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An engine for an Imp


For starters

When the Hillman Imp was first conceived under the guidance of B.B. Winter, then Technical Director of Rootes, it was towards the end of the post-war economic squeeze in the U.K. and Europe. The idea was a really cheap car for four adults, economical to run and requiring minimum servicing. It had a horizontally opposed twincylinder air-cooled engine and non-synchromesh four-speed gearbox, which was designed by and planned to be manufactured by Villiers of Wolverhampton.

As development proceeded, Peter Ware took over after Winters's retirement and this coincided with increasing demand for more sophistication in the minimal vehicle. It also so happened that there was a young development engineer on the Imp project by the name of Mike Parkes, whose weekends were spent helping David Fry with his competition cars, one of which was powered by an 1,100 c.c. Coventry Climax single o.h.c. four-cylinder engine. Parkes proposed to the management that this was the type of engine needed and there was a smaller 741 c.c. version of this unit, which had been successful at Le Mans.

With its all-aluminium construction and installed weight of only 160 lb. for an output of 65 b.h.p. it could be mounted in-line at the rear without affecting weight distribution and would obviously have considerably enhanced performance. An engine was obtained from Coventry Climax, a new synchromesh gearbox designed and this is how the new Imp started life.

It is therefore not surprising that the Imp power unit is still remarkably similar to the famous Coventry Climax 'fire pump engine which wins motor races'.

I is what I is

When first introduced the Imp engine had a capacity of 875 c.c. and a bore and stroke of 68 x 60.4 mm. For the Rally Imp the bore is increased to 72.5 mm (998 c.c.) and in this form when fitted with twin l.50 in. diameter Stromberg C.D. carburetters produces an installed power of 60 b.h.p. at 6,200 r.p.m. To achieve 10 b.h.p./litre/ 1,000 r.p.m. when fitted with car exhausts, air cleaners and driving all auxiliaries is a very creditable standard; a figure barely achieved by the Fiat 124 Sport Spider, for example.

Complete engine weight of the 998 c.c. version is 190 lb., equal to around 3 lb. per horse-power-achieved by only a handful of highproduction engines. Much of the weight-saving comes from the use of die-cast aluminium alloys for the block, head, tappet block and valve cover. Designed as a high-pressure die-casting, but still produced on gravity dies with all-metal cores, the interior of the crankcase is very smooth without any webbing, the necessary rigidity being obtained by external webs and 'u' section external channels at each main bearing panel; this also permits the use of near-constant metal thickness throughout-most important when die-casting. Also for onepiece core withdrawal, the block has an open top deck; centrifugal cast-iron liners are mechanically bonded in during the casting process.
The aluminium overlaps the liners top and bottom to ensure rigid location and also eases final machining of the top joint face. In its fully-machined state this three main-bearing component weighs a mere 22 lb.

Head design is almost pure Coventry Climax practice; the valves are angled at 20 degrees to the cylinder axis in a fully-machined wedge-type combustion without overlapping the cylinder bores. Efficiency of the combustion can be judged from the fact that both versions of the engine operate satisfactorily on premium fuel with a compression ratio of 10.0 to 1. Individual ports on the same side are used for exhaust and inlet, the latter having long constant diameter tracts for good ram filling before they join up with similar length tracts in the straight-rake intake manifold.

The three-bearing chilled cast-iron camshaft of the Hillman Imp engine is mounted in a separate die-cast aluminium tappet block, having split caps for the replaceable Babbitt-lined bearings.

Cast-iron valve guides have stem seals on the inlet valve to control oil being sucked into the port when there is a negative pressure in this region such as on over-run with a closed throttle. Valves seat on sintered iron inserts and clearance between the valves and tappets is obtained by selecting from a range of varying thickness biscuits retained in the upper part of the valve collar.

The simple roller chain drives the camshaft directly from the crank, the distributor and in-line oil pump being driven direct from the front of the crank by a pair of spiral gears. Chain tensioning is another example of the well-tried Weller design on the slack side with a nitrile-faced damper pad opposite to it. Generous water passages in the head eliminate the need for a separate water gallery, there being a single take-off on the rear face by means of a small casting containing the thermostat and by-pass pipes.


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File started: 29 June 2013
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