With a reliable engine in the house, a sorted suspension and running gear, two things were needed for a successful XK 120C, the C for Competition. Although the sinuous, voluptuous body of the standard 120 was a sales sensation, it wasn't all that good at high speeds, creating a lot of drag without a lot of downforce. So William Lyons and his gang turned Malcolm Sayer loose on the problem, and he came up with a flat-topped body with slab sides and a shortened XK 120 grille. The C version or C-Type had a one-piece flipover hood and fender setup with two sets of exhaust louvers, top and side at the rear end and leather hold-down straps augmenting the side-mounted release levers. The car's midsection and rear fenders bore little or no resemblance to any Jaguar of recent vintage, but they sure worked with the rest of the package in the wind tunnel. The headlamps were covered, the taillamps were vestigial, and there were no bumpers, front or rear, to interrupt the flow of air up and over the broad, flat expanses of front end. All of the early factory cars used a single flip-up Brooklands windshield.
The C-Type package was tidy, with an overall length of 157 in., a 96-in. wheelbase, 51-in. front and rear track, an overall height of just 38.5 in., with a scant 5.5 in. of ground clearance. The factory specs said the car would do 150 mph, accelerate from 0 to 60 mph in 8.1 sec., and achieve nearly 12 mpg at its power-to-weight ratio of only 9.9 lb/hp. The lightweight aluminum skin of the new race car was stretched out over a triangulated, drilled tubular steel chassis with subframes carrying the engine and modified standard front and rear suspension systems taken from the road car. All up, it weighed only 2,100 lb, lighter than the standard car, but not as light as some of the competition even in 1951. The only thing that interrupted or disturbed the fore-aft flow of this lissome bodyshell was the over-and-under header and muffler combination that was tucked up underneath the left-side door, keeping the heat away from the drivers, who all sat on the right side.
The XK 120C engine was a modified 3.4-liter version of the street engine, rated at 210 bhp at 5800 rpm (an otherwise authoritative Jaguar reference book says the engine made a staggering total of 233 hp per liter, when in fact the number was only 61.7; a simple conversion mistake, we'd venture). It was designed with 9.1:1 compression, an aluminum head and cast-iron block, a seven-main-bearing crankshaft design, and chain drive to its twin overhead camshafts. The original cars were fitted with twin 1-3/4-in.-bore SU carburetors, but later models used 2.0-in. carburetors, and still later ones switched to Weber carburetors. The engine was backed by a three-disc Borg & Beck racing clutch (they had learned their lesson), a four-speed synchromesh racing transmission, and a choice of a 3.31:1 or 3.54:1 rear-end ratio.
The suspension used upper and lower wishbones and torsion bar springing media with tubular shock absorbers, and the rear end suspended the live axle with torsion bars and tubular shocks as well. The original specification for the 1951 car called for 12-in. drum brakes front and rear, a 60-gal. fuel tank for endurance racing, and skinny little 6.50x16 tires on knockoff spoked wheels.
During the 1951 season, the C-Type team won Le Mans, with Peter Walker and Peter Whitehead driving, while the other two entries DNF'ed due to lubrication problems that were fixed immediately by substituting steel oil delivery tubes for the original soft copper ones. The three-car team went to Ireland for the TT and took first, second and fourth. Moss, who won the TT race, won two more races at Goodwood.
Jaguar went into series production with customer cars in 1952. For its own efforts, Jaguar built a radically restyled long-nose, long-tail car for the 1952 Le Mans race, and all three cars entered failed to finish, owing to cooling problems caused by the low nose. But a C-Type with Dunlop disc brakes had run third before DNF'ing at the Mille Miglia, and Moss drove a disc-brake C-Type to a win at Rheims that year. Ecurie Ecosse, the famous Scottish racing team, finished third with a standard XK 120.
For 1953, the new Jaguar competition department reorganized the effort, went back to the original body design, fitted all the cars with Dunlop disc brakes and three Weber carburetors instead of the earlier SUs. The engines, though down a full point of compression, were up to 220 bhp, and the brakes would enable all the drivers to carry more speed into the corners and out of the corners, as well as stop far fewer times during the 24 hours for friction pad replacements. The tallish Brooklands screen was replaced by an integrated screen, much lower, like the one on the 1952 car.
Well, it all came together like a dream. Tony Rolt and Duncan Hamilton finished first overall, with Peter Walker and Stirling Moss second, Peter Whitehead and Ian Stewart fourth, and a Belgian-entered 1951 car, drum brakes and SU carburetors, ninth. It was the first time in the history of the event that the winning car had averaged more than 100 mph for the duration-105.841 mph, to be exact. Rolt and Hamilton also set a new distance record for the 24 hours as well.