The Imp Site
Cylinder Head
unsorted bits & pieces
If performance is your goal, then cylinder head selection and preparation is the most important part of your engine block. Cylinder head preparation is also the most direct and rewarding way to improve your engine. Ask the best engine tuners and they'll tell you that cylinder heads are the key to building horsepower.
Greetham Engineering (Andy Chessman) at Edgwick Road, Coventry had a head and camshaft combination which increased both torque and bhp throughout the normally used rev range. The head itself was done with economy in mind as much as any power increase that it may achieve.
The inlet ports were slightly enlarged and reshaped, but they were left fairly rough so as to aid fuel build-up. The inlet valve remained at the standard Imp Sport size. The exhaust ports on the other hand were enlarged quite considerably over the standard and the exhaust valve itself was increased from 1.06" to 1.125". that is a 12% increase in area. The general idea of this is to cut down pumping losses on the exhaust stroke. This will make a small contribution to bhp but more important it will also help the economy.
David Vizard used this combination and tried different carburettors with it on an 875cc: Small bore power
Exhaustive tests reveal that polishing the heads gives no increase in flow. In Vizard's opinion overall peformance may decrease due to the smooth walls.
Standard heads were made for Imps with single carburettors, Sport heads for twin carb models. The Sport heads do not have a '2' on the casting. The castings were made by the Aeroplane and Motor Company of Birmingham, and castings numbers have 'AM' in front of them.
| Standard cylinder heads |
|---|
| part no.: | details: |
| 7010031 | Fitted to the early Mk I
On the very early versions of these, the area between the spark plug holes and the top of the head was filled in. On the later heads there is a recess here, tapering from one end to the other and with strengthening ribs running across it. |
| 7010128 | Fitted to the later Mk I
Some of the later models of this head had the large valves as fitted to the Mk II (prior to its introduction in September 1965). | 7050001 |
| 7010133 | Fitted to the early Mk II models up until the middle of 1968. On the rear end of the head (at the back of the car), the area from the cam box to the block does not taper, but has a step. |
| 7010171 | The later Mk II head
No core plug at the end of the exhaust manifold (next to no. 4 exhaust port), but it is solid and machined flat as a continuation of the manifold area. Also on the rear end of the head (at the back of the car), the area from the cam box to the block tapers.
Both this one and the earlier have a large '2' cast on the top face of the timing chain aperture. | 7050119 |
| 71981533 | From late 1973 on.
Normal Mk II size valves and a '2' on the casings, but it has an oil drain like the Sport heads, and it does not use inlet valve seals. This was made necessary by the fitting of the Sport cam, tappet block and double valve springs to all cars from 1974 on.
It can be distinguised from the Sport head by having a thinner casting. | 77054265 |
| Sport cylinder heads |
|---|
| 7010107 | Very early Sport, may even be part of an initial run, prior to production of the 'Sport'. Destined for use on competition cars, including the dry liner 998cc 'Rallye Imps', produced from the end of 1965. |
| 7010162 | Fitted to the early production models, until the Mk III, mid 1968 |
| 7010180 | No core plug at the end of the exhaust manifold (next to no. 4 exhaust port), but it is solid and machined flat as a continuation of the manifold area. Also on the rear end of the head (at the back of the car), the area from the cam box to the block tapers. | 7050112 |
| Valve Sizes |
|---|
| Inlet | Outlet | |
|---|
Mk I
Mk II
Sport |
1.06"
1.20"
1.28" |
1.01"
1.06"
1.06" |
- can be modified to take Mk II size valves
- can be modified to take Sport inlets
- can be modified to take 1.40" and 1.25" valves | The ports can also be modified proportionally. The Mk I obviously has the least scope for alteration, as the ports become dangerously thin if enlarged more than a very small amount. |
Literature
Vizard, D.
Theory and practice of cylinder head modification / by David Vizard. - Croydon : Motor Racing Publications, 1971. - 152p. + 22 diagrams (drawn at actual size) without page numbers. - [MRP Speedsport]
ISBN 0 85113 066 6 (paperback) © 1973, reprints: 1988; 1991; 1995; 1999
In Pt 3 'Head designs and applications' there are a few pages (p. 132-136) on 'Modifying Imp heads'. Page 133-135 are all diagrams:
Fig. 77: Standard valved Imp Sport inlet port modification. Modification to Imp Sport chamber. Imp inlet port for full race. Imp exhaust port for full race. (4 drawings)
Fig. 78: Imp chamber shape for full race engine. (1 drawing)