Differential Upgrades

[Mikey_Boy]

Hi Folks,

Did a quick search on this but not too much came up - I'm looking at either a diff rebuild (as my car is about that mileage and I've noticed a clonk or 2 recently) or something to make the diff better.

Any diff experts out there with a view on this? Anyone known in the UK as a diff Guru?

The car is a road car used on track occasionally - I have uprated diffs on my other cars in the past to *really* good effect so keen to explore the options!

Thanks!

[Beedub]

research o.s Giken

[TomK]

I think most that upgrade from M diff go to drexler, for a road car though I'd just get yours rebuilt, perhaps with a lower final drive ratio 3.91?
https://eng.drexler-automotive.com/moto ... ferentials

[Mikey_Boy]

Thank you gents - just wondering how tough the existing diff is? Happy to get the standard item rebuilt, but wondering what the durability is like after a few trackdays? Am I right in stating it's a viscous unit as standard....? if it's tough enough then that's fine, but reading that potentially the standard item needs a serious look after 80k miles doesn't fill me with confidence...!

But... I ran a carbonetic diff in another car and that was pretty awesome both for road and (especially) on track so I'm happy with something a little more aggressive. I haven't had any luck find that diff for a Z4M application, but happy to have a clutch plate diff.

TomK, I'm guessing you are running a plate diff in yours?

[Fishy Dave]

In my experience the standard M diff is tough, it coped with a days drifting (admittedly on a wet surface) and 8 or 9 trackdays with no grumbles.
I found an explanation on the way it works here, as the E46 M3 diff is similar: https://forums.m3cutters.co.uk/showthread.php?t=71424

[TomK]

TomK, I'm guessing you are running a plate diff in yours?

just the standard one, perfectly happy with it tbh and more than tough enough for my needs so far.
If it ever needs fixing I will most likely get it rebuilt with a 3.91 cwp.
The e36 m3 evo diff is worth a look as it fits in the e46 casing we have and is fully mechanical.
From another forum...
The E36 M3 Evo LSD is a fully mechanical ramped plated diff, more torque applied to the nose of the diff more the drive is forced up the ramps and harder the diff locks, so you have a static preload, and the diff gets more progressively locked as more torque is applied to the nose.

The E46 M3 LSD is still a mechanical plated diff, but with no ramps, instead it has a higher static preload which is around 110ft.lb and the viscous unit is there to keep it from running away from itself, it kinda acts like progressive but when it gets too hot it just fails, its a shear pump and the plunger pushes on the clutch pack, in the same way a ramp set up would work (kinda).

The reason they use the Viscous unit is because when a ramped diff has no nose pressure (torque) applied the diff rely on the static preload, so if you lift a rear wheel over a racing curb the diff will open up and you can get what we call in-lift, where the wheel spins away before it gains traction and locks the diff back up.. The Viscous regardless if one wheel is in the air gives a constant load and reduces in-lift, stopping the car acting funny over curbs where no nose torque is applied from engine, it is more progressive then a ramped diff. Think of it as having constant drag.

They do swap over but be warned anyone that just says toss it in has no idea what they are talking about, it takes allot of effort and many shims to get right.

Make sure someone that knows what they are doing is building it for you or you'll melt the diff.

Check static preload on the diff before you even buy it to make sure its a good one, otherwise you'll be into more cost getting a plate kit.

[exdos]

I really cannot understand why anyone would want to swap the Z4M's Variable M Differential for something different. Just watch this video comparison in action with an open diff and a Limited-slip differential from about 2:02 into the video to the end, and see what you'd get from a downgrade. https://www.youtube.com/watch?v=CIH0V8pg8jk

[TomK]

I really cannot understand why anyone would want to swap the Z4M's Variable M Differential for something different. Just watch this video comparison in action with an open diff and a Limited-slip differential from about 2:02 into the video to the end, and see what you'd get from a downgrade. https://www.youtube.com/watch?v=CIH0V8pg8jk

No one here is talking about fitting an open diff or downgrading
The "Variable M Differential" is not the holy grail of diffs you infer, depending on what you are using the car for of course. All round though it's a pretty effective solution in most circumstances.

