Imp Site logo - imps4ever!

The Imp Site

David Vizard

Vizard, D.
CCC look-in on the Imp / by David Vizard. - Cars & Car Conversions 19??. - p.54-55,91
Gearbox and gearing ratios: specialising your Imp


CCC Look-in on the Imp / by David Vizard

BEYOND A CERTAIN POINT, ENGINE tuning tends to turn a car into less of a 'Jack-of-all-trades' and into a vehicle which becomes more specialised in its purpose,

    standard Imp gearbox, section through
Figure 1.: Section through standard gearbox, showing the basic simplicity of this unit
    breather outlet in later Imp gearbox
Figure 3.: Section through breather on gearbox, from 300,000 units onwards

Having designed an engine in which the stresses are sufficiently low to ensure a good life, the standard engine has to be mated to a gearbox which will give adequate torque multiplication to get the car moving under all reasonable circumstances, as well as providing a high economical cruising speed. Inevitably such a gearbox has widely spaced gear ratios.

Now, such a gearbox may well prove adequate for ordinary everyday motoring, for the car may be called upon to do such diverse things as pulling a caravan or going flat out for several hours on a motorway.
If we are to turn a car into a performance vehicle, then its purpose is far better defined than would otherwise be the case. If we want performance, then we need horsepower and this is the task of the engine. The conventional internal combustion engine is, however, far from a satisfactory form of motive power. To make up for its shortcomings in its power output characteristics, it must be coupled to a gearbox which will effictively utilize and transmit this power to the rear wheels in the most usable form.

Ideally, what we need is a transmission which will give us infinitely variable ratios and little or no power loss. Unfortunately, nothing like this is available for the Imp, so we must make do with the next best thing, that is close ratio gears.

Close ratio gears

With the Imp, close ratio gears start to become a desirable proposition when the engine has been tuned to a little over the stage of the Imp Sport specification.

Right. Assuming you want close ratio gears, let's see just what is available. Before delving too deeply, we should get it clear in our minds that there are basically two Imp gearboxes:

  1. gearboxes up to unit number 300.000
  2. those after 300.000.

All gearboxes after 300.000 had a different type of synchromesh assembly and the gears were modified. At this point we need only say briefly that when getting gears, check on which gearbox casing you have. Generally speaking, of course, the later gearbox is the better one.

Chrysler Comps Dept originally offered 3 distinctly different sets of gear ratios. Two of them, which are now no longer available from this source, were intended for the earlier gearboxes up to number 300.000. These were known as the close ratio set and the circuit set. The reason for mentioning these is that they still crop up on the second hand market and so it is at least worth having some idea as to what you are getting. The close set of gears had a standard 1st and 2nd but 3rd and 4rth were both lower. This had the effect of giving you the standard difference between first and second, but the gap between second, third and top was a lot less.

gear sets:close ratio setcircuit setrally set
intended for the earlier gearboxes up to number 300.000
after unit 300.000
comment:a standard 1st and 2nd but 3rd and 4rth were both lower.
This had the effect of giving you the standard difference between 1st and 2nd, but the gap between 2nd, 3rd and top was a lot less.
circuit setAfter unit 300.000, the factory seemed to standardise on what is now known as the Rally set of gears.
These ratios differ from the earlier ones in that they are slightly wider spaced than the circuit ratio gears, but utilize the same top three ratios as the close set of gears, but this is in conjunction with a higher bottom gear.



Chart of available gear ratios for the Imp box:

Jack Knight  13.5310.656.96.0

The above figures are overall ratios including the 4.857/1 final drive.
Speed with any of the above ratios can be calculated
by the following formula:

Formula: Speed = revs X tyre diameter / by gear ratios

Again, for gearboxes up to unit number 300.000, one could obtain the circuit ratio gears from the factory. As can be seen from the accompanying chart, this set of gears utilized a lower top gear, a standard ratio 3rd gear and a higher 2nd and 1st gear.

After unit 300,000, the factory seemed to standardise on what is now known as the Rally set of gears. These ratios differ from the earlier ones in that they are slightly wider spaced than the circuit ratio gears, but utilize the same top three ratios as the close set of gears, only this is in conjunction with a higher bottom gear.
If you want the close ratio 3rd and top gear conversion for your box, either the early or late one, then it is still possible to obtain these either from tuning specialists or direct from the manufacturers, that is: Jack Knight (Developments) Ltd, Butts Road, Woking, Surrey (Woking 64326).

