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XJS Performance Upgrades
by Wayne Estrada

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If you own an XJS, then you know how wonderful these cars are. However, you also may be disappointed with the lack luster sporting performance of these fine creations of Coventry. If you are interested in increasing theperformance of your XJ-S, then I invite you to read on.

The XJ-S as you know is a wonderful automobile--but it is not without its foils. For one, most people think "V-12" and naturally assume ground-pounding Horsepower in spades. If you are an XJ-S owner, you know this not true. The XJ-S, in stock form, is not a true Supercar--that's the bad news. The good news is that there are things that you can do to immediately increase the performance potential of your XJ-S. This article is a brief collection of starting steps to make your Jaguar more of a performance car, and less the luxury-boulevard-high speed cruiser it was originally designed to be.

I have owned my XJ-S for almost eight years. During that time, I have made numerous performance and cosmetic modifications to my car and have learned, through trial and error, what is effective and not effective. Please, take the following as my contribution to all of you based on my experiences about what steps are worthwhile to increasing the performance of this soon to be classic.

Most people that are into cars are interested in *performance*. But what exactly is performance? Let's define what that means first.

PERFORMANCE is an increase in an automobile's ability to do three things:

1) Go faster (either in acceleration and/or top speed)

2) Handle, over curves, rallying, or street conditions at higher and safer speeds with better control (which is related to...)

3) Stopping in shorter, more controlled distances

If you are looking to increase your XJ-S's PERFORMANCE, first ask yourself, what is it you REALLY want to accomplish? Naturally, the little boy (or girl) in us is always interested in more Horsepower--nothing wrong with that! But I feel the key focus ought to be a *balance* of all three of the above. Keep in mind that whatever you do on any car, and ESPECIALLY a Jaguar, it is going to cost money. The amount of performance you will get to some degree is directly proportional to the amount of money you can, or are willing, to put into the car. Also keep in mind that the performance return(s) [small to moderate] you will get from modifying your XJ-S will NOT be recoverable in either the sale or current value of the car. For what it is worth, I'd guestimate that you'll lose 90% of what you spend--and that is, only if you can find a fellow enthusiast that recognizes what you have done to your car and is willing to pay a slight premium for your extra investment in time, money, and love.

I'm am going to keep this as short as possible. All the details are on my Jaguar web page at the end of this article, and you can read more tips there. Following is a quick hit list of action items you should seriously consider in modifying your XJ-S for the proverbial holy grail of 'increased performance', for which many of us seek...

1) Exhaust.

The stock system is dead quiet, but also power robbing. There are two resonators and one muffler for each bank (6 cylinders) of the XJ-S V12. Take your car to a muffler shop and have them remove the front (the ones just behind the catalytic converters) and rear resonators (at the tail pipe tip.) This is good for an additional 10-15 HP and will cost only about 1 hour of labor at a muffler shop. Without a doubt, this is the best way to immediately get some "free" horsepower at the lowest possible cost. Be prepared for a more sporting (and slightly louder) exhaust. You can't have it all--choose ultra quiet, or a slight increase in sound and more HP.

While you are at it, have the muffler shop replace the stock muffler with a performance model. I put Flowmaster mufflers on mine (after the above mod) and noticed an additional slight positive performance difference. With as old as most of our cars are getting, a new muffler could not hurt and may in fact be a justifiable expense to the spouse too.Although a bit more expensive, follow the above changes, but remove the old 1 7/8" exhaust and replace with 2 1/4" pipes. This is the largest size that will fit without interference to the transmission and will increase exhaust breathing that much more.

2) Shocks

Replace the BOGE shocks with Bilstein or Konis. XJ-S owners have talked, fought, and argued about this over the years on the list, but these two names keep coming up as the preferred choices. The XJ-S suspension is a bit too soft for spirited driving, and these performance shocks should provide the payback of an almost equal ride to that of the stock suspension, but with a bit better control. Don't waste your time or money on less costly shock brand alternatives. Many XJ-S owners that have tried to save money using Pep Boys, Monroe, or other cheap shocks and have regretted it.

3) Brakes

Make sure your brakes are well maintained. The stock XJ-S brakes are *more than adequate* for street driving--even under heavy use, and will most likely exceed what you need outside of racing or heavy solo racing conditions. An inexpensive upgrade (besides brand new pads) is to replace the stock hoses with steel braided hose. They will decrease some of the loss in the stock rubber hoses and at least give better 'feel' to the brakes. A common maintenance problem on the XJ-S is leaking differential seals, which often causes differential oil and grease to get on the rear rotors. Be sure your's are not leaking to get maximum braking performance from your car. Brakes are THE MOST IMPORTANT part of any car, aside from tires. Speaking of which...

4) Tires

... need to be replaced when worn. In my experience, re fitting the stock Pirelli's are a big mistake. They are expensive, don't handle that well, are a little noisy, have *terrible* snow and wet traction characteristics, and wear faster than most. In other words, almost anything is an improvement. The bad news is that there are not a lot of V or Z rated tires that can fit the stock XJ-S rim. Good choices are (just in my opinion, and not in any order) Michelin, Yokohama, B.F. Goodrich, Goodyear, Kumho, and Nitto (to name a few). Remember, tire size/rating is a real b!tch for this car. But, oh yes, it's a Jaguar-- remember? This sort of pain goes with the territory. Consider one size narrower than what comes from the factory as again, the stock size choices are very limited. A good tire expert can help you with your application.

5) Tune up your Cat.

