Jaguar Mk. VII

Introduction

It wasn’t until 1954 that a car with fully automatic transmission and two-pedal control became freely available in Australia at a price below £2500 – somewhat affordable! The car was the Jaguar Mark VII saloon fitted with the American Borg-Warner automatic transmission similar to that used in U.S.A. on the Studebaker. It sold in Australia for £2312 including tax, compared with £2173 for the same model with conventional four-speed gearbox. Automatic transmission was not available on the Jaguar XK120 Roadster.

For those that had never driven an automatic car, the Jaguar was a revelation in that it made it practically impossible to drive such a car badly from the mechanical viewpoint. This would have likely minimised the mcchanical troubles which haunted certain drivers who were unable to manage a clutch, gear shifter and the requirement to double-clutch a non-synchro gearbox.

The motoring press was quick to let the buying public know that, while slightly better performance was on offer with the conventional transmission in the hands of a really skilled driver, the average man or woman would have done much better sitting on top of an automatic. The results were particularly good on fast cruising, acceleration at cruising speeds being highly effective in making the potential of a fine engine available without the need for judgment in handling gears. This was deemed, at the time, a good safety factor in passing other traffic. But before we talk about the self-shifting Jag, lets look back at the development of the Jaguar Mark VII.

The Jaguar Mark VII Chassis

The Mark VII chassis came from the Jaguar Mark V and the wheelbase remained the same at 10 feet (3,048.0 mm). The Mark VII's body, however, looked more streamlined, with integrated headlights and mudguards, a two-piece windscreen, and longer rear overhang. As on the Mark V, the rear wheels were partially covered by removable spats. Whereas the Mark V had a pre World War 2 pushrod engine originally developed by the Standard Motor Company, the Mark VII was powered by the newly developed XK engine. First seen in production form in the 1948 XK120, the 3442cc DOHC straight-six provided 160 bhp (119.3 kW), the same as in the XK120, and the saloon's claimed top speed was over 100 mph (160 km/h).

While the car was being developed Jaguar thought it would find most of its customers overseas, mainly because UK car tax at that time penalised buyers of larger-engined cars. However it went into production just as Britain's postwar economic austerity began to ease, and in 1951 the car's enthusiastic reception in both the British and American markets prompted Jaguar to relocate production to larger premises, at the Browns Lane plant, which had been built for wartime production as a shadow factory and was now available for immediate use.

The published performance figures for the Jaguar Mark VII were based on the standard 8:1 compression ratio, but as this was unsuitable for the UK market's low-octane Pool petrol a 7:1 engine was optional. British motoring magazines tested the car's performance with the higher compression ratio, using the Ostend to Brussels autoroute in Belgium, where 80 octane fuel was available. A Mark VII tested by The Motor in 1952 had a top speed of 101 mph (163 km/h), accelerated from 0–60 mph (97 km/h) in 13.7 seconds and returned 17.6 miles per imperial gallon (16.1 litres per 100 km; 14.7 mpg‑US). The test car cost UK£1693 including taxes. In 1952 the Mark VII became the first Jaguar to be offered with automatic transmission. By the time the model was upgraded to M specification in 1954 some 20,908 had been produced.

Automatic Hill-Holder

There was an automatic hill-holder; the brakes would come into action automatically when the car was stopped in traffic, and it was immobilised when parked. The mechanism was so effectively safeguarded that there was no possibility of disastrous mistakes being made by an inexperienced driver, or one who had recently taken over and was in process of learning a partly new technique. The design was such that the hydraulic torque converter was inoperative when conditions called for direct drive; this also ensured that fuel consumption remained approximately equal to that obtained with a normal transmission correctly handled.

While control was almost completely automatic, it was not taken entirely out of the hands of the driver. An extra-low ratio was provided for maximum acceleration or for creeping up very steep hills and through difficult situations, and it was possible also to "rock" the car out of mud or snow with alternate low and reverse even more easily than could be done with normal gears. The low could also be used to get braking through the engine. Elimination of the use of the handbrake by the hill-holder and automatic operation of the brakes when stopped - primarily intended to prevent creep due to the torque-converter action when idling - was found to be of great value to drivers new to driving an automatic transmission equipped vehicle.

