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Tuning Rootes Groups Cars for Competition

By Marcus Chambers

We are often asked about tuning equipment for Rootes products and before going into details it would be advisable to remember a few points.

Rootes products are delivered in standard form and can be relied upon to give of their best. If an improvement in performance is required, there must inevitably be a reduction in the life of the engine. Super tuning is automatically excluded under the terms of the Warranty, as changes in the Engineering Specification tend to reduce reliability. The very fact that most people tend to drive tuned cars faster, must accelerate wear and tear on all components. But it does not follow that a tuned car is any less reliable than another car of similar performance and capacity.

When recommending a state of tune it should be borne in mind that such is the specialisation of motor sport these days, that cars must be tuned and prepared for the type of event in which it is proposed to enter. For example circuit racing introduces problems which are quite different from those of International rallies. A production saloon car may be modified to take part in long distance racing, club circuit racing, International rallies, production car trials, or driving tests to name a few events.

From the foregoing it will be seen that any vehicle which has to be used in a variety of events must be tuned to some form of compromise, and it is therefore as well to decide to specialise in one form of motor sport at a time. At all times it is as well to remember that Power Costs Money.


From my file on Imp books:
• Rally Cars given the Works / Autocar, 1983. - p.18-20
• Works Team '65 / Frostic
• Hillman Imps : Tuning, Overhaul and Servicing / T. Millington, 1995
• Tuning Imps / Griffiths, 1995
• Imp Tuning and Modification / Hansen, 1995
• The Theory of Tuning / Smith, 1972)
• Why finish last? / Andrew Cowan
• Uphill Racers / Chris Mason, 1990)
• How to Start Rallying / Colin Malkin, 1970)
• Rally / John Davenport

Production engines are usually built with cost and economy figuring very largely in the specification. If one is prepared to disregard these two features, there is a much broader field to work in.
The following items show where it is desirable to concentrate the improvements.


Additional or larger carburettors which will permit an increase in the choke size. Improvements to inlet manifold dimensions with attention to matching the pipe to the cylinder head.
Polishing out the inlet tract, an operation which should always be carried out before the rest of the head receives attention so that unnecessary work is not carried out if the head should happen to be scrapped through inadvertently cutting away too much metal.
Changes in the size of the inlet valve to match the improved porting and carburation. Changes made to the valve timing to increase the length of time to be made available for filling the combustion chamber. Camshafts which increase lift as well as dwell. Equalising the clearance volume for each cylinder. Improvements to the exhaust manifolds and silencer to reduce back pressure.


Both rotating and reciprocating parts of the engine to be balanced statically and dynamically. The rotating parts such as flywheel, clutch assembly and crankshaft being balanced as a whole.

Gearbox and Final Drive Ratios

Selection of ratios suitable for the type of event, whether rallying or circuit racing.

Road Holding

The ride height may be increased by using longer springs and a firmer ride will be obtained by using higher rate springs whether shorter or longer than standard. Camber angles may be changed to increase cornering power.


This should always be commensurate with the performance of the car. It is no good increasing the maximum speed if the car cannot pull up safely and in a stable manner.



Apart from polishing out the induction tract there is very little that can be done to improve the performance of the standard Hillman Imp unless fairly extensive modifications are carried out.
The first stage alter polishing the head is usually the fitting of twin or twin choke carburettors, which, with an improved inlet and exhaust manifold, increases the performance and raises the h.p. by five or six.
The second stage is to fit larger inlet valves. This operation requires the fitting of oversize valve inserts, and is usually accompanied by a change in the camshaft. This results in an increase of power, to between 50 and 55 h.p. depending on the standard workmanship and the camshaft used.
For racing a much "wilder" cam is usually installed and twin choke Weber carburettors are employed. It is possible to obtain as much as 90 h.p. from an Imp in racing tune, when the bore has been increased to 2.856", in bringing the capacity out to 998 c.c. using the standard stroke.


It should be borne in mind that it is quite pointless to fit a highly tuned engine to an Imp unless certain precautions are taken. The following points may help the owner to decide on the specification he requires.

  1. All air that is fed to the revised carburettor equipment must be correctly filtered. Paper element filters installed in an air-tight cleaner of an approved type are essential.
  2. The engine should be fitted with an oil cooler
  3. The engine must be provided with a revolution counter
  4. The cooling system must be such as to cope with the additional waste heat which the tuned engine will lose to the water jacket.
  5. The suspension system should be tuned to cope with the higher speeds which will be obtainable by using higher rate road springs and suitably matched shock absorbers
  6. The braking system should be adapted to cope with the higher speeds by fitting anti-fade linings together with race brake fluid.


