VEE-TWIN DEVELOPMENTS

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VEE-T WIN DEVELOPMENTS

THE yee-twin .J.A.P. engine has been a very popular power unit among special builders since the early 19208 and Over the past. few years the advent of the Cooper “1,000; has resulted in further development work. Not. content with the progress made by the parent firm, a number of private owners modified current versions of this famous power unit to their own ideas and twhieved very good results. One of the keenest protagonist’s is Peter Collins, who use special J.A.P. twins in his sprint Cooper. The mechanical work is done by Peter’s father, who has long been a believer in I he vee-twiti engine.

Tile Collins family use two power units, a 750 e.e. and a 1,260 c.c., in their sprint Cooper. The first engine started life as a normal 996-e.e. unit and both bore and stroke were reduped. New flywheels were machined from solid billets-, giving a crank throw of 90 tam., and new connecting-rods were made, keeping to the basic .J.A.P. principle of one rod forked over the other on the big-end. Modifications were made to the thrust washers on the journals to control side float more positively. With the aforementioned stroke, a bore of 74 nun, was used, to give a swept volume of 749 e.e. The barrels were made in east iron by Mr. Collins himself, who made his own patterns and did his own machining. The cylinder heads were standard eastiron 350-c.c. .J.A.P., purchased as blanks and the spheres machined to suit the new lame! dimensions. Valves and ports received the usual attention. All the earn gear and valve gear remained MirMal 9.96-e.e. J.A.P., as well as the valve timing. New cams were made, but they gave no appreciable increase in power, so standard ones were refitted. The normal J.A.P. arrangement of twin carburetters and magnetos was retained and the lubrication system converted to drysump, as on the later 1 ,100-e.e. .I.A.P. engines. In this form the engine is regularly run up to 7,000 r.p.m., and at this speed some 65 b.h.p. is claimed, while the car has been timed at 1’18 m.p.h. at the end of a kilometre sprint.

The second Collime-moditied twin .1.A.P. follows the same general specification as the first, except. that the reducing process is reversed and an enlarging one carried out. Starting with a normal 1,100-e.e. .I.A.P. dry-sump engine, the enlarging process was confined to new Wellworthy ” Altin ” barrels of 90-mm. bore, which, with the standard 99-mm. stroke, gave 1,260 c.c. As with the 750,e.e. engine, new flywheels were machined, and similar big-end modifications were carried out. Pistons of Mr. Collins’ own design and manufacture and standard J.A.P. cylinder heads are used, the latter re-machined to suit. the enlarged bore, the valve and cam gear remaining standard. Hun at 5,500 rims’. this engine is claimed to give 100 b.h.p. These two units are both built so that either can be fitted into the sprint Cooper chassis, and the same Norton clutch and gearbox is used for both. Not content. with this, Mr. Collins is working on a 500-c.c. ee-twin following the same principles, and has got as far as running

the engine on the test-bed, obtaining 49 b.h.p. ‘Vork is still progressing on the heads and the breathing of the engine, which lie hopes will eventually give sufficient power to deal with present successful 500 c.e. **singles.” This unit is intended for use in the .I.11.S. chassis, at present. raced by Peter, a chassis with which both father and son are well pleased, especially after replacing the original and rather flexible wire wheels by the latest Dunlop alloy ….. Another keen vee-twin exponent. is Ken Wharton, who has been well to I he fore in sprint events with his 996-e.e. Cooper-.I.A.P., and this year has been almost unbeatable with the car in supercharged form. NVIutrion decided to increase the power output of Ids .1.A.P. by supercharging rather Mos enlarging its eapacity. A Cetitrie slipereltarger was Mounted in front. of the basicallystandard 906-e.e. .I.A.P. engine, with its rotor axis parallel to the crankshaft line. A drive was taken from the engine by making up a double mainshaft-drivingsprocket, one chain running forward to the blower and the other rearwards to the Mach, In all other respects the Cooper remains unchanged. In fact, the whole_ power unit. closely resembles those blown engines used in motor-eyeles for many years by Ilaragwanath. Pope and the late Erie Fernihough. NVit.11 a boost

of some 5 in., Wharton should have well oVer 100 b.h.p. available, for Fernihough had as much as 110 b.h.p.

from his blown pre-war.

