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Jaguar XK’s

Jaguar XK's Published: 17th Aug 2017 - 0 Comments - Be the first, contribute now!

Jaguar XK’s
Jaguar XK’s
Jaguar XK’s
Jaguar XK’s
Jaguar XK’s
Jaguar XK’s
Jaguar XK’s
Jaguar XK’s
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Jaguar’s 70 year old classic still has lots of potential to make it both better for modern roads or a fine classic for competition work

The fact that so many XKs are still seen in competition shows the quality of the basic blueprint, but any improvements and mods have to be balanced against spoiling the original design and as a result the car’s provenance. Here’s some mild rather than overly wild suggestions.

Before you start

Chassis rust has to be dealt with before any racing and the nature of the body design means potentially lots of it. If you can see any bubbles on top, then the underneath will be much worse. The rear region is even more rot prone and during any restoration is more often than not replaced as a complete entity.

The XK bridged the separate chassis and monocoque eras and suffers as a result, with lots of places where water can collect and cause corrosion. While engine oil leaks over the years will have saved many a chassis, the rear can still corrode badly if left to the elements.

Still in use in the ’80s, the XK unit is an all time great if properly maintained although 3.8s can crack their blocks; the 340 engine from ’67 is the strongest of them all. Oil consumption will be up to 200 miles per pint. Use a classic oil with a modern ‘spin-on’ filter conversion from David Manners at under £90.

An XK unit can be fully re-built by a specialist for £3000-£5000, and for £6000 plus you can have a 300bhp road/race tune unit but anything much higher will be unreliable. The cooling system has always been marginal. Expect to find sludging and rusting coking up the galleys which only an overhaul will clear out A re-cored radiator, with good quality anti-freeze and perhaps an electric cooling fan, is almost essential. Watch it! Early XK engines fitted to these cars feature the thermostat set in the rad’s header: leaving this out will make it worse…

The drum brakes on the 120/140 models – if kept correctly adjusted (the self adjusters on later cars are unreliable) – work reasonably well and can be okay even for mildly tuned cars; it depends how hard a driver you are. Wheel wobble on the XK120 is pretty common and hard to dial out. Check the ride height: if low at the front, then the torsion bar suspension can be reset. Wishbone wear means a new replacement, but slack in the XK120’s steering can be successfully adjusted out by XK specialists.

Hotting one up

When it comes to that classic powerhouse, it depends on how much speed you want and bear in mind Jaguar successfully did it for you during the XK’s production run, so just a simple ‘step up’ to the next tune may suffice for road use. On the XK140, for instance, power was increased to 190bhp, with an SE version (boasting even better breathing) 210bhp. The XK150 retained the 140’s 190bhp from 3.4-litres although few were made, most being SE’s 210bhp. The ‘S’ model has 250bhp and in 1959 a 3.8-litre version was added with a regular 220bhp or a rousing 265bhp in ‘S’ tune.

Starting with a budget tweak, simply ensuring the engine is in good tune plus electronic ignition works wonders. An E-type/420 straight port head with its carbs and camshafts makes a cost-effective upgrade as is fitting similar from a fuel injected S3 XJ6 along with its pistons if possible; they give a 8.75:1 compression ratio, slightly lower than the standard 9:1, but are of a better design. With a little skimmed from the head, the 9:1 will soon be restored (if you want it). Spot-on ignition systems are worth more than a set of fancy camshafts – go electronic if competition regs allow and if you can spend more money, have the system mapped; it costs big time (around £1500) but works beautifully we’re informed.

Further tuning of the ‘top end’ is popular although valve sizes needn’t be dramatically altered, if at all, while on 3.4s it’s not worth fitting E-type carbs unless the engine has been significantly uprated to suit, ditto substituting Weber DCOEs on the 3.8 – even 2in SUs are considered to be excessive by some XK tuners. Best camshafts remain D-Type profile types available from Guy Broad at £300 a pair.

Xtreme Jaguar of Kent has developed a special (equal length) inlet manifold as well as a ‘cross-over’ exhaust manifold for the B-type head. The former (developed by Jaguar, when it planned a ‘Mk2 GT’ during the 60s, equalises the ‘tracts’ for better gas flow plus is slightly longer than standard to gain cleaner air. Xtreme’s Carl Gannon says it makes little difference to a standard XK engine but really helps on tuned ones. The exhaust is a take on a 1980’s BMW design to ‘harmonise’ the pulses and improve extraction. XK bottom end is strong but anything more than 300bhp and 6500rpm really demands Cosworth pistons plus steel connecting rods and crank. Xtreme uses a jet engine specialist to balance its engine ultra finely so it can take higher revs, safely. The standard flywheel is a hefty 28lbs effort can be shaved by some 10-19lb or you could opt for an even lighter alloy version.

This is only a guide – speak to a known XK expert for further advice, particularly if you intend to enter FIA controlled racing or rallying as only period mods may apply.

Handling the power

According to Jaguar specialist Guy Broad (, “you can’t reinvent the wheel with an XK”, meaning that tried and trusted tweaks are still the order of the day and it warns owners not to “overdo it” and make their XKs worse than standard.

The company markets Broadsport range of tuning parts that were introduced around 15 years ago encompassing engines, gearboxes, suspension, brakes and latterly even creature comfort upgrades such as interiors, lighting and the electrical system. A 30 page Selection Catalogue and a XK Engine Catalogue are available. According to Tom Norton the secret to a good handling XK is “a loose rear end” enabling the car to handle in a neutral fashion or oversteer at will.

Accordingly, the XK’s handling is improved by fitting AVO dampers all round (including an optional rear telescopic conversion for XK120s) along with a beefier 7/8in front anti roll bar to sharpen the front end, all costing just over £700. He recommends Broad’s own specially developed polybush kit to tighten chassis to an optimum setting but warns against specifically stiffening the sturdy chassis in isolation as it will upset the handling.

Brakes can be uprated to discs, either with original Jag parts or by using kits developed by specialists, which can include vented discs stopped with four-pot callipers.

Again, Broad has developed a range of brake upgrades, starting from under £1000, which includes front discs for the XK120. “For average daily driving in a 120 or 140, discs on the front and drums on the rear is adequate. For more strenuous use, discs all round is the best option.

“Once again, correct pad material and fluid will cope easily with normal driving conditions. In our experience, especially with 120 or 140 roadsters, with drums at the back and discs on the front, if you require servo assistance, we recommend its use on the front only as the rear drums have a tendency to lock-up quite easily.

“In any case, we do often recommend trying a 120 or 140 without a servo first to see if you are happy with the feel; if not, it is easy to fit a remote servo later without too much extra work”, advises Broad.

Not many opt for power steering but Guy Broad markets hydraulic (as opposed to an electric) conversions utilising later XJ hardware. What the company does advise is having the geometry set by a known XK expert as many are incorrectly set – perhaps even from new – as the transformation is startling and perhaps one of the best (and certainly best value) ‘mods’ of them all!

The smoother action XJ6 manual with overdrive gearbox is a good fit for less frantic high speed cruising and a nicer change quality, or you can opt for a specialist fivespeeder instead. The S3 XJ6 five-speed transmission also fits although be warned – none are a straightforward swap! There are several axle ratios to choose from: 2.88:1 for very high speed touring to a low 4.27:1 that’s ideal for hill climbs or quarter mile sprints, either with or without limited slip differential.

Whatever you go for, they must in turn work with the type of gearbox chosen plus also the wheel and tyre sizes which affect the gearing, advises Broad, who adds that while a limited slip differential is useful for track work, watch the halfshafts which are known to snap under very hard use.

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