Making your Jaguar XK do what you want
“What can I do to improve my XK”, is one of the most common questions we get. The first question we ask is “What do you want?” Replies generally fall into two main categories – comfort/convenience or better performance but often both. On one hand we get things like lighter steering, brighter lights and better gear change, and on the other; more power, better gearing, more gears and better brakes with a lot of variations in between.
However there is another factor in the equation we need to consider before we can answer, and that is the driver. There are basically three types of XK owner: the purists who want to keep their cars as original as possible, the pragmatists who want to overcome the problems that Jaguar could not overcome when their cars were new and the performers who want every part performing to the highest possible level irrespective of originality.
I do not intend to debate the rights and wrongs of modifying cars away from their original specification – that would take forever and we would never all agree. However as I am constantly asked by owners from the three groups what advice I would give them to make their cars better, faster, easier to drive, stop quicker, be warmer, have better lights, etc, etc, I will try and give some ideas for you to mull over. In other words what is possible. However there is not room to go into great detail in just one newsletter so I have put in some links to where you can find more information. But of course we are always at the end of a phone or email to help you. If you have something you want to follow up on – just let us know.
This may sound obvious but if you have an issue with your car – be it poor performance, bad brakes, wandering steering or whatever – you should first make sure that those issues are not due to lack of maintenance or assume that they are normal. For example drum brakes often get criticised for their poor performance when all they need is servicing. Likewise steering and suspension can spoil the enjoyment of a car because a bush or bearing needs replacing. Particularly now that the MOT test (the UK annual safety inspection for vehicles) is no longer applicable to all but a few late XK150s, worn components can easily be overlooked.
When the XK120 was first unveiled in 1948 it featured the latest technology of the day: in particular the engine design. Having said that brakes, steering and gearbox had progressed little since the 1930s, nor had the electrics or lighting. Its performance made the 120 a popular car to race, and racing is always good for the breed. Working with Dunlop, Jaguar became the first car manufacturer to line-fit disc brakes after having developed them with the C-type in racing. Rack and pinion steering found itself a standard feature on the XK140 having proved its merits in the C-type too. However I don’t propose to go through all the mods that Jaguar introduced through the XK range. If you want to find out more details on this, Philip Porter’s excellent book Original Jaguar XK is your best bedtime reading.
So let’s look at some of the areas where upgrades are possible/available starting with wheels.
The most common modification on early XK120s years ago was to change the disc wheels to wires. However today disc wheel cars are quite sought after but if you do want to do this then you will need to replace the wheel hubs with splined ones and kits are available. There are also body mods to be done as the rear spats can’t be used with wires. There is a wide choice of wire wheels including varying widths and finishes. This means that you can fit wider tyres, which is particularly important if you are racing.
One of the most effective ways to give an engine more power is to get more air and fuel into it. Fitting larger carburettors or more of them is the easiest way to do this. SU carburettors were fitted to all XKs with twins on all but late 150 S models, which had three, as on the E-type. There are several possible conversions to larger carburettors or triple fitments and the ultimate is to fit triple twin choke Webers if you like power and pumping gas.
The wiring on XKs is hidden away and old wiring poses both a fire and reliability hazard. Rewiring is often the answer and for this our replacement loom is a must. As to upgrading, then replacing dynamos with alternators or fitting high power lightweight starters are two popular mods. Converting from positive to negative earth is also something that can be necessary in some cases. Electronic ignition means no points to worry about and the ultimate ignition conversion is the fully adjustable electronic distributor. Improving the car’s lighting speaks for itself.
There is not a lot you can do to the front suspensions of XKs if all is in good order. Polybushes are a low cost and simple way to improve the front end, with a pair of adjustable front shock absorbers considered a must by many. Our front suspension upgrade kits include poly bushes, AVO shock absorbers and a heavier anti roll bar. These transform the handling and are great mods. Various rear axles as well as many different axle ratios were used through the years but these are not as interchangeable as might be imagined – so check before you try to swap an axle or differential. Fitting our uprated links to 120 lever arm shockers is another good upgrade but better still change to telescopic rear dampers.
Moss gearboxes are not everybody’s cup of tea. Not only do you need a certain expertise to be able to change gear without crunching but also the ratios are far from ideal for today’s traffic. Four years went into the development and testing of our five speed gearbox conversion with its direct bolt on case and it features all new components. This upgrade makes gear changes a joy – unless of course you are a purist and just love to play tunes on the throttle double declutching through the box.
Cylinder heads and exhausts
Although the XK engine was produced for decades, it was regularly upgraded and over the years with several different cylinder heads fitted – each giving a bit more power than the previous one. Fitting a better head is a good way to upgrade but it is not always a direct swap. Our exhaust manifold options and stainless exhaust systems give more power and improve the looks of your car.
Drum brakes were standard to XK120s and 140s with disc brakes introduced on the XK150. Although well maintained drum brakes are pretty efficient, if used heavily they do suffer from fade (diminished performance when hot). They can also be prone to pulling to one side or the other. The answer is to fit front discs on 120s and 140s. The disc brakes on the 150 were on all four wheels but modern calipers are far superior. Our front disc and caliper upgrade includes the three piston caliper as used in our 120/140 kits, which is far superior to the original 150 unit. Steel braided brake hoses are not only more durable but also they make the pedal firmer. A brake servo (booster) kit reduces the pedal pressure needed so makes driving more pleasant and our kit comes with all necessary parts to fit it.
As well as the major items mentioned, there are numerous smaller upgrades we offer to improve performance or make maintenance easier. For example replacing the old paper oil filter with a modern cartridge filter means less mess and better filtration. Lightweight alloy fuel tanks and radiators save weight and in the case of the radiators help cooling, as do our special uprated engine cooling fans. XK120 and 140 wipers are like something from the dark ages but our wiper upgrade kit has a high performance motor and can be wired for two speeds. The heaters too on the 120 and 140 were grim but we offer a heater replacement to make winter driving far more tolerable. I hope this has given you a bit of a better understanding of what is available to update or upgrade your XK. Whether this will enable you to upstage anyone remains to be seen. Our kits are available for you or your garage to fit but we can of course do the whole job for you. If you do replace any of the original parts then do put the old items away in a safe place. One day you might find a buyer for your car who is one of the purists I mentioned. He may well want to bolt back those parts that you have taken off or at least keep them with the car.