X-ray spec: Jaguar XJ13

Starved of support, XJ13's potential stayed hidden for years. Keith Howard talks with the man who tested it against its main rival, and still rates it

As many have discovered, recapturing past glories in motorsport is no cakewalk. The relentless pace of technological progress is such that that just a few years off the treadmill can put you hopelessly behind the game.

One marque to have pulled it off is Jaguar. Its golden era at Le Mans lasted from 1951 to '57 and comprised a remarkable five wins, including those by the blue cars of Ecurie Ecosse. The works team withdrew at the end of 1956 and didn't return to La Sarthe until '84, in the lead-up to its emotional win in 1988, emphatic 1-2 finish in '90 and oh-so-near 2-3-4 of the following year. But the fairytale story of Jaguar at Le Mans might well have had another chapter during the 1960s when the XJ13 — Malcolm Sayer's last design for Jaguar, and by common consent one of the most beautiful racing cars ever built — was developed in secret for a possible return.

In the event, XJ13 never turned a wheel in competition. Although a 'new GT prototype' was mooted for 1963 or '64, Bill Heynes did not issue a specification for it until 1965 and the only example that Jaguar built didn't run until '66. In fact the car only ran then in direct contravention of Sir William Lyons' edict, a transgression for which Jaguar's test driver Norman Dewis — his official title was Chief Development Test Engineer — famously received a dressing down from the Old Man.

The XJ13 project might well have ended after that surreptitious outing at MIRA, under a dust cover in some corner of a Coventry workshop. But Dewis's enthusiasm for the car was such that Lyons permitted its development to continue, although at weekends only. In a prequel to what would happen two decades later with the XJ220, the project advanced only because of the out-of-hours enthusiasm of the Jaguar personnel who believed in it.

Ultimately it was wasted effort and emotion. A final decision was made not to return to Le Mans and the XJ13 project was laid to rest for keeps. When the car broke cover in 1971 to appear in a film promoting Jaguar's new road-going V12, a mechanical failure caused it to crash heavily. Although Norman Dewis escaped serious injury, the car was badly damaged and there must have been those in Jaguar who questioned whether this cul-de-sac in its racing history was worth restoring. But restored it was and it now forms a prized exhibit in the Jaguar Daimler Heritage Trust Museum. In 2001 an anonymous Japanese collector offered £7m for it but was rebuffed; reportedly it is insured for a similar sum.

Setting the car's rarity and beauty to one side, a body of dispassionate opinion holds that the XJ13's fate was exactly what it should have been. That the car might have been competitive had it raced in 1965 or before, but not by the time it was ready to run in '67. That, not to mince words, it was no GT40 beater.

But Dewis — who still drives the car on demonstration runs — reckons otherwise, and is in a unique position to judge. Because not only did he do the development driving for the XJ13, he also had the opportunity to compare it back-to-back with a GT40 at MIRA in 1966, at the beginning of the XJ13's year-long leisure-time development programme. He was in no doubt which was the better car. Despite Jaguar's car being almost totally undeveloped, he reckons it was clearly the better of the two.

"We were way ahead of the GT40 in many respects, in the performance values alone. The XJ13 was faster accelerating and possibly we'd have seen a higher maximum speed down Mulsanne Straight. We were looking at 200mph-plus. I took it to Bruntingthorpe aerodrome and pulled about 195mph — it was very stable at those speeds, superb. Had we gone to Le Mans I'm sure we would have put up a damn good show."

If he's right, then the XJ13 doesn't deserve to be remembered as a half-hearted effort that delivered too little, too late so much as a car that could, with appropriate support from the bean-counters, have spearheaded a triumphant Coventry return to endurance racing.

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XJ13's aerodynamics were excellent. Dewis: "I still class Malcolm Sayer as one of the best aerodynamicists we've ever had in this country. Apart from doing wind tunnel work with him at Farnborough, I learnt a lot wool-tuft testing the cars with him. We used to stop in Nuneaton to buy a big skein of wool and a roll of Sellotape. When we got to MIRA we'd cut the wool into 4in lengths and stick it all over the car under Malcolm's guidance. He would then be driven in another car alongside, in front of and behind to watch the tufts, and photographs were taken too. Also I would record everything I could see from the cockpit. We did this on all the cars. XJ13 was pretty much spot-on straight from the box — it had a very low drag coefficient. But then his shapes were always so good, every time."