New owners were delighted that there was no clutch pedal - just an accelerator and a large brake pedal, both for the right foot. On the dash, above the steering wheel, was a lever which could be moved into five positions along a plain quadrant (no gate). These were marked P (park), N (neutral), D (drive), L (low), and R (reverse). The only difference between P and N was that at P a pawl in the transmission was engaged, preventing the car from rolling; otherwise both corresponded with neutral in a normal box. With the lever set elsewhere the starter would not work. With the engine running, the lever was moved normally to D and stayed there. In all ordinary circumstances driving then became the business only of accelerator, footbrake, and steering. Open the accelerator and the big Jag would glide off on the torque converter, climbing up automatically from its intermediate epicyclic gear as conditions become suitable.

Normally only intermediate and direct drives and the torque converter were used, but the L position - protected by a safety catch to prevent accidental engagement - was available for abnormal conditions, and could be used for starting at any time if maximum acceleration was desired. R, the last position on the quadrant, gave reverse, and its position next to L made possible the "rocking" of the car mentioned earlier. This was particularly easy, as no damage could be done by going into reverse with the car moving forward. A similar hydraulic locking-out mechanism prevented damage due to the parking pawl coming into operation should the lever be accidentally moved to P when the car was moving. The hill-holder is also worked from the hydraulic pump system in the transmission, which supplies pressure to the normal hydraulic brakes when the car comes to rest. There was an ordinary handbrake as well.

With all this array of safeguards and automatic pilots looking after the driver, the only real worry was the out-of-work left foot, which for most drivers having only experienced a manual transmission, would have found that it would move of its own volition - like the frog's leg in Galvani's electrical experiment - looking for something that wasn't there. Using the D position throughout, the take-off was completely smooth and there was adequate flexibility for traffic driving. All ordinary driving was done in this position. The manner in which the accelerator was used affected the type of getaway. Taken gradually, as you would normally drive out of consideration for tyres and general expenses - to say nothing of the police - the change to direct drive through torque converter came in at about 18 m.p.h. But with a full-throttle start the revs kept on building up in intermediate until about 58 m.p.h., when the change was made. Thus there was a big measure of performance control still left to the driver.

They could, in fact, take things a stage further by going beyond the full-throttle position to a "kick-down" position, where direct drive came in at nearly 70 m.p.h. The same mechanism enabled you to revert instantly from direct to intermediate drive at any speed below about 62 m.p.h., thus getting torrid acceleration for passing and the like. The following performance comparison figures were obtained by road testers of the day (in seconds) compared with those for a manual gearbox Mk.VII.

From rest:

• 0-30 0-40 0-60 0-70
  • Automatic: 4.8 7 13.4 18.4
  • Manual: 5.0 7.4 14.0 17.5
• In top gear: 40-50 50-60 60-70
  • Automatic: 3.2 4.65.4
• Manual: 3.5 5.0 4.5
  

These starts through gears were made using the L setting, then shifting to the D position, this being the procedure for maximum acceleration from a standing start. The dual-personality character of the automatic transmission could be applied to hills, as well as to acceleration on the flat. You could cruise up them quietly or storm them in intermediate, according to the manner in which you put your foot down. In either case, your wishes were carried out with complete efficiency. The basic units consisted of:

• A single-stage, three-element hydraulic torque converter, resembling a fluid flywheel and containing a freewheel.
• A single-plate disc clutch which took over from the torque converter when conditions required a direct straight through drive. This prevented slip and fuel waste.
• Two epicyclic helical gear trains, one of which can be locked out of action, when necessary, by a multi-disc clutch.
• Two oil pumps which supplied the necessary pressure for operating the clutches and the brake bands controlling the epicyclic gears, and also to maintain pressure in the torque converter.
• A control valve assembly, to distribute pressure to the various points as required.
• A centrifugal governor connected with the accelerator pedal. This acted in conjunction with the torque converter to select gears in relation to speed and load. But when the throttle was depressed hard, its action was reduced, thus providing the "dual personality" performance mentioned earlier.

