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Jaguar XJS

Published: 16th Feb 2012 - 0 Comments - Be the first, contribute now!

Jaguar XJS
Jaguar XJS
Jaguar XJS
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Trying to follow in the tyre tracks of an icon like the E-type was never going to be easy, but the XJ-S had a tougher time than it really deserved to. Oddly styled and, under British Leyland, poorly made, by 1980 Jaguar was ready to pull the plug on what ws hailed as the World’s best GT and even stopped making it for a while.

Happily the XJ-S survived and prospered for another 14 years, aided by the new range of straight six engines for those who didn’t want to go the thirsty V12 route.

And it is this breed of classic cat that we are looking at, to give it sharper claws, not simply for road use but to also compete in the excellent one-make Jaguar racing series where road going cars can compete.

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The quality of the engine means tuning isn’t that easy or cheap

The XJ-S remains a bargain basement Jaguar that has yet to soar in value, like MK2s and the E-type. What the XJ-S lacks in style to the E-type it makes up for with a superior XJ6-derived chassis, which can be made to handle really well.

Rust is the biggest worry with an XJ-S and neglected ones can rot like mad – terminally so. The situation is made worse by owners rarely spending proper money on repairs.

The engines are bombproof if looked after. Head gaskets could be a problem (between five and six cylinders) but it’s not a major worry. Ditto cam box oil leaks are irksome rather than dire but do watch for timing chain wear.

The rest of the running gear is proven but still needs watching. Wheel bearings, front wish- bones, rear suspension bushes all wear, while the inboard rear brakes on pre ‘93 cars can be seriously neglected and expensive to put right. Jag electrics were never great on the XJ-S and can play up, especially the ECUs.

Hotting one up

We spoke to one of the leading lights on the AJ6 engine, Roger Bywater of AJ6 Engineering, and to say he’s a fan of the power unit is a bit of an understatement. In his mind the straight six streets ahead of the famed XK unit and is also superior to the V12, “It’s the best engine Jaguar ever made”, Roger told us.

Sadly, so good was the development that it doesn’t leave too much for improvement, for road use anyway. And it’s not exactly the cheapest engine to upgrade either.

Assuming your car, either a 3.6 or 4-litre, is in good order, the first step on a tight budget should be a superior air filter system from the likes of K&N or Piper cross. Along with a spot of power chipping this should release a few extra horses painlessly on the pocket an make the engine feel much more responsive.

Now it gets a bit more expensive. AJ6 Engineering sells a new inlet manifold with modified tracts. Called Plus Torque, along with a bored out air flow meter (75mm for the 4.0) recalibration, Roger says a healthy 13 per cent more torque is provided right where it’s needed. However it costs over £1100. A racing version which puts the extra grunt higher up the rev range is some £40 more.

Incidentally if you have an XJ-S fitted with a 3.2 engine from an XJ40, perhaps to keep it going, a good tweak is to fit a flow meter from the 4.0.

Roger says cam and head tuning should only be done if track days or racing is in mind as the benefits for pure road use aren’t great for the costs involved. The cylinder head is well designed from the outset and so just improving and polishing won’t give a lot of gains and the valve sizes are already more than adequate. He advises a cam timing of around 280 degrees duration; try Piper and David Newman cams to see if they will cut you a pair from a blank or re-profile yours.

Now having got more air and fuel in, the job is to extract it more quickly and AJ6 Engineering markets a range of exhausts. Start with replacing the more restrictive of the silencers with Roger’s ready-to-fit substitute pipes, replacing the first silencers under the rear seat area for a power gain of about 15bhp and a slightly raised exhaust tone. A larger bore is the next option but because it only gives a small gain AJ6 only recommends this replacement if the complete system is now in the need of renewal.

The best is the TT system which augments mid-range torque by about 12 per cent. This replaces the downpipes with a tuned length twin pipe system merging at the back of the transmission which then divides again to each side of the car to join up with the original rear section. Standard and Large Bore (for those who require slightly more power at the top of the speed range) rear sections are available in addition if required, although it must be said that some people will find the Large Bore rear section a bit too noisy for daily road use. Made from long-lasting stainless steel it costs over £1200.

Naturally, if you have a worn out 3.6 then the logical step is to fit the superior 4-litre and AJ6 markets a kit to adapt the smaller engine’s ECU.

Back in the late 80s and early 1990s Roger experimented enlarging the engines, and by using a 99mm bore saw a staggering stretch on the 3.6 to 4.3-litres, while the 4.0 also gains another 700cc! Both provide a mine of torque that would easily beat the V12 but sadly high costs and lack of demand saw the idea die off. Still, even a standard capacity AJ6 engine should be good for a reliable 300bhp.

The AJ6 doesn’t seem to suffer from overheating woes, which blighted the XK, but with all these mods it’s still a very good idea if the rad is past it’s best to fit an uprated one form the likes of Radtec. A rolling session to fine tune the engine rounds the conversion off nicely.

How Did It Drive?

The XJ-S had its faults but few people complained how a good one motored on. Acclaimed as the best GT around in the late 70s, the excellence of the XJ6-derived chassis still shines though with a supple ride and fine handling for its era; today it’s somewhat soft and wallowy while the finger-light, feel-less power steering was always one of the criticisms of the car when tanking on a bit.
How the XJ-S would have performed with the 4.2 XK we’ll never know but the AJ6 was worth the wait. In either form they go extremely well while the 4.0-litre is almost a match for the V12. Best of all these engines ushered in five-speed manual transmissions which, for a Jag, are still a joy to use.

Handling The Power...

With more than adequate power in standard form, any standard XJ-S could almost be left alone – except that the handling and steering could be usefully tightened up although don’t go too mad and spoil that serene ride. Stiffer dampers all around (Gaz is proving popular) plus upping the wheel rim size to 16in not only gives a wider tyre selection, it also allows a lower profile to be fitted.

Sports bushes help firm up the steering but don’t ‘poly’ advises leading XJ-S light Hyper Engineering (01844 278481) who markets a more compliant nylon type. A rear anti-roll bar is something to consider though. This was deleted on some later cars and is worth retro fitting, along with the above-mentioned superior bushes to improve feel.

Hyper makes anti-tramp bars to keep the axle rigid for a reasonable £300. When the 3.6 was introduced, it was initially available as a manual only car, proving almost as brisk as an auto V12. Lending themselves well to slight stiffening of the suspension, these later, lighter AJ-powered cars are a popular choice in JEC club racing if you want to go further.

Brakes on all models are fine for most needs – even racing – although EBC Green Stuff pads are worth fitting, plus last a good bit longer too. If you want to go further then try looking at the XK8/R set up for guidance.

Finally tyres – you can go up to 18” x 6” for that custom look, without fouling the arches but don’t overdo it. Reforming the hydraulic PAS pipes to make them slightly smaller along the run effectively lessens its assistance and so promotes better feel… but careful how you do it!



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