How to make an old Jag fast.

I have had a lot of questions about setting up old Jags for race and track day action, so here are a few answers for those that feel the need to experience the glory of a V12 on track.

 

The starting point, an eBay XJ-S V12...

The same basic principals apply to both XJ-S and XJ6/12 cars. First up which car to go for.

The V12 in 5.3 form is fantastic on the race track with a 6500rpm rev limit and over 300bhp readily available, although the standard cooling system is dire. The first V12s had flat cylinder heads and can be tuned up to over 600bhp (at great expense), later models had the High Efficiency HE heads which limit power but drastically improve fuel ‘economy’ and is still good for well over 400bhp. Early cars had a 4 speed manual gearbox as an option but these are hideously expensive, most have the immensely tough GM TH400 auto box which can also make a good race box when fitted with American drag racing parts. I raced an auto with manual override and it was superb. The last XJS cars had the 6 litre V12 with a 4 speed 4L80E auto which can be modified to work in manual mode either mechanically or electrically (paddle shift style), but these cars are rather expensive.

The 6 cylinder engines in the XJ-S were either the AJ6 in 3.6 or 4.0 forms or the four valve AJ16 version of the 4.0. All are powerful with over 300 bhp quite feasible. Available with either the 5 speed manual or the 4 speed ZF auto, again the auto can be modified for racing but the manual is a simpler option.

The final version, it had a 6.0 litre V12 and an extra gear to play with.

TWR offered a manual version available through the official Jaguar dealer network, this used the Getrag 260 gearbox as found on larger BMWs of that era, but as the gearbox has an integral bellhousing you will need an adaptor plate to fit a non-v12 sourced box.

 

Basic track mods:

Brake pads, race brake fluid, jack the bonnet open an inch and fit good tyres. Give it a full service and off you go!

 

The more complicated version:

All cheap cars are rotten, so plan for welding. The front subframe which holds the engine up and holds the suspension on rusts from the middle out, good second hand ones are over £250 and quite a big job to change, so make sure you get a good one. The smaller cross member under the radiator also rots out but can easily be replaced with a strip of suitable metal and is not a deal breaker.

The office, and a very nice place to be when doing 150mph down the back straight at Silverstone.

The front of the sills rots behind the ally splash guard but is reasonably simple to repair. The back of the sills is a very complex construction and includes the rear axle radius arm mount, this is a sod to rebuild but for racing it can be simpler to just cut the whole lot out and weld in a simple sill and convert the radius arms to a ‘cotton reel’ bushed rod that is mounted into a fabricated box intruding into the rear passenger foot well. This mod cuts out some weight too and also improves axle location.

When viewing a car pull up the rear seat base, rain water leaks in from the quarter light seals and pool in the seat base/ inner sill area. The race car solution to a rusty bottom is to cut out the set base and weld a simple plate over it. Also check the front foot wells where water from leaking screen seals can pool and rot the floor. Many cars have been undersealed which is unhelpful as the rot starts from the inside of the car and the underseal can hide it from inspection.

The Jaguar V12 remains one of the best engines ever made, and the noise at full throttle with a race exhaust....

Mechanically the cars are strong, but bushes, bearings and ball joints wear. For racing I replace bushes with polyurethane and budget for new bearings and ball joints. Brakes get extremely hot so we use BNS grease in the wheel bearings which copes with the heat. The gearbox mounting is a cunning and complex unit which will be worn and make clonking noises when driving hard but can be replaced with a simpler rubber mount. The steering rack has very soft bushing so fitting pollybushes sharpens up the steering considerably, some cheapskate racers just limit movement on the bushes by simply putting tie wraps round the bush edges!

The cooling system on V12s is dire, I flushed the accumulate rubbish out then fitted coolant made of about 1% water wetter, 10% anti-freeze and 89% water which has better heat transfer ability. Then I removed the visco fan and associated heavy bracketry, the fan cowling and the original electric fan. I fitted a large electric fan instead as the fan is only needed in the pits. Jacking the bonnet open an inch lets the hot air out which is just as important as letting cold air in.

Keeping it simple, SportsPack springs off a 6 cyl XJ-S, Gaz race dampers, rcae brake pads and race brake fluid. Job Done!

The brakes fade horribly on track, I used EBC Yellow Stuff race pads and Motul RBF600 brake fluid. The fluid is vital, it has a higher boiling point but has to be changed more regularly. To get a bit more cooling air round the rear inboard brakes I took the access plates out of the boot and removed the boot seal to let the hot air out, another approach is to cur the boot floor out completely which further improves cooling and makes access much easier to the rear axle as well as saving weight.

Weight loss is key, the sound deadening is everywhere and it is a good days work ripping it out. A tar based substance is glued onto the floor and has to be chiselled off. The interior heater system is very heavy and can be largely thrown away, although leaving the drivers side screen fan helps demisting. I used RainX anti-fog on all the glass to prevent misting, much lighter solution than a heater. The standard XJ-S seats only weigh 7kg so make a good cheapskate racers choice. Door lock solenoids can be junked to save 2kg, but the door cards weigh naff all so leave them in. I would also leave the electric mirrors on as they are less than 1kg each and work very well.

