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Andy Dawson

Dawson, A.
Dawson's Dodges, Part 2 / by Andy Dawson. - Cars & Car Conversions 1976, June. - p.39, 41, 43
A series on how the experts prepare competition Imps, by Andy Dawson, the man who probably knows more about it than anyone else. This month he ventures into engines of over 998cc's.


Dawson's Dodges

LAST month I waffled through the Imp engine, going from basic road trim to race 998 units. This month I'm going to run through the various 'big' engines and describe the mods that can be done to the engine ancillaries and the transmission.

1132cclong stroke The most successful over 1000cc units
based on the factory prototype long stroke blocks
some 20 were produced but only 6 got machined
last one left (ex-Harper rallycross) see CCC 1974 Sep.
996cc68.53mm stroke cranks factory prototype long stroke blocks
1001 units68.5mm Fraser race
five bearing cranks and 'deep' heads
Ian Carter got hold of the last of these
Ian Carter, Paul Emery & Andy Dawson made 1000+ engines by taking basic components out to their max:
(grounded std. blank crankshaft)
std. 998 bore 
(grounded std. blank crankshaft)
(max. possible)
(max. possible)
Paul Emery tried these with cast iron inserts in the
normal block, but tight tolerances and
differential expansions made these invariably unreliable.
1150long stroke cranks75mmDawson: wet liners, 103 thou proud of top of block
1220cclong stroke cranks
suitable con rods
(w/ DAD wet liners)
Chesman: cranks & con rods fit for a standard block
Clark Dawson, FaceBook, 13-Jan-2014: 'takes careful boring; all blocks differ'. (Not tested in a car yet)
  70mm 875 block, using 930 pistons; dry liners
  72mm 930 B1 block; (thicker) dry liners

All the Imp tuning experts have tried to produce engines of over 1000cc, and have succeeded in producing screamers or a pile of little bits. The most successful units have been the 1132cc ones based on the factory prototype long stroke blocks, of which about 20 were produced although only half a dozen were machined. The origin of the blocks, with their 68.53mm stroke cranks, was to give 996cc for production, but this never came off and the blocks and cranks found their way into Chrysler competitions.

The Fraser race 1001 units were based on the 68.5mm stroke but with five bearing cranks and 'deep' heads. To my knowledge there is only one of these units left, the ex-Harper rallycross engine, which is now residing in the back of the Imp which was featured in CCC in September, 1974.

The deep heads are, as the name suggests, a deeper version of the standard head to allow better breathing. Ian Carter seems to have got hold of the last of these and has made very good use of them. Ian also produces larger capacity engines by taking the basic components out to their maximum, something done in the past by Paul Emery and now by yours truly.

The standard crankshaft blanks can be ground to give a stroke of 65mm, which with the normal 998 bore gives 1073cc, and with the maximum possible bore of 75mm gives 1149cc. Paul Emery tried to get the big bore to work with a fancy cast iron insert in the normal block, but tight tolerances and differential expansions means that these are invariably unreliable.

The current trend, and the way that I am building 1150s, is to use 75mm wet liners which, although very fragile before fitting, are plenty strong enough for the job when fitted. To accommodate the longer stroke, the liners are 103 thou proud of the top of the block, so we use an aluminium spacer plate to fill the space with a gasket above and below it. To seal the head I have had some very thin Wills rings made, which sit in a circular combustion chamber giving very good top end breathing. One advantage of this unit is that existing 998s can be converted without the need for too many new parts.

Andy Chesman has attacked the problem of big engines from another angle, and has had long stroke cranks and suitable con rods made to fit into the standard block. Now that my 75mm bore has been found to work reliably, he can produce 1220cc units by combining the stroke and bore. My 1150 gives 125 BHP on an R20 cam, so we expect over 130 with an R23 cam and perhaps 140 brake with 1220cc. Who said the Imp was dead?



Carburettor Settings

Single 125 CD875Standard6K needle & natural spring
Single & twin 125 CD875 Sport6K needle & blue spring
Twin 125 CD875Standard6L needle & blue spring
Twin 125 CD998Sport6S needle & blue spring
Twin 125 CD998R176S needle & red spring
Twin 150 CD998Sport6F needle & blue spring
Twin 150 CD998R176F needle & red spring
Twin 1¼inch SU875SportNo 1 needle & red spring
Twin 1½inch SU998SportNo 7 needle & red spring
Twin 40 DCOE Weber
with air cleaner
998R17/R20Main 130/ Air 180/ Tube F16/ Choke 32/
Pump 40/ Idle 45F9/ Return 55
Twin 40 DCOE Weber
no air cleaner (race)
998R23Main 115/ Air 160/ Tube F15/ Choke 30/
Pump 35/ Idle 45F9/ Return 55
Single 28/36 DCD
on heated manifold
875SportPrimary: Main 105/ Air 220/ Choke 21/ Idle 50/ Pump 50
Secondary: Main 110/ Air 220/ Choke 22/ Idle 55.
Single 28/36 DCD
on heated manifold
998R17Primary: Main 115/ Air 230/ Choke 22/ Idle 40/ Pump 50
Secondary: Main 130/ Air 200/ Choke 24/ Idle 50.