[exdos]

Serious question: so what improvements are realistically available over the OEM Variable M Differential for a Z4M other than perhaps adjusting the ratio (if that could be considered as an improvement)?

[2skiddy]

If you want a plated only diff, it might be worth talking to Geesport, they built me a copy Motorsport item for my Z4GT from a mixture of E36M3 and E46 M3 diffs, and made a lovely job. I am not saying this is what you require exactly, but you could give them a call for advice. They installed a higher ratio MS C&P at the same time.

[TomK]

Serious question: so what improvements are realistically available over the OEM Variable M Differential for a Z4M other than perhaps adjusting the ratio (if that could be considered as an improvement)?

The main reason people usually change from the variable diff is because of the slowness of the viscous mechanism to lock and unlock. It's nice and smooth but a proper torque sensing clutch type mechanism will be able to lock/unlock far quicker and therefore have a more immediate connection to the throttle pedal.

[Mikey_Boy]

Thanks for all the help! This is why forums like ours are soooooo good!

My reasoning/conclusions so far (in no particular order) are as follows:

1. A well set up plate diff is really a thing of utter joy, but can be a real pain to get right.
2. Viscous diffs haven't been kind to me in the past (Escort Cosworths mainly).
3. I want to be able to use it on track and road with confidence that it isn't going to wither and give up (controversial but M cars have many weak spots!! ) - given I like going to places like Spa, durability is important!
4. Some reassurance a rebuilt standard diff could be up to the job (from what I see above, that's a distinct possibility)
5. Understand what options are out there - quite a few it seems, but no definitive 'right' answer.
6. No need to change the final drive ratio - I understand the reasoning for it, but I think for my use the current final drive ratio is spot on.

The internet is a dangerous place of course, but I did read that M diffs need a rebuild around 80k miles? if that's true and given the mileage on my car, then a rebuilt standard diff should be a significant improvement?

As always, I appreciate any views/ideas!

[exdos]

Mikey Boy,

I've done around 500 laps around The Ring in my Z3MC and about 200 laps in my Z4MC and the Variable M diff in the Z4 is a noticeable improvement on the 25% LSD in the Z3 and both have been able to take the abuse with no problems. Is the diff a known weakness in the Z4M requiring preventive maintenance other than oil changes?

[Mikey_Boy]

Thanks Exdos - all I've been doing is reading honestly! (and drawing from my own LSD experiences, good and bad from a variety of cars, both FWD and RWD).

An example of my 'research' here: https://www.zpost.com/forums/showthread ... 888&page=3

Like most internet discussions, there's a pretty equal split of opinion. Also, I'm looking at where I'll be taking the car in future - Spa and N'Ring are both possible, but also places like Oulton, Donington and Anglesey which have very different cornering requirements. Plus road trips requiring a motorway schlep as well as mountain roads/fast sweeping 'A' roads. A 'catch all' solution here is tough.

Is the M Diff a weak spot? Possibly.. But I only form that opinion from what I've read to date - I'd hope that a (fresh) standard diff would be able to provide many years of track and road use with just more regular oil changes than BMW recommend.

All of the above is pushing me towards getting the standard unit refreshed and trying that (given how broad the use for my car is), but I'd hate to miss out on a really good upgrade if it exists...

[mmm-five]

With regard to diff longevity, I’m on my 3rd.

First one was replaced under warranty when the dealer mentioned it was ‘a bit noisy’ at about 78k miles (full BMW MDSH at that point).

My 2nd replacement was when I had my accident, but it wasn’t noisy before and had 70k on it at the time. Independent investigation showed the wheel to kerb/barrier impact pushed the drive shaft into the diff (and bent both RTA) - but the insurance engineer did mention that “these M3 diffs only last 80k miles”.

So I’m not expecting another until well over 200k miles