Jaguar XKR GT3

For the first time in some 14 years, Jaguar enthusiasts will now be able to cheer on an officially-backed Jaguar sports car in European racing. The last Jaguar sports car team to enjoy factory backing was Rocketsports in the North American Trans-Am series, running the old shape XKR, and prior to that was the TWR XJ220C back in the early 1990s. Now it’s the turn of Apex Motorsport with the new XKR GT3.

In September 2006 Richard Lloyd’s Apex team signed the contract with Jaguar to build and run XKRs in the new FIA GT3 championship. This is not, however, a factory motorsport programme and the Apex race activities are independent of Jaguar. At the time of the programme’s announcement in September, Stuart Dyble, Jaguar Board Director, said, “We welcome this initiative which will result in production Jaguar sportscars being seen once again on race circuits across Europe. We have just launched the all new Jaguar XKR road car, and GT3 is the natural race series in which to compete.”

Thanks to the peculiarities of the GT3 series, whilst the XKR GT3 remains remarkably similar in looks to its road going cousin, much has been done underneath the skin to improve performance.

GT3 is a very interesting championship because there aren’t really any technical rules. The technical rules stretch to about three pages, but the homologation paper is about 120 pages, so it’s all about being creative with your packaging. The engine has to remain standard and the pick-up points have to remain standard, but there is an awful lot that you can do to optimise the car and that is what we are doing at the moment. We are starting with a fantastically good base car, the XKR is a sensational road car and having driven it, it is simply astonishing. Therefore, we believe it will make a very good race car.

Richard Lloyd

The levelling process for the championship is done by practical experimentation. A control driver tries a car from each team and if it proves to be quicker than the benchmark time set by that driver in a Porsche 997 GT3 then the car in question, and its sister cars, are handicapped either by adding additional ballast, altering tyre sizes, aerodynamics or adding/modifying engine intake restrictors, or a combination of these.

Engine and Drivetrain

The XKR GT3 retains the same supercharged, 4.3 ltr quad-cam V8 engine as the road going car. The rules allow for alterations to some key components on the basis of durability, and Apex have made some modifications to increase power, now reckoned to be generating around 500bhp.

Cooling is achieved through the use of an air-to-water cooler mounted low down in the nose of the car to cool the water from the standard supercharger charge coolers, with an oil-to-air cooler mounted behind that for the dry-sump oil system. The main engine cooling duties are handled by an NRP aluminium radiator mounted in the standard factory position.

Engine intake air is drawn from vents mounted on the front corners of the car’s nose through a custom carbon fibre box holding the air filter before being fed through to the supercharger.

Power is transmitted to the rear wheels via a carbon propshaft to a 6-speed Hewland sequential transaxle in the rear. Since none of the road going XKRs come with a manual gearbox, this was something that needed to be changed anyway. This change also resulted in an enhancement to the car’s balance by altering the weight distribution slightly to the rear.

Suspension and Brakes

The suspension has also been altered by Apex. Using unequal-length double wishbones on all four corners and specially fabricated anti-roll bars, controlled by Koni two-way adjustable coil-over shock absorbers, the car is given a well balanced and reasonably easy handling style. Since professional drivers are disallowed by the GT3 rules this suits the majority of drivers likely to race the car.

Braking at the front is controlled by Alcon six-pot calipers wrapped around 375mm discs, and at the rear Alcon four-pot calipers with 330mm discs. Apex use an AP Racing floor-mounted, adjustable brake bias pedal assembly, with the bias control mounted on the central column below the electrical switches.

Wheels and Tyres

GT3 specifies the use of a control Michelin tyre in the championship, though some leeway is given on width. Maximum diameter is fixed at 18 inches, since Michelin don’t manufacture 19 or 20 inch tyres. 11 inch wide OZ cast magnesium rims are used front and rear. Since the races are all one hour in length, and the drivers are non-professionals, putting wider rims on the rear could be counter-productive. The narrower rims allow the tyres to heat up faster at the start of the race.

Chassis

The car uses the same lightweight aluminium chassis as the road car. All the interior is stripped from the car and a steel roll-cage is fitted to protect the driver. The wiring loom and all computers are removed and replaced with a custom loom (the wiring alone saves 25kilos of weight).

Although the reduced weight of the all aluminium chassis is not such an immediate advantage, given the additional weight penalties imposable by the FIA, its sophisticated bonded and riveted structure makes the car very responsive to suspension and set-up changes.

On the Track

Out of the box the XKR immediately proved its pace, being only a second a lap off the Aston Martins’ time. The car still retains much of its Jaguar characteristics, feeling very solid and remarkably comfortable, with no crashing or banging over bumps and road imperfections. Through the corners the car exhibits no unwanted behaviour or dramas, remaining very stable and controlled, and its acceleration out of the apex is staggeringly quick, making full use of the power available from the supercharged engine.

The sound of the car is very distinctively Jaguar, retaining the growl of the road going XKR at full throttle but now even more exaggerated. The whine of the supercharger is particularly audible without the standard induction system to mask it, blending nicely with the sound from the straight-cut gears in the Hewland gearbox.

Achilles Heel?

The main Achilles heel for the XKR GT3 could prove to be the use of the standard Jaguar supercharger: a Roots type blower, where air is forced into the engine using two helically formed counter-rotating rotors. Roots blowers are relatively inefficient, producing the most charge air heat for a given amount of boost pressure compared with a Lysholm twin-screw supercharger. Keeping the charge air cool at high revs, over a one hour race in hot weather, could prove to be problematic. During a warm testing day at Donnington Park, modifications were being made to the air ducting over the coolers and heat reflection away from the intake box for just this reason.

 

If there’s one thing we can certain of, however, it is that the experience of the Apex team will help them overcome this potential hurdle and the Jaguar XKR GT3 will be a force to be reckoned with in GT3 for many several seasons to come.