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      09-27-2021, 11:52 AM   #89
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so if i floor it with TC fully off, the back will slide around less than with MDM on
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      09-27-2021, 01:07 PM   #90
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The only thing the diff can do is lock-up the axle, it cannot "cut power" from side-to-side.
Thanks for the clarification. I would like to know more about how the diff. works. Looks like from pictures I have seen it has a couple "clutches" that look like a motorcycle's clutch. So, when not locked up, in a straight line, can one side slip more than another (passively) and when TC is off those "clutches tighten things up?

If anybody can point me to something on the tech the diff. uses I would appreciate it.
The F8X e-diff has one multiplate cutch. How much that clutch pack locks-up determines how much the left and right rear axles are locked together. This is all controlled by an electric motor managed by GHAS control unit.

Yes, it could be theoretically possible for the e-diff to be un-locked and have the DSC system individually apply the left and right rear brakes. I however doubt that is the case, since this is how a primitive/basic "active differential" works. The F8X has a diff that can actually lock-up and it is much more efficient from a dynamics stand-point to have the diff locked-up when pulling away in a straight line.

IMO, why folks feel the rear to be "squirrelly" with MDM is mainly because of how the DSC system operates. When DSC detects oversteer, it will apply the front brake on the same side as the slide. Since MDM allows for some wheel spin, when the rear moves to one side, the DSC applies the front brake on that side sending the rear the other way. It applies the left and right front brakes in succession while attempting to keep the car in a straight line. With DSC fully off, the is much less side to side movement. When using full DSC ON, the DSC cuts back on the power more aggressively stopping the rear wheel spin altogether and hence the sliding around.

Screenshots from the F8X technical docs:
Question about GHAS failure… the manual says in the event of failure it's returned to the open position so does this mean the mechanical LSD would work like a typical LSD?
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      09-27-2021, 01:46 PM   #91
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I'm running a BM3 CS+ tune and don't really get any squirm anymore. I had tons of squirm on stock tires (MPSS * spec) with stock tune. I'm running euro MDM/traction (way less than US intervention) with 295 PS4S.

The 285/30/20's have a 9.9" tread width, the 295/30/20's PS4S's I'm running have a 11.1" tread with. It makes such a huge difference it's a joke, this is the minimum size to run if you want to put the power down IMO. BMW I feel wanted to highlight the hooligan-ness. Also, changing to the GTS rear-diff settings makes a tremendous difference, lockup is progressive, rather than instantaneous (again preventing the hooligan-ness, prioritizing putting power down/drivability). Powering out of corners is now linear and extremely controllable even with ample power.

Running this with the standard fronts but in MP4S, I only get rear squirt if I'm at some angle or I have bad traction due to environmental conditions. I get very little squirm full power in a straight line.
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      09-27-2021, 02:18 PM   #92
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Quote:
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Question about GHAS failure… the manual says in the event of failure it's returned to the open position so does this mean the mechanical LSD would work like a typical LSD?
It depends what you mean by LSD. IMO it can NOT operate like a standard mechanical LSD since the only way to lock up the axle is through the electrical stepper motor.
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      09-27-2021, 02:57 PM   #93
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Question about GHAS failure… the manual says in the event of failure it's returned to the open position so does this mean the mechanical LSD would work like a typical LSD?
It depends what you mean by LSD. IMO it can NOT operate like a standard mechanical LSD since the only way to lock up the axle is through the electrical stepper motor.
Okay that's helpful. Thanks
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      09-27-2021, 07:05 PM   #94
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On a related note, has anyone got a code for temp out of the norm from the rear diff temp sensor? I did a rear diff service and that came up and I cannot clear or delete it. It says seek immediate attention, however, there are no check engine lights or any other indicators, other than showing up during a scan!
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      09-28-2021, 08:13 PM   #95
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Quote:
Originally Posted by hellokitty View Post
I have a 2017 so my diff bushing is the old rubber one. It's not cracked, but I was going to go with the turner bracket and get two of the revised poly ones to lock it down.