During the last year the Competition Department of the Rootes Group have developed a 998 c.c. Imp which has all the features outlined above. This model is based on the successful Works Team Imps which have done so well in International events. You will find the specification further on in this article. The object has been to provide a car which can run in Club events with a definite chance of success and which can, with slightly extra tuning, by way of head polishing, be suitable for International events. By reason of the volume of cars made, it is not possible to incorporate a high finish on the cylinder head and inlet, but all parts are balanced and the car is capable of a maximum speed of 95 m.p.h. in the delivered form.

Cars are prepared to this specification for both the home and export markets, and the car will be homologated in the G.T. Category Group III in 1966. For the home market the work is carried out as a conversion on an existing car which has been bought through a dealer and sent direct to this Department.

Whilst we have endeavoured to produce a fast and reliable one litre saloon with good handling characteristics, the owner of a Rally Imp should bear in mind the following points.

The 998 c.c. Rally Imp is built in series to a simple specification which makes it suitable for high speed touring. 1f it is to be used competitively in motor sport it should be prepared in accordance with the regulations for the event in question and in such a manner as to be suitable tor that event. To quote an example circuit racing and autocrosses require an extremely different approach.

We recommend the use of the high springs (R.A.C. Type) for rougher British events or overseas events taking place on dirt or rock roads. In this type of event a full length undershield is also recommended. On the electrical side, a competition dynamo and regulator giving 28 amperes will balance the demand made by extra lights which may be deemed necessary, on special stages which are likely to be covered at night.

A 15/32 in. width special fan belt is available for events where very high r.p.m. are in use with rapid deceleration. Alternator kits are available and can be quoted for by the Competition Department. The engine can be fitted with special carburettors and camshafts for racing, but it should always be borne in mind that rear-engined cars must retain their air cleaners at all times when in use on the highway. There are possible exceptions with tarmac race tracks, but racing mileages being comparatively small, there is less risk of abrasive material entering the engine in suffient quantities to damage the engine as compared with that on the road.

Whilst all four gearbox ratios can be changed in the Imp by means of new gear wheels, it is not possible to change the hypoid drive without considerable expense. As all gears on the Imp are indirect, it is therefore possible to change top gear by changing the 4th speed in the gear box. This has been done in the case of the Rally Imp.

Competition seats with adjustable backs can be supplied to fit the car, but are only available in black. The Halda Tripmaster and stop watches are usually fitted for international events.

In Continental type events where good roads are used but occasional rocks may be encountered the Alpine type sump guard can be used. This permits good ventilation around the transaxle - an important point in hot climates. A plastic catch bottle is also advisable fitted to the overflow pipe of the radiator. This collects the water in the event of high temperatures being reached and returns the water to the radiator when the engine cools itself.

Alloy wheels are so constructed as to give turbulence over the brake drums. These improve the brakes under adverse conditions such as fast descents in mountainous country. They are also suitable for racing. The steel wheels which are provided as standard are perfectly suitable for normal and competition use although we recommend oversize wheel nuts for racing and rallying.


Whilst we are anxious that owners should have the benefit of factory experience behind them when having their cars prepared for competitions, and in view of the Competition Department's commitments to prepare and rally their own cars, we very much regret that we cannot accept this type of work ourselves. We have however made arrangements with certain companies who have first hand experience of competion preparation to accept this type work. If you care to get in touch with this Department we can then advise you as to your requirements and if necessary introduce you to one of these establishments who can undertake the work at their normal rates.

Specification details of the Hillman 'Rally' Imp



4 forward speeds - synchromesh on all forward gears

Gear Ratios - Standard   Alternative Close Ratios
(Optional Extra)
15.1 /1,000 r.p.m.
24.29 /1,000 r.p.m.
SP 41 - 155 x 12 in.
13.96/1,000 r.p.m.
22.46/1,000 r.p.m.
SP 41 - 155 x 12 in.


Girling servo assisted - Linings Ferodo VG 95 front and rear.


Monte Carlo type springs and heavy duty shock absorbers. Reinforced front suspension



Standard Imp specification

Fuel Tank

Standard is 6 gallon tank (27.27 litres)
10 gallon tank optional extra (47.7 litres)


4½ J wheel rims with SP 41155 x 12 in. tyres

Accessoires for the 998 c.c. 'Rally' Imp

Monte Carlo Road Springs
(Front and Rear)
Shock Absorbers (Front and Rear) 
Close ratio gears
Exhaust manifold
0.310 in. camshaft *)
Oil cooler kit
Pair of C.D. 1.5 carburettors *)
10 gallon petrol tank and gauge
Special Instrument panel
2 in. petrol gauge
2 in. temperature gauge
R.A.C. Road Springs (front and rear)
Heavy duty sump guard
Lightweight sump guard
Cam carrier
Inlet manifold
Pair of C.D. 1.25 carburettors
Pair reclining front seat
4 in. rev. counter
4 in. speedometer

These parts are available through the Competition Department Humber Ltd. Coventry but must be ordered throuoh a Rootes Dealer.
*) These parts are not suitable for the 875 c.c. Imp.

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