All Jag units can be tuned, but blueprint could be best 
Modern ignitions are a worthwhile mod on any Jag 
Jags can be cool - standard set up in good order first? 
Dampers and poly bushing usually works a treat 
Brake upgrades varied but see standard ones ok first 
Brake upgrades varied but see standard ones ok first 
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Got a classic Jaguar that’s in need of beefing up for modern road use? Marque expert Jim Patten suggests these top tips
Bar room talk is the bane of the classic car world. “You’ve got to have great big brakes, nine-speed gearboxes and Formula 1 tyres,” so sayeth the sage. What a lot of old tosh. As an example, steering rack conversions for the classic Jaguar saloons are to be avoided; they’re usuallyXJ saloon parts adapted to suit and have you noticed how much wider an XJ6 is? Ackerman angles are seriously affected and, badly fitted, can be dangerous. The 420 power steering box is excellent so use that instead. With some intelligent thought, any Jaguar can be sensibly upgraded, usually using original Jaguar components. But before even getting that far, make sure that your car is running to its optimum performance first - you might be in for a pleasant surprise. Check that you have the right tyres for the job, the brakes and pads are working well (don’t forget to change the fluid every two years) and that full steering alignment is spot-on - not just the tracking!
Mk1
In its original form, the first compact Jaguar, the 2.4 saloon (retrospectively called Mk1) could barely scrape through to 100mph. But in 1955 it was a good performer. By 1957, it was joined by the 3.4 giving some serious grunt. Initially all models used drum brakes before revolutionary disc brakes took over. Bringing a Mk1 up to date is relatively straightforward. Rear axle runs with a narrow track. Replacing with Mk 2 type (3.8 uses limited slip) helps, but cutaway spats will be needed. Alternatively, MWS market bolt-on spline hubs for the rear (£79.90), which effectively increase track by half an inch. When used with off-set competition wire wheels, the track is about right Front suspension is carried on a detachable subframe. Replace entire assembly with Mk2 for greater efficiency. To get the best braking, use 420 suspension uprights as the triple pot callipers can be harnessed. The 420 complete suspension beam (use Mk2) will give you a superb Varomatic power steering box option. The 2.4 engine has low lift cam, small diameter exhaust valve, A-type cylinder head. First stage; swap Zenith/Stromberg carburettors for HD6 SUs, preferably with Mk 2 B-type cylinder head or ideally, straight port head and carbs from 240/340/420. In fact, a complete engine and gearbox transplant from a manual 240 gives SU carburettors, straight port cylinder head and all synchromesh gearbox with overdrive. Larger 3.4 engine is already strong. Could be better with straight port cylinder head. For the ultimate performance, use a 3.8 with straight port cylinder head and a pair of 420 (2in) carburettors. Triple carbs can be used, but clutch master cylinder has to be altered to clear the third carb. Gearbox Various five-speed conversions aftermarket on offer include Toyota Supra, BMW-type Getrag to the Dan Moony CJ5 as fitted to new TVRs and the latest Mustang. Check with the supplier about parts for fitting. Jaguar did make its own five-speed gearbox for the Series 3 XJ6, a beefed up version of the Rover Sdi unit. Tougher than you might think, it lasts well if the correct oil is used. Don’t forget the S-Type four-speed synchromesh with overdrive - it’ll slot straight in.
Mk2
Mk2 replaced 2.4/3.4 saloon in 1959 and brought with it wider rear track and a 3.8 litre engine. Most modifications are similar to the Mk1. Model was revised in 1967. Some cost cutting was done but also some genuine improvements. Engine had the excellent straight port cylinder head while the gearbox now had decent synchromesh on all four gears, plus an overdrive. A good 340 will give a 3.8 Mk2 a run for its money and soundly beat it across country. Best conversions of all are to the original Coombs spec., unfortunately a little too detailed to go into here. Speak to Ken Bell, he’s ex-Coombs and offers replicas. Rear axle is now full width. 3.8 uses limited slip, but halfshafts wear. Tip: source halfshafts from Daimler V8250 automatic: same part, less stress. Front suspension greatly improved. Use 420 uprights to gain three-pot callipers. Avoid XJ typepower steering rack conversions, seek out complete 420 front beam and use the Varomatic steering box, as per MK1. Don’t forget to use Mk2 springs though. Classic Spares offer new three-pot callipers at £450 a pair. They bolt directly to the existing uprights without the need for 420 uprights. Engines remain the long-lived XK type, but although the 3.4 and 3.8 had SUs, the 2.4 retained Zenith/Stromberg. Now with the B-type cylinder head, the best improvement is to search out a pair of HD6 carbs. Otherwise, comments as per Mk1 with straight-port cylinder head favoured. 4.2 best avoided as there could be installation difficulties.
Gearboxes
Gearboxes as per Mk1. If considering an automatic, the excellent XJ40 four-speed is a possibility, but conversion from G Whitehouse costs in excess of £3000.
