Posts Tagged ‘gearbox’

I’m talking about that age old argument between diesel fans (who like to talk about torque) and petrol heads who claim power is most important.  My vote is power all the way, and here is why…


Say we need to tighten a nut up.  Doing this without a spanner will not get you very far, why? because you need lots of torque.  Torque includes two things-  force AND distance.  For example, slamming a door by pushing on it next to the hinge will not make it slam, that is because the force may be large but the distance between point of rotation and acting force is small, so the torque is low.  Infact even an incredibly strong man with no tool will not be able to tighten a nut as much as a child with a spanner.

So our strong man ‘Arnie’ tightens the nut using only his very strong fingers.  Now a child comes along to undo the nut, without a spanner he’s helpless.  With a spanner, the child now applies a comparitively small force to the spanner but this force acts a long way away from the centre of the nut the longer the spanner is, the easier (or less forece) required to undo the nut.  Try this with a door.  See how hard it is to close the door if you push near the hinges, compared to if you push near the handle.

Taking this example and applying it to a car, the engine has now become our strong man/weak child and  the car’s gearbox is considered to be the length of the spanner.  The nut we were tightening is of course the road applying an opposing force on the car’s wheels.


Say we need to tighten up 100 nuts, engineers refer to this job as ‘work done’ (which is equivalent to ‘energy’).  Doing this without a spanner would probably be a lot quicker, why, because you don’t have to tighten the nut up by travelling through a large arch/distance.  Speed is the relationship of distance : time.  If we have less distance to cover and we turn nuts at the same rotational speed it will always use less time.

If you don’t believe me, drive 10 miles at 10 miles an hour and then do only 1 mile at 10 miles an hour- the latter surely won’t use up as much time!

So if Arnie and the child both complete the same amount of work in the same amount of time, they are said to have the same power.

Transition to the car example.  Two cars of equal weight have to tow a caravan up a hill.  One car has a lovely (sarcasm) torquey diesel engine, the other has a racy motorbike engine.  The work done will be the same, (same hill, equal weights).  So which one will get there first?  Well it’s all about power.  Power is of course, work done per unit time.  So if you get your work done quicker you finish first.

Interestingly, these vehicles make power in very different ways.  Rember Arnie and the child, well the diesel engine is our strong man- he can afford to spin things slowly because he has lots of torque (strength).  The motorbike engine and child have relatively little strength but a gearbox/spanner provide a torque/strength multiplication.  Like all things in engineering, if you fix one issue, another one is introduced.  By using the gearbox as a torque multiplier you must operate more quickly to keep your work output high.  This is why motorbike engines have to rev so much.

For those interested, a new school SI unit engineer like myself uses the following formula to work out power in kW:

Power [kW] = Torque [Nm]  * EngSpeed [RPM] * (2*3.142/60000)

In the brackets above we are converting engine speed into radians per second and watts into kilo-watts.

To convert kW into brake horsepower, BHP = kW * 1.34

So, What’s Best?

If you want to win a drag race, you need lots of power.  Simple, that is the answer to the question.  If you want to tow your caravan to the top of the hill before all the other caravans you still need to have the most power!  The subtly is the gearbox that denotes the relationship between road speed and engine speed.  Towing caravans up hills requires lots of torque at the wheels even to get rolling.  This can be done by a weak or low torque engine geared to do many more revolutions for each turn of the cars wheels.  Or perhaps a high torque engine, geared to do similar revolutions as the wheels it is driving.  In a towing race, the higher powered engine will always win!  And it doesn’t matter which engine produces more torque (provided they both have a suitable gear box) .

Most diesel fans will claim that this means lots of revving of the less torquey engine to keep up.  Yes this is true, but so what- thats why you have a gearbox and a red line almost double that of the average diesel!  Also, in favour of the smaller less torquey petrol engine is the weight advantage.  By minimising weight you reduce the amount of work needed to complete a race/sprint.

The only real advantage of high torque is longevity.  By creating  power at low engine speeds, service life is dramatically enhanced.  It is also possible to observe fuel savings by keeping engine speed low, as frictional losses are less significant at low speeds, and are zero when the engine is stopped!

In Summary

Cars, trucks boats have all been designed to speed things up.  Whether it’s getting to work quicker in a car compared to walking.  Or perhaps transporting building materials from A to B.  These jobs took a lot longer before the benefit of powerful engines.  Low power means less work done for a given period.


Before starting this tutorial on changing and installing a new differential for your MX5, have a look at  this link if you are interested / unsure what a differential is or how it works, this provides a good explanation.

Prepared for replacement

Diagnosing the problem

Diagnosing a problem is actually quite easy once you understand what’s going on.  Anyone who drives will have at some point heard a funny noise or felt something wasn’t quite right.  This is the first stage to fixing a problem- identify that one exists.  I will now work through the logic of diagnosis.

In my case the interior of the car was filled with an awful whining sound.  I know it can’t be a wheel bearing, experience tells me they make a lower “wub wub wub” noise.  Secondly this is usually only heard when the car is rolling, (regardless of acceleration, coasting etc).   Also, expect for wheel bearing noise to increase or decrease depending on cornering loads. In my case the sound went away when i rolled along in neutral.

We now know that by removing the power the noise goes, hence, it must be ‘powertrain’ .   Powertrain components essentially consist of engine and gearbox.  It can’t be the engine, the noise was not changing with engine speed.  Stop and think, if revving the engine does not immediately speed up the whine, it can’t be connected to the engine- and my sound wasn’t.  In fact, if the diff  is failing, the frequency and pitch of the noise must be directly linked to road speed.  Road speed is associated with gearbox and in particular the output of the gearbox, consisting of the layshaft, final drive and differential.

