of this article!

As far as NA tuning of the AJ6 goes, Jaguar already did some of the hard work with the series 2 Jaguarsport XJR. This model had a longer duration inlet cam and and a higher compression head, 9.75, up from 9.5. A twin plenum inlet manifold was also added to improve breathing at high rev's, plus some exhaust changes that I'll talk about later. To take advantage of these changes a modified map was squirted into the ECU. The result of all this was an increase in power from 235 to 250bhp which may sound moderate by does result in a perceivable improvement. If a 4.0 engine is being tuned it may well be worth seeking out these engine parts as although quite rare they can be fairly cheap second hand, complete XJR engines can cost around £200.

There are the usual aftermarket air filters from the likes of K&N and Piper, direct replacements for the standard airbox panel filter are available or it can be replaced with the cone type. If you have the money or the skill and tools yourself then port matching and gas flowing of the head can give good improvements apparently. In addition, a company called AJ6 Engineering do bored out throttle bodies and modified inlet manifolds for additional small improvements to breathing.

If large gains in power are sought then forced induction is the way to go. Rare Janspeed, Turbo Technics and Chasseur versions had twin turbo charged engines so are the amongst the most powerful '40's made. The later X300 ('94 to '97 XJ) XJR was fitted with a supercharged AJ16 engine which is based on the XJ40's AJ6. This engine should be a straight fit into an XJ40 though sorting the engine management could take some significant work as it uses a different ECU. These engines can be had for around £500 in the UK so it's feasible on cost grounds and is currently a bit of a pipe dream for my car.

The air conditioning compressor runs off the engine, getting rid of all the air con equipment will gain some power and save a bit of weight.


To help put the power down, XJR's, V12's, 'S' models and cars with the sportpack came with LSD's which can be found second hand. The vast majority of '40's had the ZF 4 speed automatic gearbox, the 4.0L cars had the better electronically controlled version that has the more aggressively shifting “sport” mode feature. The sport mode box can't be fitted to a smaller engined car due to them having a different ECU. A small number of cars, often the 2.9's, were fitted with a Getrag 5 speed manual gearbox, not a popular option new but quite sought now. These manual boes can be fitted in place of the auto box but they are not usually that cheap to buy due to demand for their use in other Jag's such as modified E Types, racers, etc. Between around £200 to £500.


As standard the '40 has a twin exit stainless steel exhaust system. Following the UK emissions law change in 1992 the cars were fitted with a central catalytic converter plus pre-cats in the two branches on the downpipe.

Removing the central cat from later cars should help the exhaust flow more freely improving responsiveness and power somewhat. Earlier cars that pre-date the emission regulations have straight through centre sections that can be fitted to later cars in place of the centre cat. The series 2 Jaguarsport XJR has a non-cat downpipe that retains the boss for the lambda sensor so this can also be fitted to cat equiped cars to completely remove all cats. Bear in that the MOT requires that post '92 cars pass an emissions test, so a cat will probably need to be in place for this.

The XJ40 exhaust has two silencers per each side of the exhaust to keep things refined and quiet, a pair of silencers could be removed though by fitting straight through pipes, AJ6 Engineering make some off the shelf items for £150. Larger diameter cat-back systems can be fitted though I think this would only really be necessary for a race engine or a car with forced induction, unless your just after a more throtty sound! Companies like Paramount Performance and AJ6 Engineering make a range of performance exhausts.


The XJ40 has great suspension technology which lead to the car being highly praised for it's magic carpet ride and good handling. At the front independent double wishbones are employed with an anti-roll bar, and the coil springs isolated from the body by a foam filled subframe. At the rear, double wishbones again are used with the drive shafts forming the top wishbone in order to reduce unsprung weight.

The XJR, 'S' versions and cars with the optional "sportpack" handling kit have uprated -15mm springs, firmer dampers, plus a thicker anti-roll bar. I think that a 4.0 car with the sportpack spec is a good basis for a tuned car, though these items can be used to upgrade a '40 with standard suspension. Some of the more opulent models had self levelling rear suspension, this was unreliable and even Jaguar recommend that it is decommissioned in favour of the conventional springs. A further upgrade to the thicker than standard sportpack anti-roll bar is to fit an even thicker one from an X300 XJR.