Finally, in this development. of the veetwin, we come to the special engine built by R. It. Jackson for B. W. Merriek’s Cooper ” 1,000.” Some idea of the performance of this car can be gained by its two outright. wins in the Formula 11 races at J3orehoin and fastest lap of the day and a handicap win, from scratch, at -Risley. At the latter meeting it. was comfortably in the lead of the Formula II. event when the clutch adjustntent disappeared, causing, slip and subsequent. retirement, from the winning position. These successes have been against the Connaught and single-seaters ! The first aim in modifying the original 1,100-c.c. J.A.P. was to obtain more, power, but. at the same flaw reliability was sought, with a view to long-distance Formula II racing. Briefly, the .1. A.1′. push-rod valve gear intd heads were replaced by Norton overhead camshaft twits, wisile the bore amid stroke were altered. This entailed making new flywheels, with a stroke of 100 nun., and new connecting-rods using the well-tried J.A.P. fork arrangement, of which Jackson st rongly approves. Special alloy barrels were cast, with a bore of 85 nun., giving o volume of 1,132 e.e. Norton bi-metal ” square ” cylinder heads were fitted, wit h Norton single-vanish:a cans-boxes and rocker gear. The frost cylinder has the head and eam-box mounted in the usual manner, with a forward-faring exhaust port., but the rear cylinder head has had to be turned round to face the other way, to avoid an impossible exhaust pipe bend and the close proximity of the front. carburetter to the rear exhaust pipe. As the Norton head has its bolt-hole Centres on a square it. was a simple matter to turn the rear head through ISO deg. It was with

I.Ite m qUilSliaa IlriVe that. UR! iii 11 y arose, for obviously the eam-box multi mot be rotated, so the head-fins were cut. to allow the vertical drive to pass 111rougli and the plug hole was blanked off and recut on the opposite side. On the Norton camshaft the exhaust ealls is located by a key and the inlet cam is pegged to the exhaust cam. hly lengthening the keyway and reversing the cams the correct timing was obtainable, but. this meant that any adjitstment of the peg-vernier arrangement entailed complete disnumtlittg of the cam-box, so that iL is not surprising that it. took :Jackson some five hours to get the rear cylinder timing to his liking ! The bottom end of the vertical drives to the camshafts reveals a Most outstanding feat of modification, which speak.s volumes for Robin Jaeksoit’s designing skill ! Within the standard J.A.P. timing chest two bevel drives had to be arranged, driven from the mainshaft, miti well as magneto, filet plump. and Oil jaunt) drives. The layout also had to allow for bevel-gear thrust. With the exception Of the crankshaft pinion all the .1.A. P. -timing gear was removed and the holes in the crankcase blanked off. Straight-cut spur gears were made for this job, a large idler being driven off the crankshaft gear. From this the front half-time pinion is driven and front this another idler gear drives the rear half-time pillion. The two timing pinions have Norton bottom bevels attached, driving standard Norton vertical-shaft drives. The inner end of each timing pinion is carried on a taper roller race, while the outer ends run in needlerollers nutunted in a redesigned tinting chest cover. In this way the bevel thrust is self-adjusting against the taper bearing. From the rear pinion sluift Norton earnshift, bevels, one small and two large, supply a dual drive for the twin magnetos, while the first idler gear is fitted with a eam to drive a mechanical -fuel puinp. An enlarged .J.A.P. dry-sump oil pump is driven from the crankshaft pinion and a T-union front the main feed supplies oil to the two cam-boxes, the return being by gravity down the vertical-shaft tubes. Front the outside the tinting arrangements do not appear very difficult and it is not until one looks inside the thning-ehest and compares it with the original push-rod arrangement that one can appreciate fully the design work entailed. Kith cylinder has-its own ’11r.” Arnal earburet, ter, fed tram a 5-pent header onik, whidi fed from a front tank by the: aforementioned well:one:0 pump, Not havilig mu 1..y.A-laql availably

00Wer (Mt 1111t„ hnt tinr estimation of WO was modestly considered about 10 too high. Witatow the output, the higt remit* that the Nor-J.N.P. (*ooper is one of the InoAt potent Formula II C011t