Most people have fits when you mention the standard Imp Solex carb, but in reality there is very little wrong with it. Until you get over 55 BHP, the normal single carb can cope well and gives very good fuel economy. There is a slight improvement in general performance from fitting a single CD or SU but I doubt whether it is worth it financially. The next step is either twin CDs or SUs or a 28/36 DCD Weber, remembering what I said last month about the use of paper element air cleaners. They are absolutely vital. The element for CDs or SUs is GUD AG322A.

Twin 1¼-inch carbs are good up to about 70 BHP and from then on it is necessary to go to twin 1½ inch or twin, twin chokes. The same air cleaners fit the 1½ inch carbs.

The problem with fitting Webers is the amount of clearance between the carbs and the chassis rail. For this reason it is worth fitting 35 Del'Ortos which are quite a bit shorter and give very little less power. With 40s it is necessary to flatten the chassis rail and use very short (5/8 inch) trumpets. An air cleaner can be made up, either using a Sunbeam Tiger unit (as sold by Chrysler comps), or by fitting a cold air box to the carbs and having ducting to a remote filter. The Mk 2 Lotus Cortina or Escort Twin Cam system works very well with the air cleaner mounted inside the car. Make sure that you fit a splash guard between the exhaust and the carbs: without it you could have a stone coming up and breaking the carb body.

Inlet manifolds are straightforward and for twin, twin choke carbs the manifolds are joined together so you have no choice of exhaust. For the single and twin carb set ups, there are two exhausts worth considering: the Imp Sport system, which is readily available and the Janspeed system which gives better gas flow at the cost of a loud note.

If you want to use a really big bore exhaust on a road or rally car then it is necessary to mount the silencer across the back. Chrysler sell a kit which includes the silencer and link pipe, but the problem is that the pipe gets flattened by stones. The remedy is to cut and shut the pipe so that it comes straight back from the manifold and then curls up under the rear panel and into the silencer. The whole of the rear mounted system should have a heat shield over it to keep Sergeant Plod happy.

Before running through the various engine specs that I have found work well, I'll go through the engine accessories.

Engine accessories

Fan belt / Pulley

Perhaps the biggest single problem with the Imp design is the fan belt. The length of the belt and the speed at which it has to move means that it is prone to flying off. The basic problem is that the engine slows down faster than the generator or water pump which means that all the slack goes into the length between the generator and crank pulley; the whip allows the belt to touch the back panel and it gets knocked off. In competition or with a tweaked road unit it is possible for the belt to whip enough to get knocked off when lifting off at over 7000 rpm.
There are two remedies: the first is to use a top quality fan belt (green tag on the back), and to use pulleys with high flanges on them. Chrysler sell aluminium pulleys, but they tend to wear out very quickly. The ultimate remedy being toothed pulleys and belt.

Front mounted radiator

One easy way of gaining 7 BHP is to remove the fan assembly and fit a front mounted radiator, but this isn't as easy as it sounds due to the pitfalls. The normal rear radiator can be used as a header tank and the front rad fitted between the head and the rear rad. The problem here is that the head doesn't bleed properly. So the remedy is to fit a steam pipe from the top of the thermostat housing in the head and connect it to the top of the rear rad/header tank.
I use quarter inch bundy tubing with proper connections at the head end and nylon tubing to connect that to the header tank.


If you remove the heater and by-pass plumbing then you must also leave out the thermostat. The easy way to plumb in the heater is to fit a thermostat into a T-piece which sits just before the radiator, and have the heater in parallel to the front rad. The thermostat will only open when the heater can no longer keep the engine below thermostat temperature.

Radiator: type and airflow

The front radiators themselves can vary between the small auxiliary type (which is just a heater matrix) to full house Group 2, which is half an XJ6 rad. Basically any rad which is good for 1½ to 2 litres of normal car is sufficient for the front of an Imp. It is just a case of finding something cheap which will fit.
To give the necessary airflow through the rad, remove the horn/air box and either cut a slot in the bonnet just behind the top of the front curve or remove the rubber seal from round the sides and back and pack the hinges up one eighth of an inch.

Water-pump hose

The only other mod worth doing to the cooling system is to turn the input side of the water-pump housing through 90° to allow the bottom hose to come into the pump from the top, thus making it less vulnerable on rally or race cars.


A good inexpensive set up for a Mk 2 is to fit a Sport cam and exhaust, with possibly a stage one or two cylinder head. Next come twin carbs and Janspeed exhaust manifold with stage two or three head. Staying with 875cc, a Sport head modified to stage three with an R17 or preferably R20 cam should give a comfy 65 BHP.

Above this I recommend going to at least 915cc or preferably 998. Standard Sport bits bolted to a 998 block give a very nice torquey motor for road use, with stage two or three heads being on the priority list, along with R17 or R20 cam. Surprisingly, an almost full race unit with big valves, R20 cam and twin CDs is more tractable than an R17 cammed, Sport headed 998, and obviously it has much more top end power. The ultimate stages of tune are obvious.