To further lower the XJ40 lowering springs are available from Avo at the cost of around £200, or custom springs can be made to suit for a similar amount. Dampers can be upgraded, the XJR's had Bilstein dampers which are available for about £80 each, Gaz and Koni make adjustables, the Gaz items can be had for £60 each.

One of pet dislikes about XJ40's is that they seem to eat suspension bushes, especially earlier cars. Luckily several companies produce polyurethane bushes, these give better suspension control and handling at the expense of some ride comfort plus last a lot longer. Polybush do a kit for doing most of the front suspension for about £160, Powerflex produce front wishbone and lower damper bushes, Superflex also make a selection of front and rear bushes. I found a company selling on ebay called Jagbush who do front damper top bushes plus rear damper upper and lower bushes for about £35 so between these companies the pretty much the whole suspension can be polybushed if desired. In my experience the Polybush items are softer and more compliant so will maintain the ride quality that the XJ40 is renownd for, the Powerflex items are harder so will give more control at the expence of a firmer ride The front and rear subframes are located to the body with large bushes, these can be upgraded with the stronger, stiffer ones from an X308 ('97 to '01 XJ).


XJ40's came with a variety of wheel styles with 15” and 16” rim sizes, having a 5 x 120.65 PCD, a 35-45 offset and a 73.9 centre bore. The metric wheels on early cars are difficult to get reasonably priced tyres for so it's a good idea to swap to imperials. The XJR's had wider 16” Sportline wheels with lower profile Pirelli P6000 tyres, they have a nice sporty period look about them. The later 'S' models had a 16” five spoke design which look good and meaty.

Wheels from newer Jaguar models also fit and can come in larger diameters. The X300 XJR had a nice chunky 17” five spoke design, the X308 XJR had several 18” wheel styles, wheels from the XJS and XK8 also fit. For those who want to go even bigger and have money to burn there are 19” and perhaps even 20” aftermarket wheels out there that'll go on. Several other non Jaguar car models have very similar wheel fitments so can provide wheels, the BMW 5 Series and Holden Commodore are two examples.


If well maintained the brakes of the XJ40 are very good, though there are various products out there that can be used to improve them. Companies like EBC make drilled and grooved discs that help dissipate heat and brake dust, they cost around £150. Aftermarket pads are also out there, like EBC's so called Greenstuff and Yellow stuff pads for fast road driving. Goodridge make braided brake hoses, these don't expand like rubber ones can so improving feel, plus they look quite cool too. If the brakes wish to be improved further then those from an XJ40 V12 will provide an upgrade. Going further, big brake aftermarket kits are out there if you've a couple of grand to part with.

The brakes on early '40's have a vacuum pump that ran off the engine, this uses up a couple of bhp's. Brakes on the post '90 cars were driven with an electric motor. Alternately, the braking system can be binned off in favour of a conventional servo set-up from the series 3 XJ.


I think the two main guises of the XJ40 are the Sovereign and the XJR. The Sovereign had the prestigious luxury appearance, the XJR's had the sporty forty look, and there were various models between these in the spectrum. Which look you have is obviously dependant on what car you have, which is preferable is down to personal taste. I'll detail all the sporty appendages here that can be source second hand and used to modify the look of the car to a more sporty one.

There were two styles of OEM bodykit, the TWR series 1 XJR type and Jaguarsport series 2 XJR type. The TWR series 1 XJR type has GRP side skirts and body colour GRP bumpers that replace the rubber and chrome ones usually fitted. The Jaguarsport series 2 XJR had more sculpted side skirts, rear side spats under the conventional rear bumper, plus a front spoiler / splitter under the conventional front bumper. These bodykits can be found for around £100 or so secondhand, the bumpers are around £150 each new.

