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Jowett Jupiter Specifications, Performance and Statistics

Jowett Jupiter rolling chassis first appeared in the London Motor Show, October, 1949, having been designed in four months by the famous (of Auto Union pre-war, and Cisitalia immediately postwar) Austrian engineer Dr. Robert Eberan-Eberhorst. Bodywork was designed (also, astonishingly in view of its complexity and beauty, also in four months) in-house by Jowett's body stylist Reg Korner. Two complete Jupiters were on the road March 1950. The third Jupiter raced at Le Mans in June 1950 and won its class. First production batch of five cars were built in October 1950. Last Jupiter left the factory in November 1954.

See below for the Jupiter's standard tool list and images showing what some of the items look like


The Jowett Jupiter engine is a good example of British engineering eccentricity. It was Gerry Palmer's (he of the Jowett Javelin design, and later the MG Magnette, Wolseley 4/44 and variants) first and last engine design. It is a horizontally opposed four cylinder unit of bore 72.5mm x stroke 90mm swept volume 1486cc. (90.6cu-in). Compression ration 7.6:1 or 8:1 dependent upon destination country's petrol.

Left: rear view of a Jowett Jupiter engine without the two carburettors. At the top is the updated (throwaway) oil filter. Below one of the heads the left side exhaust manifold can be seen. The clutch housing is detachable; gearbox and starter motor attach to the clutch housing. Beneath the black cover-plates on the top of the engine the pushrods can be accessed. Aluminium covers (extreme right and left) hide the rocker gear.

The whole engine is sealed and runs under a slight vacuum - positive crankcase ventilation - universal today but  a novelty when the engine was designed in 1946.

Right: view of the engine installed in a Jupiter. The water pump pulley can be seen at the top, with oil filler pipe and dipstick below. One of the carburettors and the Lucas distributor can be made out on the left, as can the aluminium water transfer pipe which connects the head coolant to the crankcase front water channel - the water then flows round the front timing cover to the water pump at the top.

The strange white object top left is part of the Jupiter bodywork, since the whole of the bonnet is hinged up for the photograph. Red stripe is wing piping

Overhead valves are operated by rather long pushrods from a central camshaft mounted directly in the crankcase above the crank. Camshaft end-float adjuster can be made out in the above engine-installed photograph, just below the filler pipe.

Another view of an engine installed in a Jupiter.

This is in the last Jupiter built, the Mk1a owned by Pat Lockyer. He rebuilt the engine himself after welding up a crack that was letting water into the sump.

As with everything Pat does, this engine runs silently, superbly, and the car travels swiftly.

For the standard engine, maximum power occurs at 4,500rpm (Mk1, Mk1a) and 5100rpm (R4 which had a slightly raised 8.5:1 compression ratio). Crank is 3-bearing hardened type. Block aluminium split vertically down the centre-line so that the main bearing housings (and the camshaft bearing housings) are half in one side, half in the other, with some big big bolts clamping it all together.

The cylinder heads are cast iron - fortunately for us 50 years on as this makes them relatively easy to repair and convert to lead-free juice. Oil filter housing is a separate casting bolted to top of engine block via an obtuse-angled vee - always a conversation piece as it is such fun to keep oil tight. The water pump is perched above the engine as a separate unit that also carries the fan spindle, thus the water pump has to suck hot water from the engine, or rather, cylinder heads. Since the pump is joined to the top of the engine via a small piece of hose, and since the pump is triangulated to the dynamo and engine for fan-belt tensioning, you have to ensure that the pump does not get too close to the distributor. Radiator is mounted above and behind the engine in a compact but unusual layout.

Nevertheless the Jupiter's power of over 60 genuine BHP was quite good for the period from only 1486cc. Good enough for 10 major victories in 3 seasons, that is.


Compact transmission is separate from the clutch housing. It offers 4 forward ratios (synchromesh on 2nd, 3rd and top) plus reverse. Gear lever is steering-column-mounted.


Steering is by a solidly mounted rack-and-pinion unit, rare for it's time. As a result the steering is light and positive.


Apart from the very early Mk1 Jupiters, the Girling brakes are all-round hydraulic with twin leading shoes at the front. Very early Jupiters were hydraulic leading/trailing at the front, mechanical rear, which meant that should the hydraulics fail you still had some braking! Brake shoe area is very adequate except for fast mountain descents with heavy braking, when fade can be experienced.


Front suspension is independent, unequal length wishbones, with torsion rod springs and telescopic shock absorbers.