Dewis's crash in the XJ13 at MIRA in 1971, while driving the car for a promotional film, was caused by the catastrophic failure of one of its lightweight wheels. "Magnesium alloy wheels were new to us then and what we and the manufacturer didn't realise is that magnesium can corrode internally. The wheels were fresh from the stores — the car had been on the others so long that I'd said to fit new ones. They looked perfect but even while they were in storage the corrosion was taking hold. When we found the broken pieces of the wheel and took them to the laboratory it was like black coal dust inside. It was a lesson we learnt the hard way."

As soon as the first prototype of any new Jaguar was built, Dewis took it over until its suspension dynamics were signed off as satisfactory. "I always started carefully, never went straight out and drove a car quickly. The first thing was to go on the 108ft radius steering pad at MIRA. You're only doing 35mph or so round that but you quickly find out the bad habits of a car, its breakaway behaviour. If you spin it there, there's no damage done. Whereas if you do that out on the circuit you're likely to bend the car and set back the whole programme. Then I'd take the car around the bottom of the banking, where you can get to 100-110mph and find out what the high-speed handling is like. Again it's pretty safe because you've a lot of space. The XJ13 had very rapid oversteer originally—the rear end broke away extremely quickly. But we'd sorted that by the end of our year's development and had a nice-handling car."

If the MIRA crash taught Jaguar anything other than metallurgical lessons about magnesium, it was that the XJ13's all-aluminium monocoque formed a tough safety cell in an impact. Although the car rolled end over end and barrel-rolled, the unbelted Dewis escaped without serious injury. "I'd already had a roll-over with a C-type. That was worse because I was pinned underneath that, and couldn't move. I was thinking, any second now there will be a loud 'woof' and that'll be the end of Dewis. The XJ13 crash was a bigger mishap but I was all right tucked in under the scuttle. I just switched off and got down there. The outside of the car looked like it had been in a crusher."

Codenamed XJ6 (until appropriated by the marketing department, XJ had always stood for eXperimental Jaguar), the 60-degree quad-cam 24-valve V12 had a bore and stroke of 87x70mm to give a swept volume of just under five litres. Compression ratio was 10.4 to 1. Eight units were made, the first of which was originally fitted to XJ13 but later replaced with XJ6/7, the engine still in the car today. The principal difference between them was that X16/1 had chain drive to the cams whereas XJ6/7 had gear drive. There were also small (and largely ineffectual) changes to /7's port angles and it had a 12-cylinder Lucas Opus ignition system rather than the twin six-cylinder units fitted previously. Peak power output was around 500bhp at 7600rpm with 378Ib ft peak torque developed at 6250rpm.

Suspension was what we now regard as classic Jaguar — double wishbone from a bolt-on subframe at the front and twin lateral links at the rear, plus long radius arms. The fixed-length driveshafts formed the upper lateral links, just as in the E-type.

Wally Hassan, who moved across to Jaguar from Coventry-Climax in 1966 to develop the V12 for road use, expressed disappointment in his autobiography that the racing version developed only about 100bhp per litre (he was used to achieving 130bhp per litre at Climax) and had poor low- and mid-range torque. George Buck, who worked on the engine, agrees with the criticism and explains why: "The angle of the inlet port was basically unsound. It came in from overhead rather than from the side, so instead of a downward curve in the port you had a curve the other way. We never did achieve good flow figures with it. We had to do it like that because we couldn't get the intake manifolds and throttle bodies accommodated any other way — it was a space issue."

The five-speed transaxle was supplied by ZF. Including 41 gallons of fuel and allowing for a trim driver, the XJ13 weighed 2724lb (1235kg) distributed 44 percent front, 56 percent rear. Overall height was 39in, undercutting the GT40, and the centre of gravity height mid-laden was 15.06in.