Gearbox Operation

Here is a basic explanation of how the gearbox works.  The top picture shows the actual cross section of the gearbox, while the second below is diagrammatic showing the main physical components. I spent several hours sitting down with the manual and a stripped gearbox working this out!

The engine drives the input shaft as per any car.  The gears transfer the torque to the output shaft.  This shaft has a pinion on it which drives the ring gear around the diff assembly.  These two parts determine the final drive ratio of the vehicle,  and as it is before the diff they can be changed independent of any other ratio.

The ring gear is around the centre diff.  This diff splits the torque between front and rear axle.  Think of it purely as a normal diff,  with two output shafts for now.  One of these shafts goes to the front diff,  which acts in the normal matter to split torque between the front two wheels.  The other goes to the rear diff,  just like a rwd car.

That's all easy.  The problem is the packaging.  This is where it gets tricky...

The front diff (driving the front wheels) is inside the centre diff (there is no "shaft" as such going to it, I lied earlier!).  This is impressively designed,  but the diffs are very small.  That doesn't seem to be a problem however.  The front diff is driven at a 1:1 ratio.  One front CV shaft flange plugs straight into the fwd diff,  there is a ~25mm shaft going to the opposite flange (through the centre of everything else).

A hollow shaft leads from the centre diff to the transfer.  The transfer is a ring gear and pinion which changes the angle of the drive through 90 to the driveshaft which leads to the rear diff.  This is (from memory) a 2.933 ratio.  The rear diff is also a 2.933 ratio,  again resulting in a 1:1 final ratio to the rear.  This ratio is identical in all GT-Fours so the gearboxes are interchangeable.

When you have no tyre slip that is basically how the gearbox transmits power.  Now,  what's that viscous diff thing?

Well,  it is a viscous coupling,  not a viscous diff.  It is a pack of plates with viscous oil between them.  Half the plates are connected (effectively) between the front and centre diffs,  and the other half are between the centre and rear diff.  In reality this is a toilet roll sized can within the transfer.  If you get wheel spin (assume both wheels one end) these plates turn relative to each other.  The drag of this oil limits the differential speed,  and hence limits the power that can be transmitted to the end with no traction.  By preventing them spinning the centre diff transmits more power to the opposite end.

You can uncouple this which gives an open diff.  This will make it the same as the stock 86-87 GT-Four,  however that gearbox probably was strengthened where necessary to cope (splines etc).

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