Rear suspension (Mk1, Mk1a) is live axle, well-located by Panhard rod (so - no axle tramp!), with transverse torsion rod springs and telescopic shock absorbers. Jupiter R4 is the same except semi-elliptical leaf springs instead of torsion rods.

Interior and hood (ragtop)

The single bench seat accommodates two, or three at a pinch. It is upholstered in leather (either a mid-tan or Bordeaux) while the rest of the interior is covered in a matching ambla (rexine). Winding windows are provided. The hood folds out of sight behind the seat, and can be raised rapidly in the event of a sudden downpour. Hoods are either beige (all body colours except white) or black (white body). White cars had Bordeaux interiors only. Red cars had mid-tan interiors only. Other body colours could have either interior colour.

Production Totals

It is confirmed that 897 cars (all bodies) were built on the Eberhorst Jupiter frame. But another five MAY have been built, making the maximum possible the total of 902 cars on the Eberhorst Jupiter frame

The Jupiter type R1 was on the Eberhorst frame but the Jupiter type R4 was not.

It is known that 829 standard-bodied Jupiters once existed, of which a good 50% still exist in some form.

  Mk1 Mk1a R1 R4
Right Hand drive 498 57 3 3
Left hand Drive 233 37 0 0
Special bodied Jupiters about 68 1?    

These diagrams are of the Jupiter Mk1a version, taken from a 1953 AUTOCAR magazine.

The dimensions in the above diagrams are in inches, with the seat in its central position. Jupiter Mk1 boot locker is 25" x 10" x 31"

The main differences between the Mk1 and the Mk1 are:

Boot: Mk1 - internally accessible by tilting the seat back. Mk1a - externally accessible, slightly greater capacity.

Spare wheel storage - For the Mk1 is reached through a special small door at the rear, but in the Mk1a you have to unload the boot.

Tankage: Mk1 - 11 Imperial gallons, Mk1a - 8 Imperial gallons.

Hood (soft top): Mk1a hood was raked further back than that of the Mk1.

Instrument Panel: Mk1 - walnut veneer panel with recessed instruments in direct view of the driver. Mk1a: metal panel painted body colour with chrome bezel instruments placed central to the car's axis.

Leg room: Mk1 - Plenty OK for a six-footer. Mk1a - slightly less.

In sum, the Jupiter Mka has the externally accessible larger boot, and is even rarer than the Mk1 and of course had all the latest mechanical updates. The Jupiter Mk1 has a nicer instrument panel, instruments, and instrument layout, and greater legroom and tankage - and is still pretty rare!







Length 168in 427cm 159in 404cm
Kerb weight 1895lb 860kg 1658lb 711kg
Height 56in 142cm 54in 137cm
Width 62in 157cm 62.5in 159cm
Wheelbase 93in 236cm 84in 213cm
Track Front x rear 52 x 50.5 132 x 128 52 x 49 132 x 124
Power 62.5bhp 46.6kw 64bhp 47.7kw
0 to 60mph 14.2sec      
standing ¼ mile 21.0sec      
max speed 88mph 142km/hr 100mph 161km/hr
Fuel consumption 30mpg 9.4l/100km    

The Jupiter R4 was speed-tested at MIRA (Motor Industry Research Association) test ground at Nuneaton. It just exceeded 100mph with overdrive top engaged and the hood removed. It was the cheapest 100mph car at the 1953 London Motor Show.

Here is the list of tools, issued by Jowett cars Ltd. with every new Jupiter - this actual list went out with E1 SA563R on 23 January 1952

Starting handle

Spare wheel, tyre pump with connection, tyre valve key, wheel brace, jack, tommy bar

Grease gun, oil can, axle drain plug key, sump drain plug key

Cylinder head spanner. open-ended spanners 3/16 x 1/4", 5/15" 3/8", 7/16" x 1/2", box spanner  5/16" x 3/8", spark plug spanner

Screwdriver, pliers, ignition screwdriver, tube for brake bleeder valve, budget lock key

Tool roll

This shows the position of the tool-box in a RHD Jupiter. The lid is made from the fabric (rexine) of the car's upholstery. The box itself is steel.

Some tools, the steel tool box, and the bag for the tyre pump.

The Jupiter's tool roll, a rare item indeed as their survival rate is poor.


Some of the Jupiter's tools - these are to the correct pattern Close-up of the correct jack supplied with the Jupiter


Here is a rear view of the Mk1a showing opening boot and symmetrical metal instrument panel

Radio was an optional extra of course

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