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A Feather Weight engine for an Imp

When the Rootes Board had rejected the second idea for their potential baby car, they suggested to use some form of water cooled power unit. Rootes themselves did not manufacture an engine small enough to be fitted into the prototypes. From 1931 to the early 50s the smallest Hillman engine had been an 1185cc sidevalve unit, designed for the original Minx. For their Imp they looked outside of the company.
The answer lay in the Coventry Climax FWM unit, related to the FWE that was used for the Lotus Elite.
A set of drawings was sent for, and much time was spent in trying to find a way of installing the four cylinder 750cc (741cc, 653cc ?) Climax engine in a compartment which had originally been intended for a two cylinder 600cc unit. Eventually it was made to fit by tilting the engine over at an angle of 45 degrees and slotting the radiator in alongside it.

the engine compartment in a Stiletto (non-standard) showing the 45° tilt of the engine. (44 kB)

Productionising the Coventry Climax

It wasn't so easy to transform the sand-cast iron blocked Coventry Climax engine into a die-cast alloy unit. It was to be the first time that a British volume car manufacturer would use an alloy engine. And Rootes engineers had little knowledge of working with this material. But the benefits to be gained from using alloy (low weight, low noise, allied with good performance and good economy) were felt worth the additional effort of developing suitable die casting techniques.

   Leo Kuzmicki
Leo Kuzmicki at the Imp's introduction
The job of redesigning the original Coventry Climax engine so that it could be manufactured in alloy fell to Leo Kuzmicki. The original engines used in the Apex prototypes were made from steel and had been manufactured from single purpose machining equipment at Coventry Climax. This type of equipment, however, was quite unsuitable for mass production, and many detail alterations had to be made to the engine's design in order that the components could be manufactured on dual operation machines. It was to be tooled for manuafacture at a hoped-for rate of 3000 units a week.
There was a period during the development of the Imp alloy engine that a cylinder block with no liners was considered. It was proposed to use a specially hardened alloy that would permit the pistons to run directly on aluminium cylinder walls. In particular, the aluminium was to be alloyed with approx. 10% silicon and no doubt heat treated, etc. to maximise the wear resistance of the cylinder walls. This way the engine could dispense with ferrous metal cylinder sleeves, making it lighter, easier and cheaper to manufacture, etc. A big breakthrough in lower cost automobile manufacturing.
An engine was built using pistons with deep cast iron rings and tested with negligible cylinder wear. The reason why this route was not followed was because of the degree of precision needed during the initial machining operation. Thus all the production 875 cc Imps had alloy cylinder blocks with cast iron dry liner cylinder inserts. These inserts were cast in situ when the block was die cast.
Die casting of the alloy components was done by Alumasc of Northamptonshire by a whole variety of new-fangled methods.
The cylinder heads were supplied by the Aeroplane and Motor Company of Birmingham, as this company already supplied the alloy heads for the Sunbeam Rapier and Alpine.

The basis of the design was an aluminium pressure die casting cylinder block, with open-cored top deck contruction, and centrifugally cast-iron liners mechanically bonded into place. It had three crankshaft main bearings. The water pump was combined with the cooling fan, set at the other side of the engine bay, behind the cooling radiator.
The cylinder head was also a low pressure casting, with a line of valves angled even further away (20 degrees) from the vertical position, operated by a single overhead camshaft, coil springs and inverted bucket tappets -the classical solution- with the shaft chain-driven from the front of the engine.
There were wedge-shaped combustion chambers, with a 10 : 1 ratio even on the most mundane Imp saloons. (For certain export markets the ratio was reduced to 8.0 : 1.)
The whole unit, complete with accessories, weighs 170lbs. (77 kg).

Not much stretch was build in, sizewise it was much at limit. This meant that bigger bore versions, that could have meant much to both the Sunbeam and the Singer nameplates within the Rootes Group (and other buyers on the outside), could not be produced. So over the years (1963-1976) the same 875cc engine was used.
This all-aluminium, splendidly detailed, single-overhead camshaft, inclined valve design was a true classic engine, simply because it was so ambitiously engineered without much regard to cost saving or to versatility.
It has been stated by several that there is no smoother high-revving unit in the business, then or now.

"If the engine has any fault, it is its willingness to rev. The power peak is at 5000 rpm, but it can be wound up to an easy seven grand with most of the noise coming from the Rootes engineers"
Hillman Imp: road research report. - Car and Driver, 1963

Model Size Max. bhp Mpg MphCarbs
Imp 875 37 (DIN)@4800
39 (net)@5000
38.1 81single Solex
Imp Sport 875 50 (DIN)@5800
51 (net)@6100
33.1 90
Stiletto 875 50 (DIN)@5800
51 (net)@6100
32.4 87
Rallye Imp 998 60 (DIN)
65 (DIN)@6200
30 92
Chrysler Sunbeam 1.0 928 42 (DIN) 31.5 77
Clan Crusader 875 50 (DIN) 34.3 100
Hartwell Clubman Imp998 65>100Weber 40 dcoe
Rootes were pessimists: usually an Imp could do better than its supposedly peak power output and top speed. In 1963 the factory had been disparaging about higher stages of tune.

Two photos of the engine in the back of a Ginetta G15: one with the lens at the level of the radiator and the other looking down on it.

Sunbeam 928

The Sunbeam 1.0 LS that appeared in 1977 had an Imp engine in it. Or rather an engine based on the 875cc Imp engine. The casting is same, by and large. The sump and the tappet cover are a little different.
Stroke is the same; bore is 2mm larger. It has a lower compression ratio (9:1 in stead of 10:1). It can deliver more than 41 DIN bhp@5000rpm.
Torque is reduced to 52 lbs ft. @ 2600rpm.

The Imp Site
Design of the Hillman 875cc engine
The Coventry Climax file
© Franka