Identifying the component making a loud whining noise is not easy, the noise fills the cabin.  In the end I made an informed guess!  With some input from my Dad and a work colleague, it was decided that the most likely culprit on my car- a Jap import , was the early spec viscous limited slip diff (LSD).


So I left work early on Friday and nursed it 250 miles in the slow lane, hanging out with the lorries.    Next morning, I headed to MX-5 Heaven, to collect my reconditioned diff.  It just had to be another LSD, I love the way the car handled with the viscous diff, a generous helping of opposite lock mid corner on the power was commonplace!

The upgrade employs a torque sensing design, ‘torsen’ diff.  This was originally fitted to a later model 1800cc NA MX-5.  It is actually a nice upgrade for those considering fitting a turboA far more robust and consequently larger form factor.

The recon

To upgrade an NA MX-5 (Miata) diff you must have the following:

  1. Replacement diff (obviously)
  2. Replacement drive shafts- shortened torsen spec  (due to larger size of the torsen replacement)
  3. Replacement prop shaft- shortened torsen specific
  4. 0.65 litres API GL-5 Differential/gearbox oil
  5. Goggles (I had to have rust removed from my eye, by the eye hospital during the job!)
  6. 23mm socket (Diff filler plug)
  7. 24mm socket (Diff drain plug)
  8. 14mm- removal of rear wheels & drive shaft to diff flange
  9. Hammer and chisel or equivalent to remove staking from hub nut
  10. 29mm socket (remove hub nut)
  11. Breaker bar and almost certainly some kind of extension
  12. Jack & axle stands
  13. Wire brush for cleaning duties
  14. Assortment of ‘standard’ spanners and sockets for other bits and piece
  15. Replacement exhaust gasket (as you will be splitting the exhaust)
  16. Long flat screw driver or equivalent (to remove rubber exhaust mounts)

Replacement parts

The night before starting, it might be helpful to spray the areas you need to loosen with some penetrating oil.  Once equipment is available you are ready to begin.

Start by simply slackening the hub nut (using your 29 mm socket), here is a picture of me in the process.  I applied so much torque, the wheel was rotating on the driveway despite the car being in gear with handbrake on!  Have someone stand on the foot brake to prevent this.

Remove hub nut

Once those nuts have been cracked, get the wheels off and car in the end.  Then remove the driveshafts from the diff using your 14mm socket.

Gain access to the diff for it’s removal.  This requires splitting the exhaust, up near the catalyst and dropping it off it’s rubber mounts (I used a long flat blade screw driver to lever it off).   Also, don’t forget to unplug the small plastic sensor found in this area.  My Jap import has a metal bar connecting the lower wishbones together for added rigidity.  Here I am working it loose with Dan’s help, the bolts were all corroded.

Remove lower arm brace

Disconnect the driveshaft from the diff flange, in preparation for their removal as shown (below).

Now to get the drive shafts out disconnect the top wishbone from the upright (below), then drift the splined shafts out allowing them to drop onto the waiting ground below.


The final problem is the Mazda power-plant frame (PPF)- this is designed to give good throttle response, by ensuring that the gearbox and the diff are rigidly connected.  I found that by loosening it, it is possible to slide it off the diff and with some string tie it out of the way.  Don’t be afraid to get rough with this sturdy old bit of girder!

Now you have access to the diff, it is simply a case of disconnecting the propshaft from the diff and removing the old unit.  Take note here, as far as I can remember the propshaft just slides into the gearbox.  Expect gearbox oil to seep out, when you pull it.  To avoid any further mess I would recommend supporting the car at an angle, (one hole higher on the axle stands at the rear) this helps keep the gearbox oil in the gearbox when you do remove the prop shaft.

The time had come and because Dan wanted to be a hero he bench pressed it out.  We loosened the single 17mm and  two 12mm bolts attaching the diff ‘arms’ to the vehicle.  Then, getting underneath it and undoing the final bit lowering it out by hand.  It’s fairly awkward but gravity is working with you.

To replace is the reverse of above, this time I would suggest utilising a jack so that you can line up the bolts without having to hold the heavier diff in the air.  Prepare for some swearing as you try and mate the PPF with the diff, whilst simultaneously offering up to the car for attacment.  Once up get it tightened up quickly, you don’t want to be looking for your 12mm bolts when its in position!

The offering

Now would be a good opportunity to replace the diff oil, for peace of mind.  I used 0.65 litres of API GL-5.  Unscrew both plugs and allow to drain.  Replace the lower plug and with the car level fill to the top of the filler (do not over fill).  Now replace the top plug, torque to approx 45Nm.

Fix the shorter driveshafts back on.  Smear the upright to top wishbone swivel nut in copper grease and reattach.  Reattach exhaust and all other things which are lying around on the floor!  Put the wheels back on and put the car back on planet earth.

I kept my old driveshaft nut, it is advised to replace however.  Now torque up the driveshafts to 260Nm and re-stake them.

All done

The scary thing now, was driving the car in hope that the noise did not persit- thankfully it didn’t, all was well in the MX-5 and the new diff gave the car a slightly different handling characteristic.  I have found that turning through a chicane has the effect of locking the outside wheel, this is great, except when you wish to finish the chicace, turning in the other direction.  It is impossible without lifting off.  Without lifting, the diff remains locked up and is effectively preventing the wheels from taking alternative radii, if this happens to you, you will feel the car’s front wanting to push on forward and the steering wheel becomes incredibly difficult to turn.

Hopefully this tutorial has been helpful.