Cross bred diff

buddah · Post by **buddah** » Fri Oct 31, 2008 2:50 pm

Howdy folks!
Some more questions from someone who doesn't know what they're talking about

I was just trying to remember which diff I was told to get, that would be the best diff for a cross-bred set up on an 84 MY (ej20t, rs 5speed, 5 stud conversion....)

When I orignially spoke to the guys at perfourmance motorsports in Briz, I think Rob told me to try and get the diff from a legacy rather than from the rs because it had a better ratio or something?
I could be totally wrong, but I definately remember him saying that there was a particular one that was better for this purpose than the stock rs diff.

Does anyone know what I'm talking about, or have any other opinions on which is the best diff for this purpose.

Any thoughts would be most appreciated,

cheers

brumbyrunner · Post by **brumbyrunner** » Fri Oct 31, 2008 3:18 pm

There are many and varried opinions on which is the best diff ratio. Having a Crossbred 5-stud conversion will have no impact on your gearing decisions. Perhaps if you focus on what diff ratio you'd like rather than what models to look for. If you have no clue, tell us what ratio your gearbox is and we'll go from there.

Outback bloke · Post by **Outback bloke** » Fri Oct 31, 2008 3:36 pm

Also, the diff ratio doesn't matter too much as they can be changed easily. What you need to decide is if you want a clutch type diff or a viscous type. Then you need work out how much trouble you want to go to to fit your cv joints.

The easiest way to go is an early RS diff that has the male stubs on it. All you have to do then is swap the ratio if need be. It will be a viscous diff.

You can try and find a mechanical (clutch type) diff from an RX or Vortex which will be the same, although they are harder to find.

You can get a WRX or later Legacy diff in either style centre but they have push in cv joints. You can use these but then you have to modify your shafts to suit. It is doable but not just a simple fit and play like the first option.

As Brumbyrunner said, let us know what box and or ratio you want to use and then the advice can be more precise.

buddah · Post by **buddah** » Fri Oct 31, 2008 6:21 pm

Thanks fellas,

Well the gearbox is from a 91 rs, so the ratio is 3.545.
Not sure exactly what I want, because I don't know enough about the benefits of changing gear ratios.

As for the diff. The previous owner said that he had it reconditioned as it was playing up, and now I here the occasional knock coming from it on take off.

I found a cheap second hand diff from an rs and was wondering if I should just get that. Or wether it might be worth getting something else?

Cheers,

brumbyrunner · Post by **brumbyrunner** » Fri Oct 31, 2008 10:43 pm

Is the knock coming from inside the diff or is the diff just moving around because of worn-out mounting rubbers? MY rear diff mounts were never designed for the sort of torque you're putting them then so the'd be moving around a lot anyway.
You'll find that most people change their diff ratios to compensate for a change in tyre diametre.
I think you're current gearing would suit a 25 or 26" tyre. How acurate is your speedo?
How happy are you with you're acceleration compared to your revs at cruising speed? These are the 2 main things that are affected when changing your gearing.

AndrewT · Post by **AndrewT** » Sat Nov 01, 2008 9:11 am

You can't really change your rear diff ratio unless you are willing to modify the ratio in your gearbox to match (which is a pretty big job) or change your entire gearbox for one with a different ratio. The rear diff ratio has to match the final drive ratio at the rear of your gearbox.

Basically if your rear diff is buggered, your best off getting it repaired/reconditioned by a diff place or swapping it for a good one which is the same ratio.

If your tyres are roughly the same diameter as what a standard RS liberty comes with then you are running the perfect ratio already.

Agreed its well worth checking the rubber mounts first, it's probably the front diff mount bush has flogged out causing the knock.

buddah · Post by **buddah** » Sat Nov 01, 2008 2:01 pm

Well I'm pretty happy with pick up at crusing speed, and I don't think I want to go to all the trouble of changing the gearbox as well.

The speedo I reckon is about 8-9% out, quite a bit?

It has 16" rims with low profile 40's, so I guess that's why it's so far out.

I have had a look under there but i didn't look closely at the diff rubbers, I will check again.
However there seems to be quite a bit of play in the diff when I rotate the drive shaft back an forth. Around 1/6 of a turn of free play before it catches either side, is this ok?
So if I reverse in somewhere and then drop the clutch too quickly (by accident of course), I can hear the knock.
I thought that it might be because the clutch is fully released whilst the drive shaft is in free play and then it catches inside the diff and transfers power through to the wheels. And it's that 'catching' that is making the noise

Does this sound plausible?

I'm not some crazy d*ckhead trying to do awd burnouts, but I don't drive like my grandma

I'm guessing that if that is the problem it will be doing damage and before too long I may blow the diff?

I should probably double check the rubbers again today though.

Thanks again,

buddah · Post by **buddah** » Sun Nov 02, 2008 12:06 pm

Yup, diff rubber is really worn. I'm sure it will help once I replace that.

Oh and I exaggerated about the play, it's more like 1/8th of a turn. Still not sure if that's ok though

YouTube · Post by **El_Freddo** » Fri Nov 07, 2008 10:28 pm

buddah wrote:Oh and I exaggerated about the play, it's more like 1/8th of a turn. Still not sure if that's ok though

For an LSD that sounds fine to me... The knocking/clicking sound you hear on take off can also be the LSD letting slip. I've seen an impreza with front and rear LSD's on a slight turn having the LSD's wound up so tight that the wheel sudders as the LSD clicks (letting slip through)... Sounded nasty but it was a rally car so it was pretty much just short of welded diffs...

Cheers

Bennie