The infamous 'Book' (available for free on line at under the XJ-S section and written by Kirby Palm) refers to the frozen distributors of the earlier (pre Marelli/1988) cars, problems with aftermarket distributor caps, and the importance of good spark plugs and wires. Study this carefully. All that and a good air filter, plus a new PCV valve and also replacement Oxygen Sensors, oil and gas filters, can and will noticeably increase the pick up and go and 'seat of the pants' feeling in your car--as well as help to improve the dismal mileage that most of us get.

6) Neoprene Streering and/or Suspension Bushings

John's Cars sells a kit for the XJ-S steering (among others) to tighten up the truck-like luxo-barge steering that Jaguar tries to pass off as that of a performance car. I really loved the change in my steering with this modification! The car immediately had better control, but more driver involvement is definately required. The car does not automatically track straight by itself--constant steering correction is required, but the reward is instant steering response.

7) Transmission Shift Kits

...make the torque more direct in the transmission, resulting in more power transmitted from the engine to the rear wheels. This will make your kitty shift less Cadillac-smooth and more Corvette-like direct and will return a LOT of smiles per mile at a very low cost. Most transmission shops can put a kit in your (GM) transmission easily and fairly inexpensively in a couple of hours. The result will be positive shifts and less slippage in the transmission. Be forewarned--shift kits come in various 'stages' of hardness. Most owners will prefer the first stage kit, but more radical kits can increase the stall speed higher and provide very firm, even brutally hard shifts between all shifts. Any good transmission shop has in stock or can order a kit for your GM400 transmission.

The above are the first tier of the 'moderately priced' upgrades/modifications. The following will cost a bit more, but are do-able and should be considered at some point in the future.

8) Gearing Change.

The differential axle ratio on most production XJ-S is 2.88, and in tandem with the GM 400 3 speed transmission makes for a very highly geared car--especially in first gear. THE NUMBER ONE PERFORMANCE PROBLEMS WITH THE XJS IS HIGH WEIGHT AND VERY HIGH 1ST GEAR GEARING RATIOS.

As you know, MPG on the XJ-S is bad enough, so lowering the rear end ratio will make it even *worse* AND lower overall top speed. The good news is you'll have more punch off the line. This is not a cheap modification, but is perhaps one of the most dramatic improvements for 'return' on performance for the dollar. It requires dropping the rear suspension cage--a mostly labor intensive job. The XJ-S uses the same differential as the XJ6 Series 1, 2, and 3, so rebuilt or used 3.54 ratio differentials are available from many Jaguar parts suppliers. While you are at it, you could also...

9) Transmission Change.

Keisler Engineering sells a 5 speed manual conversion and Grand Tourisimo Jaguar sells the hefty Richmond 6 Speed manual. Both companies also offer lighter flywheels and heavy duty clutches to complete the job. John's Cars also sells a T700 GM 4-speed conversion kit. Whether you chose an upgraded automatic or manual, there will a healthy improvement upon the ratio/gearing problem previously mentioned and faster acceleration, often with improved mileage in top gear over stock to boot. Our friends in Australia and Canada are fond of the Toyota Supra 5-Speed transmission swaps, and much success has been reported with this tranny as well.

10) Increase intake Breathing.

There are two major ways to do this.

AJ6 Engineering in the UK sells a tuned intake manifold (they call it "Torque Plus"), larger throttle bodies, and a normal and (additional) cold air air filter casing and low restriction air filter. They also market a recalibrated ECU (Engine Control Unit computer) that brings on the engine torque sooner in the rev range. The last three items are a 2 hour bolt on operation. The tuned intake runners are also a bolt on, but require taking off the top of the engine--about a 10 hour project in all. I have installed all these AJ6 products and am very satisfied, however the cost -vs- payback is not as high for what you pay. However after installation, my car will now strongly pull strongly to red line (6,200 RPM) whereas before it would start gasping at around 5,000 RPM.

An alternative to AJ6's tuned intake runners is to buy a spare set of XJ-S intakes and cut open the inlet side and weld in TWO throttle body openings. Of course, this will require fabricating some special linkage and replacement air cleaners for two throttle bodies *per side*. I've seen it done and the results are very, very impressive in increasing horsepower and performance. If you are not skilled at metal working, Keisler Engineering in Knoxville Tennessee also sells this as a bolt on kit. Having twin, instead of the stock single, throttle bodies per side opens the door for some *serious* intake breathing improvement adn will result in slightly more horsepower and increased willingness for the engine to reach peak power sooner.

11) Go to a 16", 17" or 18" wheels and tires.

The fabulous (but expensive and exclusive) Arden of Germany used to sell their own specification 8" wide wheels for the XJS, therefore up to 18x8 inch wheels are a proven combination that works on this car without clearance problems. There is not enough time here to go into what or where to look for other wheel options. Check the Jag-Lovers XJ-S archives for more information.

I used 18" x 8" Forgeline wheels on my car and am very happy* with the looks and performance. Of course, you'll also need new tires. All this is not going to be inexpensive...figure $3,000-$4,000 USD for this upgrade alone. The handling will improve, but the ride will change from luxury standard more towards sports car feel. If this is what you want out of your XJ-S, it is a highly recommended step.


Well, that's about it. All of the above is based on my experiences and is a brief synopsis of what XJ-S owners around the world have collectively discussed over the years. When properly applied, these modifications should help you to enjoy your XJ-S more and more as time goes on.


About the author - Wayne Estrada is President of, a company specializing in executive level seminars on helping executives understand the technological world, and is also a master teacher on the UNIX Operating System and the Internet. He can be reached at His personal Jaguar web site is at


Posted: 6/6/2002