For most road purposes and the occasional bit of competition, the 3rd and top gear conversion presents an excellent compromise since it gives much more suitable ratios than the standard box and yet does not cost a vast amount of loot.
If any serious Rallying is to be done, then the works Rally set of gears can be considered a very good choice.

Circuit racing

However, for circuit work, we find a slightly different situation arising.
Generally speaking, one does need slightly closer ratios than the Rally set provides. The versatility of the Imp box is such that Jack Knight (Developments) Ltd. are able to supply a vast range of gears from which one can choose one's own ratios. The advantage of so doing is that one can select one's gear ratios to suit the engine characteristics, and also make some allowance for the sort of tracks one is to be driving on.
The only snag to this, and indeed most other close ratio boxes, is that there always proves to be one corner on every track which makes nonsense of your choice of ratios.

Quick change box

To overcome such a situation, Jack Knight can offer an Imp box modified such that ratios can be speedily interchanged. This means that if you find a corner for which you have no suitable ratio, you can quickly remove the gears and substitute them with another pair to give you a more suitable rate of gearing. This quick change box has been achieved by doing away with the standard lay shaft and substituting it with a splined shaft upon which the gears are mounted. This makes it a simple job to slide the gears off the shaft, and replace them with alternate ones.

However, each gear must mesh with its correct mating gear. To make the main shaft gears quickly interchangeable, the synchromesh as such has been done away with and face dog engagement is employed. The lack of synchromesh, however, does not affect the gear changing action or the speed with which gears can be changed, as the closeness of the gear ratios gives little for the synchromesh to do in any case.

Needle rollers

In the interests of efficiency, the gears on the main shaft in the Knight gearbox run on needle rollers as opposed to the bronze bushings of the standard box.

5 speed gearbox

Apart from the four speed, dog face engagement quick change boxes, Jack Knight can also do a five speed version which must be considered about the ultimate in transmission assemblies for Imps.

3rd and top gear conversion: modify the shaft to take straight cut gears of closer ratios
3rd and top gear conversion entails modifying the standard shaft to take straight cut gears of closer ratios.
Such gears are available from most Imp specialists.



As we have already mentioned there are basically two types of gearbox, those up to the 300,000th gearbox and those after. There is a little more to it than just that. The factory policy of continuous improvement has brought about a number of changes since the introduction of the Imp. Some of these changes are concerned with a gearbox.

Early casings could be had in 0.100" or 0.120" thickness castings. Avoid the thin ones.
Later casings had extra ribbing which is a better choice.
Latest type casing with even more ribs to support final drive unit.

Gearbox casing

Let us consider the gearbox casing itself. This has significant importance because it is the casing which has to withstand all the stresses and strains produced by the transmission of the engine power.

Early production gearboxes had a general casting thickness of 100 thou. Although these were found about adequate for production engines, they proved to be a little unreliable with tuned engines. Because of flexure in the early casings, it wasn't long before the thickness was increased to 120 thou.
Even with a thicker casting, the box could still suffer from flexure around the hypoid gear and output flange area, when tuned engines were used. Because of this, the works competition box was introduced with extra ribbing - although the casting thickness remained the same at a hundred and twenty thou (which it does to this date). Not too much time was to pass before the extra ribbed competition gearbox casing was introduced as a production item, and as far as the extra ribbing goes, this gearbox is in production even today.

However, this is not the end of the line as far as gearbox casings are concerned. While some dies were producing the Comps. Dept. replica box, as we could call the extra ribbed box, some new dies were introduced to produce a casing which had even more ribbing around the hypoid area than the Competition Dept. replica box. Apart from the extra ribbing, this latest casing, which must be recog nised by the ribbing, also has a shorter reverse selector rod, so interchangeability of some parts may be affected.

If you are contemplating a gearbox for a tuned Imp, then obviously the early 100 thou casings must be avoided. The 120 thou casings without the extra ribbing should also only be used as a last resort. The ones with the extra ribbing, those which are a replica of the Competition Dept. casing, will of course be alright to use, and have been quite successful on even highly tuned cars. The best to use, though, must be the latest casing, as this is the most rigid of all.

