Final Drive Ratios:
XR3i - 4.29:1
RS1600i - 3.84:1
S1 RS Turbo - 4.27:1
S2 RS Turbo - 3.82:1
Mk3 and Mk4 Gearbox Casings
The mk3 and mk4 spec gearboxes are to all intents and purposes the same unit with different ratios used for the different engine types fitted. A gearbox from a 1.1L for example can be successfully fitted into a 1.6 Ghia though the ratios will be different to those of the original gearbox.
However, due to revised mounting, the gearboxes from the Mk4 cars cannot be fitted to Mk3 cars. The reason for this can be seen below.
The Mk3 gearbox is shown on the left with the Mk4 gearbox on the right. As can be seen, the attachment point for the forward mounting of the Mk3 gearbox is not present on the revised Mk4 gearbox. Apparently some very early Mk4 gearbox still had the mounting platform in place on the casting though without the four holes drilled and tapped. These were probably end of line Mk3 units that were carried on into the new Mk4 production and with careful drilling and tapping could be used in the Mk3. I have not however seen any of these boxes myself so cannot say they definitely exist. If you do find one and decide to drill and tap the unused mount then be sure not to drill too deep or you'll puncture the casing.
The Mk4 car's gearbox is supported by an additional bolt on cross-member running along the engine bay from front to back. This bolt in cross member, incorporating two rubber mounts and known as the gearbox cross member, is fixed in place to the shell by four bolts (two in the front cross member and two at the base of the bulkhead) and to the gearbox by lower bell housing bolts. Due to the fact that it locates onto the gearbox by utilizing the bell housing bolts it is possible to fit a Mk3 gearbox to a Mk4 car.
Limited Slip Differential
LSD - Differentials - Standard
A limited Slip Differential unit (LSD) was fitted to the top spec Series 1 & 2 RS Turbo cars to try to control wheel spin and improve traction. Fitment of these gearboxes is a good idea for high performance cars and there popularity and the fact that they were only fitted to one model makes them rather sought after so they command higher prices than the other gearboxes. However, there are no external physical differences between the LSD equipped gearbox and the standard gearbox. So how can you tell if the gearbox you are buying actually does have an LSD fitted? If the gearbox is still fitted to the car then jack the front end up so both front wheels are clear of the ground. Turn one of the wheels by hand and watch the opposite wheel. If it turns in the same direction then the gearbox has an LSD fitted. If it turns in the opposite direction then the gearbox has a standard differential gear fitted. Use the same method if the gearbox is removed from the car but still has the drive shafts or C.V. joints fitted.
If the gearbox is on it's own with the drive shafts removed then you obviously aren't going to be able to turn anything. However, you can still check the LSD is present as you will now be able to see it.
If you look through the output/drive shaft holes in the diff housing of the gearbox you will see one of two possible things:
If you see a metal bar across the hole as shown in the view below then the gearbox has a standard differential fitted. (the bar can be at any angle as it rotates with the differential gear)
If you see the view shown below with no bar present but what looks like a thick washer deep inside then you have an LSD equipped gearbox!
There are differences between the Series1 (Mk3) and Series2 (Mk4) RS Turbo gearboxes though that go beyond the mounting differences detailed above.
The Series 1 RS Turbo was a proper homologation car with true motor sport routes and received the LSD as a result. The LSD unit could be harsh in operation and the final drive ratio was lower geared. The series 2 RS Turbo however was not a homologation car so it was toned down and re-tuned to be more civil for everyday driving. Therefore the LSD unit was altered to be smoother in operation and the final drive ratio was revised to give it longer motorway cruising legs. The other change was the way in which the crown wheel on the LSD unit was secured in place. On the Series 1 gearbox it is held onto the LSD by six bolts whereas on the Series 2 gearbox it is secured by eight bolts. Therefore the crown wheels cannot be swapped between the two LSD unit's. This means that if you whish to fit a Series 2 LSD to a Mk3 gearbox you also have to fit the crown wheel and final drive pinion. In the same way, if you wish to fit the Series 2 crown wheel and pinion to your Series 1 RS Turbo to get the higher gearing then you also need to use the Series 2 LSD unit.
For instructions on how to fit an LSD into a standard gearbox, look in the upgrades page of this chapter.
Apart from complete gearbox swaps for either 4 speed to 5 speed or for different stock ratios, there's not a lot that can be done transmission wise that I know of except for the fitting of a limited slip differential.
Changing from a 4 speed to a 5 speed should as far as I know be a simple task of swapping the two complete gearboxes over. You will of course also have to swap across the gear selector linkage, though chances are your original 4 speed linkage is probably pretty worn out by now anyway. I don't know for definite but I seem to recall hearing somewhere that the gearbox and engine mounts might have been tweaked slightly when Ford started fitting 5 speed boxes to the Escort in order to move the whole engine and transmission assembly closer to the drivers wing. This would make sense as it would provide a little more clearance for the 5th gear housing of the 5 speed box. However, I have not yet had this confirmed. It is just something to bear in mind. If you do carry out such a conversion and discover that this is or is not the case then please do let me know so I can amend this page of the site.
Fitting a Limited Slip Diff
Fitting a limited slip differential gear to your gearbox might sound like a bit of a daunting task and many might argue that it would be easier to simply fit a gearbox from a Series 1 RS Turbo that comes with an LSD already fitted and bolts straight into a mk3 Escort.
However, the Series1 RS Turbo has a relatively low geared final drive which isn't ideal for higher speed motorway cruising etc..
The Series2 RS Turbo had a higher geared final drive for more relaxed high speed driving making it the preferred choice. Unfortunately, as already covered in the compatibility section, being a mk4 car the Series2 RS Turbo's gearbox won't fit into the mk3 Escort.
There are two way around this problem. The first is to simply fit the mk3 gearbox outer casing to the Series2 RS Turbo gearbox allowing you to mount it into the mk3 shell, or remove the required innards from the Series2 RS Turbo's gearbox and fit them into your own mk3 gearbox.
The first option is the easiest option and will work fine as long as the donor gearbox is in good condition. However, many RS Turbo gearboxes have led a hard life and the gearbox is known to be the weak link in the performance Escort.
My original gearbox has been recently reconditioned when bought so I knew it's internals were good. I therefore opted to take the second option, keeping this known good gearbox and transferring the required internal parts to my good gearbox.
This was achieved in the following steps:
From the donor gearbox:
Bare mainshaft with final drive gear (single piece unit)
Gearbox rebuild kit (gaskets, seals, bearings and spring rings)