Final Drive identify?? Middle Diff broke??
 

Hi guys

I just wanted to rebuilt my rear diff since the last owner kind of ****ed up the ratios.

It is a MY03 STi box with DCCD.
Original FD was 3.9.

Now inside was a pinion with 9 teeth and a crown with 40 teeth.
This should come to a 4.444 FD, am I right??

I do have a second diff with a 10 teeth pinion.
What FD will this equal to?



Then I do think I have a problem with the DCCD system.
If one wheel is in the air I get no power on the ground.
Is this due to malfunction of the DCCD or could it also be a malfunction of just the rear LSD?


Help is hugely appreciated.

Thanks

The drive to both rear wheels has nothing to do with the dccd. It controls the drive gien to the prop shaft, so good news!

The issue is the rear diff. You must have the right one for the box of course.

Seems you need a 3.9.

You have the alloy cover off the diff as you counted the teeth, however, look on the side face of the crown wheel and you will see 2 numbers. This is the diff ration, divide the last by the first.

Unless that arithmetic = 3.9 you have the wrong diff.

To the lsd element, and loss of drive with one rear wheel in the air:

The diff you have might be a viscouse type, or even an open type.

Easy way to tell:

Does one of your drive shafts have a splined extension to it compared to the other, or are both shafts the same length where they enter the diff?

If one is about 20mm longer (o'side IIRC) then the car should have a viscous diff fitted. Now look into the centre of the diff with the cover off and you should see a 'module' about 40mm long and 60mm dia where you would expect to find two simple/normal planet gears. If all you can see are 2 planet gears, then the diff is open and not correct. Will work, but you will loose drive as you describe. To a limited degree, the viscous can also give similar grip loss, they are not very good!

Thanks mate, you are always great help. :notworthy

I currently do not have the two diffs here. They are at a friends shop.
However they have the following tooth numbers:
Nr. 1 has 10 pinion, 40 ring
Nr. 2 has 9 pinion, 40 ring

Nr. 1 is a 4.444
Nr. 2 is a 4.0

Is this correct?
So I would probably need a ring which has 39 tooth?


Concerning the power loss with one wheel in the air.
From looking at the LSD's we took out both are proper LSD, no planet, no viscous. From outside they look just like the Cusco RS I just bought for my Ae86. So I guess they are the original suretrac LSDs which supposed to be in there.

So what could cause this loss of power?
It happens when one wheel looses ground contact. Then you get no more power on the ground, feels like the clutch is slipping completely.

Maybe the wrong ratio from rear to front messed up the middle diff (Dccd)?

Driving the car with the wrong rear diff ratio is going to lead to tears!

To my knowledge, there is no rear diff ratio of 4.
4.111/ 4.444/ 3.9 but never read about a '4'.

The issue with the drive could be they need a striop and re-set? The best place I know to check and fix are Welcome to the API website who did my plated 4:444 R160. Not expensive either.

Without sounding silly, BUT, are you sure the centre diff is not broken? You could have a '2 WD' Impreza and the rear is doing nothing because no drive is going to the rear.

I did not twig that the car had been driven much with the wrong diff.

The FD of 4 was just by calculation of 40/10.
Sounded odd to me too.

We checked with the car on the lift.
We turned one front wheel and the prop was turning too. Other front wheel and same result. SO the middle diff can not be completely done.

API would be a good choice.
Is a little far for me though from Switzerland.

So could the power loss on the wheels result through a bad rear LSD?

If the lsd is not functioning then it would be lost if a wheel was in the air while trying to get the power down.

You could talk to API and simply buy a correct ratio diff with a known working lsd.(of the type you want.)
David there has a rack of diffs.
My rare 4:444 plated diff, checked and re-set for competition was £350.

It is so good a condition that there was no rust on the diff whatsoever.
Most of their parts are from Japan direct.

This way you can get what you need and fit and forget it.

I will have a chat with the guys at API.

So you think that if one rear wheel is in the air and the rear LSD is done then also on the front wheels will be no power?

does this help?
The SPDA - Transmission Chart

I already have this.
And it only helps to a certain point.
But thanks anyway.

I still can not figure out what a FD it is with 40 tooth ring and 10 tooth pinion.

And still do not know if only the rear LSD is gone.

But all your help is hugely appreciated, seriously.
Thanks

If the rear diff is done for and a rear wheel lifts, then power will be lost in the rear to the furiously spinning lifted wheel.
If the two fronts are on the ground then drive is totally present.

When you can get to the diffs, it will be good to know the engraved numbers on the side face of the crown wheel, but if those are 10:40 then that is a ratio unknown to the Impreza world I think. So:
the diff is not an Impreza one (Subaru share Nissan parts a lot) or dare I say it, it is not 10:40.

Looking at the gear box list above, have you checked the box number against the chart? I presume you have and you believe the transmission is as specified.

Yes I checked the gearbox number and it is a MY03 STi JDM with DCCD.
The box should be standard with no mods.

Thing with having one rear wheel in the air is that there is also no power on the front wheels.
So middle Diff gone too?

So the car 'stops' driving forward when a rear wheel is off the ground?

If that is the case then the ceter diff is done for and you up for a re-build. The transmissions I've read, will not tolerate the diff ratio difference for long.

Prob best to change the whole box?

Not quite Graham, if you remove or break a rear drive shaft [ or lift a wheel off the ground ] on a Non Dccd car it will go no-where as the weight of the car is greater than the ability of the 4Kg grip of the viscous to transfer drive to the front diff. Downhill with the wind behind you it might roll on to a bit of movement, but any resistance it will stop.

If you have dccd and put it to full lock then it will drive home on the front diff only.

David APi

I keep on learning David!
:D