Differential assembly

    Imp differential assembly
Figure 2.: Section through differential unit (early type)
Early differentials can be updated to later specifications. All have the 4.857 final drive ratio
    Imp differential gears
Figure 4.: Early differential gears (sun and planet) - at the top - and output shafts
Later units are as shown in the seond set-up

The differential assembly and output shafts have come in for a fair degree of change since introduction.
Again, we can safely take it that the later type diff is the best. Early type diffs have the sun and planet wheels bearing directly on the diff cage. On a hard pushed car, that is one where the diff is continually called upon to work, as it would around sharp corners when the inside wheel could be spinning, it was possible for rather more wear to take place than was necessary.
To overcome this, one should fit the thrust washers from the later type diff, and these must be used together with the later type planet and sun wheels, or it will be found impossible to re-assemble the unit.

At the time of the thrust bearing modification to the sun and planet gears, a further modification was made to the other end of the output drive shaft, that is the spider end of it.
Early models had a large nut, which secured the spider on its spline. During an interim period, the spider was press-fitted to the spline and the circlip on the inner end was deleted, but the clinch nut was still retained. This particular type can be identified by the paint on the nut. After a short period of production of this type the nut was dispensed with altogether. Other than updating the diff assembly and output flanges to the latest design, there is little we can do in the way of improvements for normal use.

For serious competition purposes, a limited slip differential can be considered an advantage. These, like virtually all other Imp transmission components, are available from Jack Knight (Developments) Ltd. Although a limited slip diff can be of advantage when traction is the prime consideration, the cam and peg ZF type (of which the Jack Knight manufactured item is an example) does have the disadvantage of making the car a little less predictable in handling.


The subject of reliability must rank high on the requirements for a competition car, be it a Race or Rally car. When the engine power output gets around the 100 bhp plus mark, we find that we can be faced with certain problems concerning the transaxle.

Oil temperature

Circuit cars which have been lowered a lot or Rally cars which have a sump shield can find that the transaxle oil can get very hot. If the oil overheats, then the bushes on which the gears run tend to seize.
The problem is slightly reduced by grooving the bushes such that they can hold a little more oil. This, however, is far from an entirely satisfactory solution, and is nothing more than a help.
The ideal way is to fit an oil cooler, and pass the oil through it from the transaxle by means of an electric fuel pump. The Jack Knight dog face engagement close ratio box does not suffer with bush trouble, simply because the gears run on needle rollers. This does not mean, however, that the oil temperature need be ignored on these boxes, as it could lead to premature failure in spite of the needle roller bearings.

As far as lubrication is concerned, the best oil for the job seems to be Shell EP 80. Other oils have tried, but they inevitably lead to premature failure, so it's not worth taking a chance with anything else but the Shell oil.

Build your own

We can also consider the building of Imp transaxles under the heading of reliability. For if they are not carefully built up, they will not last. To completely disassemble and rebuild Imp gearboxes, you need a fair number of special tools, so unless you anticipate building a fair number of units, you should entrust the building of the box to a reputable Imp specialist.

Although the Imp gearbox is in itself simple, it relies on it's accuracy of building for smooth and reliable operation. Within the gearbox are a number of selective spacers or washers, and it is upon these that gear position and bearing preloads depend. If you do decide to build your own Imp box, then do it exactly as it says in the manual, or you could be building yourself a load of trouble.

Choosing ratios

Before finally winding up, we should say a little about selecting ratios.

normal road work 3rd and top gear conversion is a good bet
a serious bit of Rallying you can hardly do better than to use ratios as for, or similar to, the works Rally gear set
circuit work  just remember that the Imp motor revs, so don't try to overgear it in an effort to get top speed.
It is best to select gears which are low enough to make use of 9000 revs plus. If you get the Knight quickchange box, then of course you can dabble around with ratios until you get the right set of gears for a particular circuit. The idea is to select top gear so that you pull somewhere between 9000 and 9500 revs on the fastest part of the circuit and the other gears to suit the principle corners. Ideally the gearing needs to be such that you change up just after the exit of the corner.
Whilst catering for corners and top speed, we must not forget that Races can sometimes be won or lost on the start line, so first gear needs to be low enough to give you a good start.

It may be as well to remember that the Imp transaxle can be a little more demanding than most other gearbox assemblies. However, if it is carefully built, treated right, and used in an intelligent way, it will give good service.

oo - 00 - OO - 00 - oo


The Imp Site
   Imp Anatomy (index)
   Imp technicalities (assorted bits of knowledge)
   Tuners & Builders
      David Vizard
   Imp Gear Shift
      John Lewis, co-developer of the Imp transaxle


© Franka
Version of: 5 Oct. 2013
File started: 3 Oct. 2013