[4][5] Bailey's original 8.0 L design used double overhead camshafts heads sharing the same basic layout as the inline 6-cylinder XK engine, in order to allow for a relatively high redline.

After the XJ13 project was cancelled the team of Hassan and Mundy designed a new single overhead cam head, with the camshaft lobes acting directly on vertically-inclined valves through bucket tappets.

[7] The revised head design by Hassan and Mundy also had longer, more restrictive inlet ports sacrificing top-end power but which—along with an increase in displacement to 5.3 litres (5,344 cc) (90 mm bore x 70 mm stroke)—greatly improved performance at lower and mid-range engine speeds, which was more desirable in heavier luxury cars.

Initially the OPUS ignition amplifier unit was secured directly to the engine between the cylinder heads and had problems due to overheating.

Originally the V12 was supposed to use an advanced fuel injection system under development by AE Brico but this plan was cancelled at a late stage, possibly due to concerns that the design was too similar to Bosch products.

Series 3 XJ12 and Daimler Double Six cars used the CEI system until the end of their production in 1992, but it was superseded in the XJ-S in mid-1989 by another from Magneti Marelli.

[11] The 6.0 litres (5,993 cc) engine on X305 used a new Nippondenso distributorless crank-fired ignition system with coil packs very similar to Ford EDIS-6 units.

And with drivers such as Tom Walkinshaw himself, Briton Win Percy, West German star Hans Heyer, and budding Formula One driver Martin Brundle, the big Cats would soon become the cars to beat and in 1984 TWR had not only won the ETCC but had also won the 1984 Spa 24 Hours with Walkinshaw, Percy and Heyer driving.

However, the XJS V12's were brought out of their early retirement for one off events over 1985, 1986 and 1987 with their best result coming at the 1985 James Hardie 1000 at the famous Mount Panorama Circuit in Australia.

Not only were the Jaguars easily the fastest cars in the race (in what was Australia's first year of running to Group A rules), but local Jaguar driver John Goss teamed with TWR regular, West Germany's Armin Hahne in the team's 3rd car to win the race with Walkinshaw and Percy finishing 3rd.

By 1991, the V12 was good for 7.4 L inside the XJR12, developing an impressive 750 bhp (559 kW; 760 PS) TWR also upgraded production Jaguar cars (usually XJRSes), with a variety of styling, handling and performance modifications.

Lister Cars, a well-known Jaguar tuner with a long history of technical collaboration with the British automaker, made frequent use of this powerplant.