Barry Green at Hartwell, 1971
Barry Green at Hartwell, 1971

The Imp Site

At the Hartwell tuning shop

Improving performance. - Car Mechanics 1971, March. - p.75-77. - [Hot Shop]
"we comported ourselves to the doorstep of one Barry Green, of Team Hartwell in Bournemouth - and found one of the most efficient tuning shops it has been our pleasure to disrupt for a day."

The Imp engine is light, powerful, rugged, and relatively trouble-free. Furthermore, it lends itself to quite dramatic tuning procedures without flying apart.


If you want to tweak an Imp without major modifications:

  1. carburettion
  2. exhaust pipe
  3. camshaft

On a 875cc engine this could gain 13 bhp. (from 52 to 65 bhp).

Undo the engine's rear mounting bolt and remove the battery.
With the rear mounting slack, take the weight of the engine by shoving something underneath.
Remove the rear bumper and then the rear body panel.
This will come off after you undo the bolts around the side.
Before it is free is a good time to disconnect all the electrics and cables, etc.
The rear panel comes off backwards, which disposes of the rear mounting
and leaves the engine clear to slide out to the rear.
The fanbelt, dynamo, water pump etc. come out with the engine.
Disconnect the hoses,
particularly the top hose, which is rather well hidden in the front of the engine compartment.
The bellhousing bolts must now be undone,
and in doing so, the starter motor will come away in your hand.
Be sure the engine is properly supported, with all the conncetions released.
First treat is an inlet manifold and a pair of Stromberg CD125 carburettors.
They cost over £40, but you get a 2 ½ sec. improvement on the 0-60, even if you do nothing else.
A four-branch exhaust manifold comes complete with a silencer at £19.
The bigger bore on the left is for stage III 998s etc.
A High Torque camshaft is well worth while after you got the carbs and the exhaust.
Hotter 998s can use an R17, which need a special cam carrier to match.
Other cams are available, too.
Stage II cylinder head (behind) Stage III (front).
The 875 can take a Stage II head; a 998 can use a slightly modified stage II or the stage III.
All 'straight edge' Mk II blocks can have 998 liners fitted.
Early Rootes 998s (forground) often gave haed gasket trouble.
You can't modify your block if there is scoring on the rear main oilseal facing.
Grit gets caught in the crankshaft scroll.
The crankshaft can be lightened and balanced, as can the rod / piston assemblies.
Flywheels can be lightened to either road or racing weights. Don't use racing unless you are racing.
Fit a competition diaphragm clutch and heavy duty plate if you have a hot 998.
Close ratio 3rd / top are available, but fitting is a pro job.
If you use a 998 with a Stage III head and an R17 cam,
you will need this 'static' distributor.
Advance curve is changed, and there is no vacuum advance. Statis timing is 10 btdc.
A hot Imp needs an oil cooler.
This the Rootes kind, but you can also fit a Serck of 8 or 13 rows,
depending on what you did to the engine.
Superfluous for the road:
a full race head plus a set of 40 DCOE Webers plus the big bore exhaust
at a total cost of about £200
The handling!
Shorter and stronger springs can be had, plus stiffer dampers and front anti-roll bar.
You can lower the front suspension pivot points on the old Mk 1, too.

If you want to tweak an Imp, the place to start is the carb. Replace the standard carburettor, manifold and all, with a Twin Stromberg set-up. This package comes from Hartwell's complete with a brace of CD 125s, manifold, gaskets, air cleaners, and all the necessary connections. You can see the price and performance gain on the chart below.

Parts     0 - 60 mph     £
standard 875cc Imp   19.7 sec.    
Twin-carb. kit
(2 Stromberg CD125s & manifold)
  17.0 sec.   42.50
4-branch exhaust system + silencer   15.5 sec.
(exhaust + twin carbs.)
Hartwell high torque camshaft   14.5 sec.
(cam + exhaust + carbs.)
Stage I head
improved ports, improved valves, incl. high torque cam
      42.50 exchange
64,50 outright
Stage II head
improved ports, improved valves, incl. high torque cam
larger inlet valves
  13.2 sec. (on 875)
12.0 sec. (on 998)
  47.50 exchange
73,50 outright
Stage III head (998 only)
improved ports, improved valves, incl. high torque cam
larger inlet + exhaust valves
more mod.s
  12.0 sec. (on 998)   52.50 exchange for 998
R17 cam
(1 stage up on HT cam)
+ carrier to fit
Static distributor       10
Full race head       75
Twin 40 DCOE Webers       90
+ cold air box
Oil cooler kit       20
Large bore exhaust       20
Lighten & balance crankshaft + clutch       9.75
Lighten & balance pistons + rods       16

The next step is a decent exhaust pipe - the rattlesnake thing produced as standard being a bit of a restrictive practice. This again comes complete with silencer and clips and things, as priced below.

As a final bolt-on goody before we reach the stage of ripping the motor down, you can use a Hartwell High Torque camshaft on the 875 engine. These three things together - and they are mentioned in order of appearance, so it's no good popping the HT camshaft on your standard engine and expecting to go any faster - will up the power output of an 875 Imp from 52 bhp to 65. Using the HT cam, Barry reckons to set all the tappets up to 0.009 in.

