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Jaguar XJ40

Published: 20th May 2013 - 0 Comments - Be the first, contribute now!

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Jaguar’s maligned XJ40 can make a fun car with a few cost-effective tweaks. Sam Skelton explains how to give the big cat some sharper claws

Reviled by many traditional Jaguar enthusiasts, the XJ40 has never quite managed to shake off the stigma of its conception. Yet they’re not all bad – and can even make a left-field candidate for fast road and track use after a few small modifications.


The XJ40 can be a rewarding car to own – and prices appear to have just passed banger territory. XJ40s have a pair of Achilles heels. We asked an XJ40 collector about them in order to avoid the oft-spouted clichés, and yet when quizzed about rust spots and electrical glitches, he replied with “Everything!” Headlamp modules and heaters have been known to give up the ghost, whereas electric window, digital dash and central locking issues are par for the course. Rust can break out on the bootlid, but more serious is the threat of A-pillar rot spreading to the sills and front inner wings. The corners of the rear screen surround rot through, and there’s a reason why so many XJ40s wear chrome arch cappings – they’re using hiding the flaky brown stuff.

Serial XJ40 owner Alex Sebbinger recommends checking everything over underneath, too. “Front and rear shock absorber top mounts can wear, as can front A-frame bushes. These will notably ‘clonk’ by way of a warning.”


At launch, XJ40 was available with a choice of engines: a 165bhp 2.9 SOHC or a 221bhp 3.6 DOHC, both derivatives of the AJ6 slant-six engine These were replaced in 1989 (3.6) and 1990 (2.9) with 245bhp 4.0 and 200bhp 3.2-litre units respectively.

The first tip for added power is surprisingly obvious. If you have a 2.9, sell if and buy any of the twin-cam units.

Assuming your car, either a 3.6 or 4-litre is in good order the first step on a tight budget should be a uperior air filter system from the likes of K&N or Pipercross. Along with a spot of power chipping this should release a few extra horses pretty painlessly on the pocket.

More complex, AJ6 Engineering offers modified venturis which counteract the effect of the original, restricted airflow meter and ECU reprogramming. The result is useful extra power – AJ6 reckons in the region of 15 per cent. A modified air filter can be added to create AJ6’s £495 Plus Flow conversion. AJ6 also offer an improved inlet manifold, which can be added to the aforementioned extras to form the £962.50 Plus Torque kit, which offers 30bhp and 13 per cent extra torque.

Of course, engine and gearbox swaps are perfectly viable and are a cheaper option – find a 3.2 manual, and a 4.0 AJ6 will be plug and play if you so desire. You could also buy a manual gearbox and clutch setup but finding a donor will be difficult and you may as well simply find a decent manual transmission car to start with.

Incidentally if you have a 3.2, a good tweak is to fit a stock flow meter from the 4.0 while AJ6 markets a kit to enable a 3.6’s ECU to run the bigger engine, too.

The outfit adds that any 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.

Now having got more air and fuel in, the job is to extract it more quickly and AJ6 Engineering has a range of exhausts. The best is the TT system which augments mid-range torque by about 12 per cent.

Ambitious men would find an accident damaged X300 XJR – the AJ16 is a slightly more involved swap, but is perfectly possible. Over 320bhp and nearly 380lb/ft of torque could be at your disposal if this were to be your route of choice – but is it worth the hassle? Alan at McMillan Jaguar of Motherwell has done this conversion, and thinks it’s easier than some would have you believe. “We did this a few years ago for a customer. It’s basically a very easy installation as the X300 shares the XJ40 crossmember, brakes and so on.” Don’t be tempted to buy an engine – get the whole car. Alan reckons conversion is easier with a full donor car on hand – depending upon your fortune, bank on anything from £500-£1000 for a donor which will never see the road again.

• AJ6 Engineering’s High Flow and/or High Torque kit
• Improved induction set up
• ECU chip, rolling road tun

• Engine swap – AJ16S from supercharged XJR
• Engine swap – 4.0 AJ6
• Fuller flowing exhausts


Make sure it’s all in good condition – all mountings and bushes should be in excellent order and are the first things to replace. Early cars with self-levelling rear suspension can be a nightmare, and are best avoided. The optional sports pack gives bigger brakes and uprated, lowered suspension – but these are rare, finds.

Equally, the rare Jaguarsport cars tuned by Tom Walkinshaw Racing have better suspension for performance use – a thicker anti roll bar and better dampers combined with re-valved power steering to give a meatier feel mean that the big cat gains some useful claws (some achieve the same gain by slightly kinking the PAS pipes to restrict it – but don’t overdo it!). You won’t find a TWR car in a scrapyard – nor will many enthusiasts be breaking them.
The only option for meatier suspension and steering as upgrades for your XJ40 is thus the X300 Sport or XJR. Yet many would find upgrades totally unnecessary for fast road work – the XJ40 chassis is so composed that little improvement is made even with the factory-developed parts.

It’s worth finding a donor XJ40 V12 (Also known as XJ81) – the front hubs and brakes are an invaluable swap on uprated cars due to their twin-piston brake calipers. Standard brakes are capable; but couple these with fast road pads such as EBC pads and the twin-pot calipers, and the braking system should be more than up to most performance mods.

Wider wheels and tyres are a wise idea. Jaguar X300, XJ8 and XK8 wheels all fit, and there’s a wide range of secondhand wheels to choose from. Always fit good quality tyres, mind.

• V12 brakes
• Uprated pads and discs
• Full geometry check and adjust
• Poly bushing suspension
• Decent tyres on wider wheels

• X300 XJR suspension
• TWR Jaguarsport parts
• Revised power steering set up
• Dedicated racing tweaks

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