Thursday, 6 October 2016

Jowett flat-four engines: Developments for sports cars and racing

In 1949 the Javelin engine was developed for completion motoring and class victories were obtained in events such as the Spa 24 hours race and the Rheineck-Walzenhausen Hill-Climb. The Spa Javelin produced approximately 57 BHP and was fitted with the oil cooler and copper-lead bearings etc. Flywheel weight was also halved from 28lb to 14lb. These Javelins also raced at Silverstone.
Class winning Javelin saloon at Spa in 1949

Jowett had also decided to put into production the two-seater sports Jupiter, the chassis of which was based on a tubular-frame design evolved by Leslie Johnson in association with the German engineer Eberan von Eberhorst.

The Javelin engine was required to be developed to give 60 BHP at 4750rpm for use in the new car. The compression ratio raised from 7.25:1 to 8:1 by reducing the volume of the combustion chamber and by changing the shape of the piston crown. Javelin port sizes, bearings and camshaft were used unaltered, but heads and ports were polished. A Delaney Gallay oil cooler was fitted, initially located behind the fan. Later a Bowman block cooler was mounted on the front of the off-side cylinder block. Instead of 23mm carburettors, 26mm Zenith 30 VIG carburettors were used. Later these were replaced by the easier-to-tune Zenith 30VM units. The oil sump capacity stayed at 9 pints. As there was more room under the car, the off-side exhaust pipe joined the main pipe at the rear of the near-side manifold and not in the manifold as on the Javelin. The shape of the Jupiter body called for air-extractor louvers which were not used on the Javelin and when overheating was experienced in Continental driving and Alpine work, the radiator was increased in size. Louvers were also added in the bonnet top. Intake noise was less of a concern with the Jupiter, so no air silencers were used for the carburettors and eventually the Vokes air filters were replaced by AC units.

Jowett issued “Competition Tuning notes” to Javelin who sought an increase in performance. Port-polishing and relieving was covered and stronger outer valve springs were recommended. Special pistons were declared available that increasing the compression ratio from 7.2:1 to 7.6 and 8.0:1, a reduction in combustion chamber volume of 2cc and further 3 cc respectively. The standard Zenith VM4 and 5 carburettors could be replaced by 30 VM Zeniths and it was assumed the hardened crankshaft, copper-lead bearings, larger water and oil pumps, later oil filter assembly and oil cooler would be employed.

Subsequently similar tuning of the Jupiter was permitted with a compression ratio increase from 8.0:1 to 8.5:1 for 80-octane fuels, this being attained by the use of thinner gaskets. Stronger inner valve springs were recommended and the flywheel could be lightened. It was assumed that correct fitting and assembly would be ensured and that modified h.t. cylinder-head studs, Lucas DVX4A distributor and Champion L 11S or LA11 spark plugs would be fitted.

Then next step was to prepare the Jupiter for racing. For Le Mans in 1950 a compression ratio of 8.5:1 was employed by using the thinner head gasket and with the stronger inner valve springs, high-duty ignition distributor and lightened flywheel, the output was 64 BHP. To obviate gasket trouble the strength of the cylinder-head studs was increased from 45 to 60-65 tons tensile, but the number and position of the studs were unchanged. The spark plugs were up from L 11S to LA11. The car was a great success; winning the 1500cc class at 75.84 mph.

For 1951 the porting and valve timing were improved and after further experimenting with compression ratios of 8:1, 8.5:1, 9:1 and 9.25:1, the latter ratio was employed. Just over 100 mph was obtained from the R1 Jupiter, but after six hours at Le Mans the C.A. gasket collapsed. A composite copper, asbestos and steel gasket was found satisfactory, after experiments with solid copper, laminated aluminium and corrugated cupro-nickel gaskets etc. This gasket went on to be used on all production engines but, eventually, for racing, a gas-filled metallic sealing ring at 600 lb/ pressure in a circumferential recess on the liner top flange stood up to the highest compression ratios. A Plexseal gasket was used as a water joint. The gasket failures were finally traced to sinking of the cylinder liners and this was cured be redesigning the liner bottom seal; a rubber ring trapped between the liner bottom flange and the crankcase permitting metal-to-metal contact between liner and crankcase, obviating liner sinking and enabling the initial liner interface on the gasket to be maintained.
R1 Jowett Jupiter competition version of the Jupiter sports car in 1951 guise

The 1.5 litre class was won again at Le Mans in 1952. A 9.25:1 compression ratio was utilised with flat-top pistons. The serrated-face big-ends were used and the top piston-ring lands were increased from 3/32 to 1/8in. To reduce the tendency of piston-ring flutter and increased oil fling, pressure loading of the scraper ring was increased to 70 lb/ 2 BHP was gained by using solid-skirt pistons due to lower friction. Trailing oilway drillings were used on the crankpins to feed oil at a point of minimum pressure.