Typical Imp power curves Typical Imp torque curves


The start of the transmission train is the clutch, and as good as anything for up to full house engines is the bog standard Mk 2 unit. Chrysler sell a competition cover which has a slightly stronger diaphragm and helps the plate grip the flywheel. For full house engines the rally clutch plate and cover will only just cope and certainly don't allow full power take offs.

Sintered clutch plate

The answer is a sintered plate, either in Formula Three form or the latest AP Gp 2 design. The problem with the sintered clutch for rallying is that the release mechanism is very heavy and moves out of engagement over the rough, meaning a double pump for every gear change. The latest AP design uses standard release mechanism and cover with special plate. We have been trying one of these on an 1150 and so far it has proved to be 100% successful. The big advantage as far as you are concerned is that it will be much cheaper than the F3 design.

Lower gear ratios

So onto the grotty transaxle. I say grotty because for long enough it was the transaxle that was the limiting factor in the Imp's development for rallying. When I was rallying my own Imp I wanted lower ratios but knew that going lower than the works rally ratios would mean risking having the gears sieze on the bushes due to heat build up.

    Gear assemblies: dog clutch-type and standard
 comparing gears     standard gear assembly
 component parts of one gear of each sort
8 selector dogs on the left hand dog type
 multiple dogs of the standard, synchro type
a complete dog clutch pinion shaft assembly with first gear engaged

At Chrysler Comps we tried using carbon for the bushes to help stop seizure, but the manufacture problems and cost became preventative. The answer appeared to be to use the Jack Knight needle roller and dog clutch type of box, but experience with Peter Harper's rallycross car indicated that these weren't reliable enough. Now we have found the answer to making this type reliable, but more of that anon.

The biggest problem with a transaxle is maltreatment. Firstly the only oil that should be used in the bush type boxes is Shell EP80; in my experience anything else causes problems. The second point is that they should be assembled by somebody who knows what he is doing. The preloads and clearances are very important. There are very few actual tweaks inside a transaxle:
the detent ball should be preloaded so that the selector rod can just move without the spring becoming coil bound
the muff coupling should be modified by putting a spring behind it to stop the spring ring coming off at high revs,
and that's about it.

The gear ratios of the standard unit are far too high and, because as the final drive isn't changeable, it is the ratios which must be lowered. Chrysler sell rally ratios which have an uprated first and low third and top, but they are only made in small batches and are sold very quickly.

In the past they have produced close and circuit ratio sets, plus limited numbers of Group 6 stage rally and rallycross ratios. Jack Knight sell conversions for third and top to give a wide range which covers all likely possibilities, and with their needle roller conversion any sets of ratios are possible.

Standard 16.68.905.70 4.14 
Close16.6 8.906.184.48
Gp 613.58.906.805.04
RallyX13.510.65 7.806.00

As will be seen in the photos, the dog clutch boxes are very crude compared to the production units, but they are also very strong. Thus the problems that we used to have with these boxes puzzled me, and it was almost by accident that I found the cure. As designed, there is no preload on the sleeves which the needle rollers turn on, and I found that setting these with five thou compression and then setting the pinion preload by shimming up the pinion tail bearing, I reduced the amount of snatch in the box from power on to power off and thus reduced the shock loadings. It is necessary to reduce the pinion preload to a minimum to stop the tail bearing overheating, but so far this has proved 100% successful for stage rallying.

The actual bearings in the box can be changed for better ones, which are:
Tail bearing - SKF BK 21075/K - 21212,
Input shaft rear - SKF 6204MA C3 B20,
Input shaft front - Bremen BR 162112.

And so into the differentials.
Firstly the crownwheel retaining bolts should have their tab washers left off and the bolts should be Loctited, as I have said many times before.
Secondly the diff preload adjusters should be locked by drilling and split pinning instead of the usual clips.


The diff itself isn't strong enough for much hard work, the diff cross pin snapping like a carrot with a good snatch, and the diff cage breaking at its root after a lot of hard usage. The cross pin should be replaced with one made from EN36A or equivalent. And the cage should be polished at the root, crack detected and shot peened or tuftrided.
Similarly the diff gears and cross pin can benefit from tuftriding. The ultimate cure for the cage breaking is to fit a pearlitic iron cage as fitted to the very early transaxles.

Jack Knight locking differential

The alternative diff is the Jack Knight locking diff, which seems to enhance the car's handling but breaks rotoflex couplings. The LSD also seems to increase the shock loadings and helps the breaking of the casing, but as long as the latest type of casing is being used there shouldn't be any problems.

Differential output shafts

The diff output shafts have a habit of breaking due to the shaft being pulled out of the spider. The cure is to use either the early type which has the shaft held in with a nut, or to put three dabs of arc weld between the blunt end of the shaft and the spider.


oo - 00 - OO - 00 - oo

Next month:
the closing tome of this epic, as DAD moves onto the subject of suspension and allied matters. Don't miss it.



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© Franka
Version of: 7 Jan. 2013
File started: 7 Jan. 2014