S-Type/420/MkX/420G
S-Type/420/MkX/420G have been lumped together for convenience but they do have their own foibles. S-Type can be treated as Mk2 (although it never gained the straight port cylinder head) while 420 boasted all the good bits. Most were automatic though; a manual car makes for a lot of fun. MkX/420G all ran with 14in wheels to appeal to the American market. Substitute these for XJ6 15in wheels and tyres and use stiffer dampers.Paul Roach markets conversion to four-pot callipers on ventilated discs (using genuine Jaguar parts), almost obligatory for such a heavy car. Incredibly, the MkX had tiny Dunlop callipers as standard while the 3.8 used a bellows servo! 420G had the best brakes, but not as good as the four-pot conversion. Manual cars available but are rare - it’s a huge reach across to the gear lever! IRS (independent rear suspension) was (andstill is) class leading with a version running in Aston Martin’s DB7. The best advice is to make sure that any play is eradicated by sensible maintenance, while all rubber mounts and bushes are perfect. If replacing, never use reproduction parts – fit genuine Metalastik. Replace dampers (remember, there are four) along with the front, using the same manufacturer. Try Gaz, Spax or Koni.
XJ6/12
XJ6/12 in its original form is possibly one of the greatest Jaguars ever built. Find a short-wheelbase S1 six with manual transmission and it is an absolute delight. Engine developed to eventually become the most powerful production XK engine Jaguar built. With large inlet valves and fuel injection, it is a real flier. Fit this, along with the all-synchromesh gearbox and overdrive to a Series 1, and you’re in business. Engines were originally 2.8 and 4.2, with the 2.8 being dropped to be replaced by 3.4 in 1975. A V12 joined the range in Series 1 form. A lot of power has been extracted from the 2.8 (Toyota single seater pistons taking the block out to 3.0 litre) but it makes sense to stick with the injected 4.2. Even on carburettors it works well, especially with a well sorted ignition system set up. Being a luxury saloon, the suspension is a tad soft but this can be tightened up simply by intelligent use of go-faster dampers and, if needs be, harder springs. In principle, the very last XJS steering rack (a sports aluminium unit) could be used but in practice, sports bushes work very well.
XJ-S
Phenomenal power in standard form, the XJ-S could almost be left alone. Except that the handling and steering could be tightened up. I would suggest fitting Gaz dampers all around while upping the wheel rim size to 16in. This not only gives a wider tyre selection, it also allows a lower profile to be fitted. The rolling radius is often the same as standard, so the speedometer will not have to be recalibrated. Sports (poly) bushes help firm up the steering rack Pre-HE models could use some help on the ignition side. SNG Barrat markets a replacement to the original ‘OPUS’, (dubbed ‘opeless,) which has successfully ironed out the earlier unreliability hassles and is a worthy mod. With few genuine manual V12 cars made (fourspeed), a five-speed conversion is well worth considering. However with conversions costing at around £2500, it is a costly one. When the modern 3.6 was introduced, it was initially available as a manual only car and it proved to be almost as brisk as an automatic V12. Lending themselves well to slight stiffening of the suspension, these later XJs (especially the bigger 4-litre) are fine road cars and a popular choice in the Jaguar World Monthly/Classic Spares XJ-S race series, run under the control of the JEC. Brakes on all models are fine for most uses - even racing!
Common to all models
Electronic ignition: Luminition has been a popular upgrade for a few years and has been extremely efficient on the my E-Type, giving reliable 30,000 miles as well as better efficiency. Prices start at £150 from Classic Spares (01992 716236). Fully mapped ignition: The ultimate in ignition systems. Each sector of the system is unleashed to give its fullest potential all the way through the rev range, rather than the compromise of fixed curve conventional systems. No power gains but what power there is can be accessed, unlike a coil system where so much power remains hidden. Prices start at £1500 fitted at Paul Roach Jaguar Specialist (01787 461226). Cooling: It is a myth that Jaguars suffer from poor cooling. If everything is in top order, then all should be well. However, in some marginal conditions it helps to have a bit in reserve. N.A.R. now offers a multi-core (or aluminium) radiator with a built in shroud and electronic fan as a unit. Not only is the cooling enhanced, but with the engine driven fan removed, a few bhp can be gained. Prices start around £375. Exhaust systems: As these are perishable items, it might be worth looking at an improved system if it is being replaced anyway. Standard exhaust manifolds are enamelled and wear badly. Why not consider a six-branch? Be careful though, some are just for show. E-TYPE FABS has tested its own design and discovered an extra 15bhp for the E-Type. AJ6 Engineering (former Jaguar development engineers) has developed a TT range for E-Type and XJ, which offers great gains. Expect to pay around £650 for the system, extra for the manifolds. Suspension: Polyurethane bushes have become very popular; easy to fit, long lasting and more efficient. In some instances though, they can give a worse ride. Prices start at £22.50 for a top wishbone kit. Gaz dampers offer tremendous improvements. Prices start at £46. Wheels and tyres: It is tempting to fit the widest and stickiest rubber around. Well don’t as it will upset the chassis; seek a decent balance instead. An E-Type on 6in rims and Pirelli P4000 tyres works a treat. Jaguar advised against chrome for competition work; MWS can supply a 6in rim fitted with a Pirelli tyre, balanced for around £205 a go. Always fit the correct speed rating or your classic insurance policy could be invalidated.
