The Big End - Getting Geared Up | Classic car restoration, servicing, technical advice, surveying and valuation

The Big End - Getting Geared Up

We have all said it several times in the past. Why does this Morris drive as though it needs another gear, and I wish it would not rev so high in top.

Yes it all been said of the Morris time and time again. But what is the fundamental problem here? In a nut shell - gearing - and, if you like, the wrong combination. Just changing your tyre size can have a drastic effect on the vehicles speed and performance, as will the Speedo calibration.

All is not in harmony beneath the covers so to speak. Our little wonder is not acting as it could, and to make it worse, because the engine is getting a little tired, its a bit noisy at speed.

Gear ratio are in the main looked at as though they were unmentionable, and if you ask the chap down the road what his thoughts are on the subject you get a blank stare or worse.

In fact Gear ratio are just about as important as the fuel you need to run the engine.

The problem of having the right gear available at the right time is a very common problem and one that dictates to the motor tuning world whether the gear sets fitted are for general motoring or for sporting activities. The problem is paramount. Engineers have strived for perfection and invariably got it wrong. Perhaps no gearbox at all, only an infinite variable set of ratios would suffice. Remember the old belt driven Daff set up which, as you took up the drive from stop, a set of belt driven drums would automatically set a ratio according to your speed. It worked very well indeed and I have often wondered why it was never developed.

In principle all the gearbox does is to multiply the torque (leverage) the engine applies to the rear wheels through a series of cogs. These allow the engine to run faster and nearer to its peak power for any given road speed. Eventually the engine in each gear will run out of revs until a higher gear is selected. This process continues until you are in top gear and runs until finally flat out. The complications arrive when you wish to be in a particular gear at a particular driving condition, be it on a corner, incline, steep hill or on a flat straight road. It is at these moments that any mismatching will manifest itself.

A simple check on your Morris's resident gear ratios can be done with a small amount of effort and is as follows:

For the final drive gear ratios:
Chalk a mark on your engine pulley to count your engine revs in conjunction with the driven wheels.
Put a mark on your rear wheels.
Put the car in each gear with the hand brake off.
With some help and with the vehicle placed on flat and solid ground, push the vehicle forward, and note one complete revolution of the marked rear wheel, at the same time note the revolutions of the engine pulley.
Note each reading for each gear setting which should approximate the following table:

You should get a set of figures approximating to: 12 engine revs in bottom, 8 in second, 5.5 in third, and 4 in top. Divide each figure by top (or final drive ratio) this will give the gear ratios fitted.

For example:

1st = 12/4 = 3:1;

2nd = 8/4 = 2:1;

3rd = 5.5/4 = 1.375:1;

4th = 4/4 = 1:1 (direct)

And to check your differential ratio proceed as follows:

This can be done with the gearbox floor cover removed and with an observer sitting inside the vehicle.
With a suitable mark on your drive shaft to note rotations, mark one tyre edge at the bottom while the vehicle is standing on level ground.
Making sure you have plenty of room, release hand brake and push vehicle forward while noting one turn of the rear wheels (360deg) in conjunction with the turns of the propshaft.

For example:

3 ¾ turns on drive shaft indicates diff ratio 3.73:1

4 turns on drive shaft indicates diff ratio 4.10:1

4 ¼ turns on drive shaft indicates diff ratio 4.27:1

5 turns on drive shaft indicates diff ratio 5.38:1

and so on.

Another useful way of determining the differential ratio is:

If you have the diff out of the vehicle there is usually a mark on the crown wheel which for instance could be 9/37. Dividing 37 by 9 = 4.1 ratio.

Fitting a higher differential will be one way of reducing rpm, and as the usual ratios fitted to the Morris are either 4.55:1 or 4.221, you could fit a differential from a Midget or Sprite, of which the later units were 3.909:1. This of course will lower the rpm, but you must remember the acceleration will also be lowered. Its a trade off here, and it really up to the individual needs.

The better choice is to fit a better gearbox with synchromesh in all gears, bearing in mind a 4 speed is perhaps the realistic size for vehicles up to 1098cc. Anything fitted after this depends on the power output.

Of course fitting a different axle is also an option, but there are other components and upgrades needed with this combination. I have covered this area in the past in my articles on tuning the Morris. Please see back copies.