The standard car had chrome look grille vanes in a chrome grille surround, some later models had vanes that were sprayed black or body colour. The series 2 XJR had a single piece “egg create” type black plastic grille insert, with a recess for a “Sport” badge. Series 2 XJR's and the post '93 'S' models had a body colour plastic boot infill panel which brought the inset space between the rear lights flush, these cost around £30 secondhand. Series 2 XJR's, 'S', and V12 models amongst other had the black non-chome window frames, the Sovereign's and Daimlers had the chrome items, all can be swapped about.

The Sov's, series 2 XJR's and Daimlers had the square lights which can be swapped for the more popular round lights fitted to the other models. The complete light assembly must be changed including the relay modules. The series 2 XJR had grey rear light lens, the Sov had chrome rear light surrounds which brighten the rear up a bit and look good.

So called “leaper” bonnet ornaments can be ftted by drilling a couple of holes in the bonnet, the non-spring loaded ones are apparently illegal.


The best spec interior came in the Sovereign, XJR, V12 and Daimler models, plus the rare Majestic version. Some of the cheaper models had cloth or part leather trim which can be swapped for leather. The post '93 cars had redesigned doorcard, the two styles can be swapped between if the mounting brackets are changed over and the electrics rewired.


soniq boom said...

Great article, I always look fwd to see your progress on your 4.0s I think you are doing a great job making one great car out of two with the warmer months coming up over there you should really make good progress

Kieran said...

Thanks, your comments are much appreciated! Yes, I'm looking forward making some good progress soon, I want to get cracking with polybushing the suspension as that's the biggest job that needs doing.

soniq boom said...

When you do please go into detail, as I need to travel that road as well and would be very appreciative to see how and what products you use to accomplish this major task good luck!

Kieran said...

Thanks. I'll be going into detail over a number of blog posts as there as obviously various tasks involved in the suspension rebuild. Hopefully the reports of my experiences will be helpful to fellow XJ40 owners out there.

Anonymous said...

Keep up the good work , especially the how to do it pictures,helps take some of the mystery out of jags, as you have probably learned by now they're just like big meccano really.

I'm glad to see that youngsters are beginning to realise the joy of fixing up an old classic, it does seem as if young lads have tied themselves to expensive finance to buy cars that cost a fortune to insure and don't realise the fun to be had from creating something personal and really learning how a car works.

Have your fun with these cars now, by the time you are my age they'll probably be banned or severely restricted due to the environmentalists .

Kieran said...

Thanks for the positive comment. Retro-classic cars like the XJ40 are good fun restoration projects for sure, they cost next to nothing to buy and most parts especially used ones are very cheap. Classic car insurance is very cheap too, less than £200 a year for this car.

A lot of young lads do get into the whole expensive finance and insurance thing as you say, I have the oppostite mentality and believe spending a modest amount on an older car that's more unique is much preferable.

I do hope they'll be no stupid bans or restrictions on older cars, it's much more environmentally friendly to keep an old car on the road rather than throwing it away and buying a new one!

CGS cat back exhaust said...

There are always a lot of considerations to be taken into restoring cars. This is a great guide.

air ride kits said...

Very interesting to read. You have worked in it nicely in your car. Love how you did it mostly on the suspension system.

Sara Anthony said...

The engine, the tires, the chassis, these parts are the ones that are modified for about 90% of the time, some even to the point of tricking out their car to run at par with the fastest cars. However, the brakes should also be given a lot of attention to ensure our safety.

Sara Anthony

Nikki Benz said...

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Car remapping said...

Great Post !! I like your post.You have sharing awesome information about car jagur project.Thank you so much !!

unitedrestorations12 said...

Thenks for shearing this post there is very nice information in your post.
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watahyahknow said...

wat we could realy use is links to aftermarket supliers for the parts that are no longer availanle at the dealer like bearings and pistonrings

Shaun said...

What a well informed interesting read! Plus the help from the comments, thanks Nikki Benz! I'm currently searching for information on tuning a 1996 Jag t getting lost in contradictory advice, can anyone help?