E-Type
E-Type is still top of the tree and remains quick today even in standard form. But… Rear Axle cage houses IRS (independent rear suspension). Ratios were 3.31:1 and later 3.07:1. Some export cars had 3.54:1 and were real screamers. All (apart from later S2 cars) featured limited slip. Just make sure all is in good condition. Dana, the original manufacturer, still exists and can rebuild your unit (or swap ratios) using new parts. Front suspension is by torsion bar. This can be replaced by that used on the famous lightweight cars. Derek Watson offers a conversion kit that alters the front wishbone mounting point (as used on the works cars) and modified steering arms to maintain correct camber and Ackerman angles on hard cornering. The kit also includes four-pot callipers and ventilated discs with stronger stub axles. The whole lot is less than £1000. Various dampers can be used, with all offering better than standard performance. Engine size started at 3.8 before growing to 4.2 and then 5.3-litre V12. The 3.8 is the classic, making as much power as the 4.2 but not as much torque. It is possible to bore and stroke the 4.2 to 4.5-litre (4.7 the maximum heard of). Jaguar blue-printed its road test cars to achieve 150mph. So can you… Make sure that everything is lightened and balanced and put together as if in a hospital. If you are unsure, seek professional help. The 265bhp Jaguar quoted was wide of the mark but engineers did achieve 344 genuine bhp on a race engine. Today, 300bhp is a reality for a road-going engine while retaining SU carburettors. Spot-on ignition systems are worth more than a set of fancy camshafts - consider electronic and if you can spend money, have the system mapped. My advice would be to lighten the flywheel to 19lbs, fit Series 3 XJ6 inlet valves and pistons - they’re actually 8.75:1, slightly lower than the 9:1 as standard, but are a better piston design. With a little skimmed from the cylinder head, the 9:1 will soon be back (if you want it). Series 3 XJ6 (on injection) camshafts also have a better than standard profile. Best are D-Type profile cams, available from Guy Broad at around £300 a pair. Always use the latest highoctane fuel; Tesco is boasting of 100 octane from its brew, that’s almost five-star be gad! V12 is very quick as it is but the carburettors are awful (there to help with proposed emissions). A complete fuel injected XJS engine could be fitted, although there will be some difficulty with the ancillaries. Burlen Fuel Systems offers an SU conversion (all new, genuine SU parts) that improves both performance and economy. Reckon on around £650 for the parts. Gearbox on 3.8-litre cars best replaced by the later (late ’64 on) four-speed all synchro unit. Up to £900 second-hand though! A five-speed gearbox conversion is popular, especially with a 3.54:1 diff. Note that the gearbox on regular (ie not 2+2) cars was a short mainshaft and overdrive could not be used. Due to compact length, use of other types of ‘box is restricted (although XJ6 overdrive can be grafted on if using auto transmission tunnel. The CJ5, Toyota Supra or Getrag fits, but further mods usually needed. V12 will need the toughest ‘box in the book so look to the CJ5.
- Source: Will fit
- Mk 2 front suspension complete: Mk1
- Mk 2 rear axle: Mk 1 (needs cutaway wheel spats)
- 420 front suspension complete, including power steering: Mk1 & 2, 240/340 & S-type
- 420 front suspension uprights, including calliper: Mk1 & 2, 240/340, S-type, S1 E-Type (needs S2 E-type steering arm)
- 420 straight port cylinder head and carbs: 3.4 Mk 1, Mk 2 (2.4 needs bigger carbs), S-Type
- Straight port cylinder head will fit all models. Standard on XK 150S, E-Type, MkX/420G, 420, 240/340 and XJ6 (rear waterways will need blocking, best avoided)
- S-Type all-synchromesh gearbox with overdrive: Mk1 & 2 (make sure that clutch is correct three-part)
- Late S-Type as above (compact overdrive, needs prop-shaft too): Mk1 & Mk2
- Engines in principle will swap across the range. A 420G engine into a Mk1 will pose some difficulties while that from a 3.8 S-Type is an easy fit. For the perceived gain, best leave the 4.2 where it is
Interchangeability
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Excellent and useful article.
I agree that the 420 power steering box works well on the Mk2 jaguar but realistically this is not an option as it's now almost impossible to source.
The rack and pinion systems from some suppliers should be named and shamed.
Comment by: mike hughes Posted on: 28 Sep 2011 at 07:20 AM
Above article most absorbing and insightful. My only concern is my recent purhase from the UK for a electro-hydraulic powered XJS rack and pinion for my 3.8 MK2 which I will be installing soon.......now concerned.
Comment by: mornay van niekerk Posted on: 26 May 2012 at 06:52 PM