Cylinderhead modifications

The next step, and to all intents and purposes the last word on the 875 engine, is to chop in your cylinder head for a specimen of same modified to what Hartwell call stage I or II. Both I and II heads include the High Torque camshaft in their prices: this is the best camshaft for any roadgoing 875.

Stage I involves ports which are bigger, better shaped and polished; valves which are lightened, better shaped, and polished - and, incidentally, ground in and fitted to the head - and heavy-duty valve springs.

The stage II is just he same, except that you get bigger inlet valves and seat inserts. Since the Stage II is only £5 more than the stage I, we can't quite imagine anyone bothering with a stage I. However ...

The stage II head with twin Strombergs, the exhaust manifold and the High Torque camshaft just about constitute all it is worthwhile doing on the engine in 875 form. You can take it further, of course, but unless you are trying to stay within a capacity for some particular competition formula, you are far better off going to 998 cc if you wish to go any faster.


Going bigger is fairly painless, except on the pocket. Hartwell will equip your old block with the necessary bigger liners for £35. The only buts are that it must be a Mk II (straight edge) block or it won't take the liners - and that it isn't worth doing the job if the crankcase is scored around the place where the rear main bearing scroll seal rotates. If this is the case you might as well throw the block away, because it will never be oil-tight again.
A new 998 block costs £50, and a set of 998 pistons by Hepolite are £22. They only come in one type, which is 10:1.

Having gone to 998 thusly, you can fit a head which is the same as the aforementioned stage II. except that it has bigger combustion chambers to accommodate the bigger combustion. If you still use the HT cam, this set up with two Strombergs and the exhaust will do 0-60 in about 12 sec. You don't want to go using an 875 head because the smallness of the combustion chambers then raises the compression to 11.5 : 1, which is a bit much.

At this point you will find lightening and balancing of the crankshaft, flywheel and clutch to be a good thing. Flywheels can go down to light for the road and lighter for racing, and rods an pistons can also be balanced and lightened for £ 16.

Alternatively, you can use a thing called the stage III head on the 998. This has still bigger ports and valves, both inlet and exhaust, and it uses double valve springs. The rub is that there's no exchange service on this head unless you happen to have a reasonable Imp Sport head to give back, since there is an extra oil drain in both the stage III and the Sport head which isn't there on the more plebeian specimens. (As a matter of detail: if you use a stage III head on any but a Sport block, you will have to have the return oilway from this drain drilled into the block; provision is made for the hole on all Imp blocks, but it is only actually drilled on the Sport.)

The stage III head still uses the Strombergs, but you will be best advised to use what Hartwell call their big bore exhaust to go with it. You can use the High Torque camshaft here, but you will be better off with a device called the R17 cam, which costs £ 18 plus £6.50 for the modified cam carrier you need to make room for it. This combination also requires a new distributor which has a modded advance curve and no vacuum advance and costs £ 10. Static timing then becomes 10 deg. btdc.

Full race

Finally, you could go berserk and lash out on a full race head. The head itself is only a fiver more than the stage III, but the rub is that you need a brace of 40 DCOE Webers (£ 106 with cold air box), to take advantage of it.

There are one or two other camshafts you can ring the changes with for special uses, but for practical purposes you can take the ulti¬mate road-going Imp as being a 998 with the stage III head, Strombergs, an R17 cam, and the big bore exhaust. If you seriously want to go faster than that. give Barry Green a ring and enclose your cheque-book.

Oil cooler

Finally, before leaving the engine altogether, we ought to mention that a tweaked and hard-used 875, and practically any tuned 998, usually finds itself in need of an oil cooler. This will set you back another £20 or so. Transmission-wise, the clutch ought to be a competition dia¬¨phragm job once you're up to the 998 stage III state, and you can have a close-ratio 3rd/top put into your existing transaxle. Don't try doing that yourself unless you're so bright you don't have to read motoring magazines. You ought, of course, to do something about the suspension if you're going to start winding an Imp up to Mach 2. There are various degrees of shortened and stiffened springs available which lower and stiffen the car, and you can have heftier dampers in either adjustable or non-adjustable form. A hefty front anti-roll bar is also available.


Lowering obviously has the side effect of negativising the wheel camber, and if you are so misguided as to be tweaking a Mk 1 - or even if you are so misguided as to be driving an untweaked Mk I, come to that - you will need to lower the front suspension pivot with a suitable kit, like Hartwell sells. This also lowers the steering box to match. You won't need to lower the pivot point on a Mk II.

If you want to go rallying or pursuing some such other heathen course which involves rushing over 7 in. obstacles with a 6 in. ground clearance, Hartwell will flog you a different range of springs to the ones aforementioned. These are longer, and therefore increase the ground clearance, while being stiff enough to improve the handling a bit.


On the same sort of theme there are, of course, wide wheels available, starting at £3.63 each for 5½J 12 in. steel jobs. and progressing in one easy stage to £25.50 each for 6J Minilites. A set of spacers and nuts to widen the track is £ 1O.


You can spend £ 100 on front disc brakes (chopped around Viva units) or, rather more reasonably, £8 on a set of VG 95 linings for the existing drum brakes. The VGs will be quite adequate for road use, and you can always fit a servo (£ 13) if you find the resulting pedal pressure is too heavy.


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