The centre exhaust silencer can be replaced with a straight through tube for a few more bhp and less weight. The standard intake airbox is a little restrictive and the trumpet can be cut off and a larger hole formed with a radiused edge, it is vital to get cold air to this and running ducting from the headlight surround works well. The middle headlights on quad light models can be removed to make an excellent cold air intake point, although some bodywork has to be cut out to get into the engine bay. The standard paper air filters work well when new, no need for expensive sponge filters.

Racing is in the blood, you either understand or else there is no point trying to explain!

The engine oil needs to be able to cope with high speeds and temperatures, I used Castrol RS which is now superseded by Castrol Edge. The other fluids have to be changed for quality higher performance versions too, including power steering, gearbox and differential.

All XJ-Ss had and LSD as standard, the 6 pot models had the lower 3.54:1 ratio needed for racing and is a straight swap to replace the overly high V12 item.

With the car lighter it will sit stupidly heigh on its springs. Eibarch make a suitable race springs but they are pricey. I cheated slightly by using the Jaguar ‘Sports Pack’ springs from a 3.6 model on my V12 which worked out just right. Adjustable front dampers are a handy mod and can be tweaked to suit different circuits – hard for flat ones like Silverstone and softer for less even ones such as Croft.

Fitting 50 profile tyres on standard wheels drops the gearing a tad more and lowers the car a bit too. Buffing the tread down to 4mm will stop them going off due to heat build up, this maintains grip levels and actually improves wear rate. I used Toyo Proxes T1R tyres as they were mandated by the race series, and they seemed to work well even in heavy summer rain.

You may wonder about roll cages, but if the car is ever used on the road I would avoid them because of the injury risk from hitting a steel bar next to your head in a collision. Cages work well when the driver is secured in a race seat with a race harness and wearing a crash helmet. The Jags are strong cars anyway so a heavy cage is of questionable benefit unless racing.

Go on, search eBay now, you know you want to! 😉

All that remains is to fit a race harness to hold you steady and a race steering wheel to speed up response and it’s time to head for the track to have more fun than is decent.

 

Check out these web sites which I have found very useful:

http://www.jagracing.co.uk/index.html

http://www.xjs6000.cc/

http://www.jag-lovers.org/

http://www.mig-welding.co.uk/

 

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About Ralph Hosier

I love exploring everything the world has to offer, the fabulous beauty and intricacies of nature, the stunning majesty and grandeur of the universe, and the fascinating range of chocolates available from the local sweety shop. I have led a charmed life, sure there has been extremes, but the highs far outweigh the lows. I get paid for arsing about in very fast cars, I get to write about them and amazingly get paid for this too. My days are usually filled by making prototype and concept cars for car companies, a dream job. I have lived many of my dreams, worked all over the world, raced cars built by my own hand (and hardly ever crashed really badly), seen things and done stuff. But nothing compares to the love of Diana and my son Peter, beyond my greatest hopes. I am a chartered engineer, a member of the Institute of the Motor Industry (IMI), and of the Institute of Engineering and Technology (IET) and I am a member of the Guild of Motoring Writers. A pleasing fact is that there are now more letters after my name than there are in it ;) R.Hosier B.Eng(Hons) C.Eng MIET MIMI MGoMW
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8 Responses to How to make an old Jag fast.

  1. Mohammed says:

    A great write-up and good info, thanks.

  2. watahyahknow says:

    thanks for the info . i still have a question :
    the bigger and later jags use the 4l80e automatic , now the internals are the same as a gm box but the bellhousing is different , this mush i know
    the thing i still need to know about is the converter , is that the same one as found on gm trucks and the sort , will a gm truck converter bollt up to the jaguar flexplate and slide intoo a jaguar 4L80E

    • Ralph Hosier says:

      It will but it’s not quite that simple because the Jaguar V12 rev limit was 6000rpm and some truck torque converters may fail at the speed, you just have to pick the right sort. The 4L80E was also used by Rolls Royce / Bentley on the old 6.75 pushrod V8.

  3. Bunnet says:

    This maybe insane but always taught about doing away with the body of this and throwing a more aerodynamic aluminium and fiberglass body

  4. Miq says:

    Hi Ralph, great little write up!!

    I am not racing with my xjs but I’ve already done a few mods to it including full sportspack with Gaz coilovers all around, exhaust with only the rear silencers and a few polybushes here and there.

    I now want to tackle the ZF gearbox mount as the double spring type is a bit of a pita and imho it has too much play. Do you have any more info on this mod? I can’t seem to find anything on the net…

    ta

    Miq.

  5. James P Walter says:

    What needs to be salvaged to transplant fuel injected HE V12 From xjs to 71 e type.

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