The pistons were now solid-skirted and of die-cast silicon alloy, with the top gas ring chromium plated. Stronger valve springs allowed the engine to rev to 5500 rpm. KE965 (EN 54) exhaust valves combated a neck temperature of 700-800 degC which had caused an XB valve to break during the Silverstone Production Car race. The stems were chromium plated, 0.001 in extra clearance given in the guides, and the valve tip at the rocker end was stellited. With 0.5 ml per litre of lead in the fuel, valve life was approximately 200 hours at 4500-5000rpm.

An external carburettor balance pipe with an internal diameter of 5/8in was now required. Lodge spark plugs in waterproof covers were also used. The crankcase was stiffened by ribs radiating from the main bearing regions and the walls were also stiffened. The Marston Excelsior oil cooler, radiator and reserve fuel tank were fabricated in aluminium, with a weight saving of 45lbs. An axle ratio of 4:1 was employed instead of the former 4.56:1. The engine now had a fuel consumption of 0.51 to 0.57 pint/BHP/hour, equal to a race fuel consumption of 18 mpg. All this resulted in a third consecutive 1.5 litre class win at Le Mans.  

The standard crankshaft broke on test after only 50 hours on a dynamometer running at 4200rpm with compression ratios above 8:1. A crankshaft failed which had run approximately 200 hours during the 1950 TT race when an 8.75:1 compression ratio was used. These failures led to a mathematical investigation into crankshaft dynamics and the most probable cause of the crankweb bending fatigue was thought to be axial and torsional vibrations of the crankshaft in conjunction with the presence of an adverse residual stress in the crankpin fillet adjacent to the fracture. This residual stress was due to induction hardening of the bearing surfaces especially if followed by a cold straightening operation allied with stress rises in the form of sharp fillet radii abd tool marks on the webs. A new crankshaft was developed incorporating fillet radii on all bearings of not less than 0.1in. The crankpins were also drilled in order to reduce the off-centre weight and the magnitude of the bending loads. Great care was also required when induction hardening the crankpins to ensure that the hard zones did not extend into the webs. Experiments showed that shot peening the fillets could increase the fatigue resistance considerably.

A polar load diagram was drawn up for the big-end bearings for running above 4750rpm. Sufficiently high inertia loadings were discovered to warrant drilling the racing crankshaft with oilways at 60 degrees trailing on each crankpin. The oil temperature under racing conditions was held to a maximum of 75degC.
R1 Jupiter in which won its class in 1952 at Le Mans

Besides the 1.5 litre class victories at Le Mans, further wins were obtained at Watkins Glenn and in the 1951 1.5 litre class of the TT race.

The Jowett Javelin and Jupiter engines really were race developed, as almost all the modifications found through racing were incorporated into the mark 3 engines:

  • Crankshaft - The crankshaft was redesigned to increase its fatigue strength. Modifications included increasing crankpin and main bearing fillet from 0.05in to 0.1in. The hardening methodology was also altered to ensure hardness did not run into the crank webs. The crank-pins weight was reduced by drilling them with a 7/8in hole through them; the object to reduce the bending load on the shaft
  • Oilways – The oilways in both the crankshaft and crankcase were modified; in the the case of the crankshaft these were repositioned so that they emerge on the crankpins at the point of minimum load. In the case of the crankcase the oilways have were increased in size to prevent the potential of restriction, especially under cold starting conditions.
  • Bearings – These, with the exception of the rear main bearing, were of Vandervell manufacture and of tri-metal construction which consist of a steel backing strip onto which is cast a layer of copper-lead alloy; this layer of copper-lead is plated with an 0.003in thick coating of lead indium alloy which acts as a bearing medium
  • Crankcase –This was stiffened by the addition of radial webs on the front, centre and rear panels. This also helps to minimise noise.
  • Cylinder heads – The combustion chambers and ports are polished and the ports are aligned with the manifold ports. This was done to improve the gas flow characteristics.
  • Camshaft – An adjustable end location was provided so that individual adjustment can be carried out to reduce noise from excessive end float.
  • Cylinder liner bottom seal – This was redesigned to consist of an oil and heat resisting rubber ring trapped between the liner bottom flange and the crankcase. There is thus metal-to-metal contact between the liner bottom flange and the crankcase, which obviates any tendency to liner shrinkage due to collapse of the bottom joint. This ensured that the initial liner interference on the gasket is maintained and results in greater gasket reliability
  •   Oil pump –This is of a submerged design which ensures instant priming under all starting conditions, and the relief was by-passed to the suction side of the pump to reduce oil churning and frothing in the sump.
  • Pistons – The size of the piston ring top land was increased to improve fatigue resistance.
  • Spark plug covers – The original design of Bakelite covers with a bayonet fixing was replaced by a rubber design from Lodge.

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