V12 powered production automobiles were unusual at the time; Italian luxury sports car makers Lamborghini and Ferrari produced such models.

The buttresses behind the windows were criticised at the time as German authorities feared these would restrict rearward vision, and refused to give type approval to the XJ-S (and to the similarly designed Lancia Montecarlo) – requiring German XJ-S buyers to obtain road approval for each individual car upon registration.

[8] Italian styling house Pininfarina introduced a 1978 concept car based on the XJ-S, called the Jaguar XJSpider;[9][10] which did not see production.

[11] At the same time, the XJ-S HE received changes to its exterior and interior: there was body-coloured boot trim in place of the standard previous black, new five-spoke "Starfish" alloy wheels fitted with 215/70R15 Pirelli Cinturato P5 tyres, chrome inserts on the upper part of the bumpers, and burled elm inserts on dashboard and door cappings.

Between 1983 and 1987, the six-cylinder-engined cars were only available with a five-speed manual transmission (Getrag 265), with a four-speed automatic (ZF 4HP22) offered from 1987 onwards (along with improved fuel injection as used on the XJ40).

To accommodate this design element, the Hess & Eisenhardt convertibles have two separate fuel tanks, positioned to allow for the roof to fully retract.

The process of converting the stock Jaguar XJS coupé into the H&E Convertible included the post-production removal of the roof, cutting the body in several sections, the addition of steel reinforcements behind the driver's seat, and 9.1 kg (20 lb) weights placed just behind the headlights to eliminate harmonic resonance caused by the significant modifications to the car.

H&E XJS convertibles are easily identified by the lower folding top, as well as two small badges located just behind the front wheels.

The car had a distinctive body kit, special 15" alloy wheels, 235/60R15 Pirelli P600 tyres, a unique suspension system utilising modified coil springs and Bilstein shocks, a luxurious interior with Connolly Autolux leather along with walnut wood trim, and handling improvements.

During a comparison test conducted by Motorsport magazine in 1997, the XJR-S outperformed the other competitors which included a BMW 850i, Porsche 928 GT and a Ferrari Mondial.

[citation needed] Jaguar considered a luxury Daimler version to be marketed as the Daimler-S, notably without the buttresses and with only a single prototype built in 1986.

[18] Paul Banham did produce some custom notch back coupés without buttresses, larger rear side windows, and a narrow C-pillars.

The XJS was relaunched in its final form in May 1991 under Ford Motor Company's new ownership, as a division of its Premier Auto Group.

The main interior change was to the instrument pack; the original featured distinctive drum type ancillary gauges - this was replaced by a more conventional style binnacle similar to that found in the XJ40 saloon.

TWR body kits included a rear spoiler with a distinctive hole for radio antenna on the right hand side.

[21] In 1986, the newly-formed Lister Cars, under the leadership of engineer Laurence Pearce, began offering a high performance package for the Jaguar XJS.

The standard package included increased engine displacement to 7.0-litres, a modified fuel injection system with four additional injectors and throttle bodies, larger engine valves along with connecting rods manufactured by Cosworth, a new crankshaft, new cylinder heads, new inlet and exhaust valves, new bearing caps, and a new body kit featuring a spoiler at the rear with a modified rear light clusters and flared wheel arches to accommodate the wide tyres.

[29] Walkinshaw hit upon the XJS, rather than the Rover 3500 that BL wanted to use, as its double wishbone suspension would allow it to fit the widest possible wheels under the Group A regulations.

British Leyland was still smarting from the failure of the XJC racing programme in 1977 and opted not to provide factory backing for the new effort.

[29] TWR moved forward with the project, depending on outside sponsors like Akai and Motul instead, although Jaguar did provide some engineering support.

[29] The XJS won its first race that season when Walkinshaw and Chuck Nicholson won the XIV Grand Prix Brno, the cars simply proving too fast for their rival BMW 530/530i and Alfa Romeo GTV6 opposition with Walkinshaw qualifying 5.37 seconds faster than anyone else on the 10.925 km (6.789 mi) Brno Circuit.

During the championship season the TWR Jaguar team also won the prestigious Spa 24 Hours race with an XJS driven by Walkinshaw, Hans Heyer and Win Percy, proving not only the speed of the cars but the reliability of the 5.3 litre V12 engine.

However, the tyres failed to arrive at the circuit in time and Walkinshaw eventually qualified 10th having to use front wheels on the car as no suitable rubber was available.

The Scot was determined to come back in 1985 and win the race in the first season of Group A in Australia and in 1985 the three TWR XJS Jaguars were brought out of retirement for the James-Hardie 1000.

After having the oil line replaced, Walkinshaw resumed in 4th place but easily caught and passed the JPS Team BMW 635 CSi of 1985 Australian Touring Car Champion Jim Richards in the last laps to make it a Jaguar 1–3, with Walkinshaw following Goss across the finish line in a formation finish.

The TWR XJS Jaguars were next seen late in 1986 at the Fuji 500 in Japan in what was meant to be its Group A swansong as it ran out of FIA homologation in at the end of the year.

The Fuji race saw the Walkinshaw/Percy car lead the race until retiring on lap 6 with no oil pressure after easily holding off the 313 kW (420 hp) Holden Commodore of Peter Brock on the circuit's 1.5 km (0.93 mi) main straight, while the Hahne/Denny Hulme/Walkinshaw car only lasted until half distance before retiring with a broken differential.

While he reportedly had obtained more power from the V12 than TWR, Willmington didn't have TWR's resources to develop either the car or engine reliability and results were not forthcoming, though on occasions where the track had a long enough straight (such as Sandown Raceway in Melbourne or the Adelaide International Raceway), the Willmington Jag was usually the fastest car in a straight line.

John Goss also ran his own privately entered XJS in the 1986 James Hardie 1000 after Jaguar-Rover Australia pulled its backing of the proposed TWR return to Bathurst due to a severe downturn in the Australian car market forced him to defend his crown alone, though he did receive some technical assistance from TWR.

Their championship has now been running for over a decade, offering an affordable entry into club motorsport in the UK, and continuing to attract large grids of this unlikely racing car.