The manner in which a direct drive of 4.27:1 at the back axle and the forward epicyclic gear train of 1.609:1 satisfactorily replaced a manual four-speed gearbox was explained by the action of the torque converter. This gave a multiplication factor ranging from 2.15 at stalling speed, and falling to unity as the speed rose to 2000 r.p.m. At this stage the converter became in effect a fluid coupling, like the familiar fluid flywheel. In addition, a positive direct drive to the rest of the transmission was ensured by a single-plate clutch coming into action as explained earlier.

This multiplying factor of the torque converter, when applied to the reductions already available in the intermediate and rear axle gears, gave a lowest available gear of 13:1 in the D position, when was at maximum torque multiplication, up to 6.12:1, when the converter multiplication had dropped to unity at 2000 r.p.m. and it was giving a direct drive. Once the shift of gear had occurred, eliminating the intermediate train, the car was being driven direct through the rear axle ratio of 4.27:1, provided flywheel-torque converted speed was above 2000 r.p.m. When the low and reverse trains were called into operation by the manual control, the converter was able to provide automatically the following ranges:

• low forward, 21.2:1 up to 9.9:1
• reverse, 18.5:1 up to 8.6:1

As the process of energy conversion always involves some loss, whether the process is by gears, an electrical or an hydraulic device, a certain amount of heat is developed in the converter; hence the air-cooling system with a side intake in the flywheel-converter housing. The mechanism was necessarily fairly complicated, but not appreciably more so than a preselector gearbox-fluid flywheel system - a familiar transmission which had proved itself over many years.

Jaguar Mark VII M 1954 – 1956

The Mark VII M was launched at the British International Motor Show in October 1954. Although the engine continued with the same capacity and 8:1 compression ratio, it was uprated to 190 bhp (141.7 kW), giving the car a claimed top speed of 104 mph (167 km/h). The four-speed manual gearbox was standard, while the Borg Warner automatic, hitherto available only on exported Mark VIIs, now became optional for British buyers. Distinguishing the Mark VII M from its predecessor, circular grilles over the horns were installed below the headlights in place of the former integrated auxiliary lamps, which were moved slightly further apart and mounted on the bumper. Both bumpers now wrapped further around the sides of the car.

In 1956, with the advent of the Suez Crisis Britain anticipated fuel rationing, and bubble cars appeared on the streets. Jaguar switched focus to their smaller saloons (the Mark I 2.4 had been introduced in 1955), and neither the Mark VII M nor any of its increasingly powerful but fuel-thirsty successors would match the production volumes of the original Jaguar Mark VII. Nevertheless, before it was superseded by the Mark VIII, the Mark VII M achieved 10,061 sales during its two-year production run.

The Jaguar Mark VII In MotorSport

Both variants of the Mark VII won race victories, and an M version won a Monte Carlo Rally. In 1954 Jaguar built a lightweight Mark VII M which, although intended for racing, never participated in contemporary events. Road-registered KRW 621, it had magnesium body panels, D-type engine, Dunlop disc brakes and modified suspension. Factory-entered Mark VIIs won the Daily Express International Trophy Production Touring Car race at Silverstone five years running, and twice took the top three places. Stirling Moss won in 1952 and 1953; Ian Appleyard in 1954, with Tony Rolt and Stirling Moss 2nd and 3rd; Mike Hawthorn in 1955, from his teammates Jimmy Stewart and Desmond Titterington in 2nd and 3rd; and Ivor Bueb in 1956, with Belgian journalist and racing driver Paul Frère taking 4th.

In January 1956 a Mark VII M driven by Ronnie Adams, Frank Biggar, and Derek Johnstone won the Monte Carlo Rally. In August 1956, at Road America, in Elkhart Lake, Wisconsin, Paul Goldsmith's Mark VII averaged 59.2 mph to win a 100-mile NASCAR Grand National race for cars up to 3500cc.