The only other thing I can think of is toe change on hard acceleration which is why I am going with SPL toe arms.
Pretty sure I still have the original one on my car too and it seems fine. I think it's best you try the software route before you replace anything, BMW seem to have made numerous tweaks to it over the years.
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      09-28-2021, 08:28 PM   #96
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Quote:
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Originally Posted by hellokitty View Post
I have a 2017 so my diff bushing is the old rubber one. It's not cracked, but I was going to go with the turner bracket and get two of the revised poly ones to lock it down.

The only other thing I can think of is toe change on hard acceleration which is why I am going with SPL toe arms.
Pretty sure I still have the original one on my car too and it seems fine. I think it's best you try the software route before you replace anything, BMW seem to have made numerous tweaks to it over the years.
I already have the latest software hehe…

GTS everything with CS EDC

The problem is that I'm running full E85 so all of the flaws are coming out due to the additional power
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      09-28-2021, 10:19 PM   #97
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Quote:
Originally Posted by hellokitty View Post
I already have the latest software hehe…

GTS everything with CS EDC

The problem is that I'm running full E85 so all of the flaws are coming out due to the additional power
Take a look at my post above, try running the 295's if you're on 285's, 305's would be better for your power level. E85 usually has a lot of added timing with a lot of that able to come in strong at low RPM's. Even with larger tires you probably need to taper below 3k.
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      09-28-2021, 11:17 PM   #98
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Quote:
Originally Posted by hellokitty View Post
I already have the latest software hehe…

GTS everything with CS EDC

The problem is that I'm running full E85 so all of the flaws are coming out due to the additional power
Ah yeah that will do it I guess
I'd vote on the poly diff bushing + brace as the next step in that case.

Some G80 spec PS4S tyres would do some good too, if you have the wheels for it (they fit CS wheel sizes, maybe ZCP too?), just for the extra traction. I'm sure those tyres are a big reason for the traction and sharp turn-in that the new model is praised for.
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      09-28-2021, 11:29 PM   #99
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Quote:
Originally Posted by maddmatth View Post
Quote:
Originally Posted by hellokitty View Post
I already have the latest software hehe…

GTS everything with CS EDC

The problem is that I'm running full E85 so all of the flaws are coming out due to the additional power
Ah yeah that will do it I guess
I'd vote on the poly diff bushing + brace as the next step in that case.

Some G80 spec PS4S tyres would do some good too, if you have the wheels for it (they fit CS wheel sizes, maybe ZCP too?), just for the extra traction. I'm sure those tyres are a big reason for the traction and sharp turn-in that the new model is praised for.
Good theory, hoping someone get a chance to try them out soon.

Tyre Reviews seemed to like them:

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      09-29-2021, 07:20 AM   #100
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To the OP, I love the 04 cobra. I'm just curious what you think of the power in your cobra to the m4? Which would win in a race? I don't know if your cobra is modified.. I've been in some amazing kenne bell cobras that were just sick!
I recently bought a '18 m4 zcp and am in the same boat as you still trying to learn how to drive it. I'm tuned on bm3 stage 2 . I'm thinking of reducing torque in 1-2nd gears so I can utilize them more. At 50 mph the car breaks loose the tires on some of the old slicker roads where I live. But I absolutely love this car! Amazes me everyday!
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      10-16-2021, 08:53 PM   #101
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Just picked up a new 2020 M4 Heritage edition stock MPSS. Only time I slip is when it's cold and wet. However I never drove any other M4 -
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      10-16-2021, 09:11 PM   #102
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All I can say is get used to applying a smooth throttle. Get used to how much intervention the car applies to your driving. Once you get used to that try the MDM, M seting.

I wouldn’t not go full non-nanny unless you are really experienced.
I had a Corvette C7-Z06, I damned near recked that car burning the rear tires off on the way to get new tires.
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