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      05-03-2014, 11:00 AM   #1
///Mangler
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LSD slip fluid?

So I bought a used CPO 2010 E90 DCT and in Review of the dealer service history it shows where the previous owner brought the car in due to noise from the LSD. The service manager showed me where they changed the fluid with some that removed the noise but also removes the ability for the LSD to do its job. Anyone ever hear of diff fluid that reduces or removes the LSD to do its job. I thought the car felt really weird coming out of slower corners at vir.
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      05-03-2014, 11:16 AM   #2
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They probably put in the diff fluid with friction modifier. If it causes the diff to not do it's job, then we'd be seeing more cars in shops for bad differentials.
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      05-03-2014, 02:26 PM   #3
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How many people who take their car to the track are going to have that fluid in the car to even notice? Is there such a fluid that BMW would put in the diff that would change the way the LSD is supposed to work?
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      05-03-2014, 04:08 PM   #4
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LSD noise is, by definition, from the locking action. Friction modifier actually decreases the locking and thereby quiets things down. So technically yes, FM limits the LSD's ability to do its job, but this doesn't really increase or decrease wear in a significant way. FM'd differential fluid has been around for decades.

The FM is added to suit the owner's preference. Some people don't mind the noise.

When my car is due for a fluid change, I plan to try it without the FM and see how loud it is.
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      05-03-2014, 04:44 PM   #5
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So how much does it reduce the LSD's ability?
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      05-03-2014, 08:19 PM   #6
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Quote:
Originally Posted by ///Mangler View Post
So how much does it reduce the LSD's ability?

I don't know if there's an easy way to measure it. I don't think it's going to be anything substantial. Pretty much every M3 on the road has the FM in it now and they seem to have good handling.
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      05-05-2014, 03:25 PM   #7
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Quote:
Originally Posted by ///Mangler View Post
Is there such a fluid that BMW would put in the diff that would change the way the LSD is supposed to work?
yes! i wrote a lot about that fluid with fm booster here in the forum. just search my old postings if you are interested. i don't want to write the same things ever and ever again.
in short: just switch to a normal lsd-oil with a little FM but without the FM booster!


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Originally Posted by dparm View Post
I don't know if there's an easy way to measure it. I don't think it's going to be anything substantial. Pretty much every M3 on the road has the FM in it now and they seem to have good handling.
well, if someone does not even recognise that the variable m-diff in general is a slow-performing, indirectly acting unit that makes the car handle ugly, this person will not recognise a difference in an oil that lets the clutches lock and the one that bmw uses if someone complains about the noises.

if someone is into high level car handling and knows about the various types of lsd units out there, he will not only change the oil but change the whole lsd unit immediately! ;-)

see my lsd shootout-thread here...
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      05-05-2014, 05:01 PM   #8
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Hey driftflo, after reading your thread, it looks like you are diff expert.
If i switch my diff fluid to the normal oil without FM booster, will diff performance get back to normal? What is your thought on that?
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      05-06-2014, 01:55 AM   #9
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yes, it will. at least after driving a few miles with the new oil.
i would recommend "castrol syntrax limited slip 75w-140".
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      05-06-2014, 02:34 AM   #10
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Thanks, I've always been a bit doubtful about using the SAF-XJ with BMW added extra FM and have refused to let my dealer use it on my car.
So a little clutch pack groan is ok on tight turns especially when the oil is cold?
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      05-06-2014, 04:14 AM   #11
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Originally Posted by SenorFunkyPants View Post
So a little clutch pack groan is ok on tight turns especially when the oil is cold?
it definitely is!
if it disappears over time, this is a sign of worn out clutches inside the lsd unit! so you should only mind if this groan is gone...
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      05-06-2014, 04:33 AM   #12
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Quote:
Originally Posted by driftflo View Post
it definitely is!
if it disappears over time, this is a sign of worn out clutches inside the lsd unit! so you should only mind if this groan is gone...
Thanks for that. Its interesting that you say that this LSD is "slow"...I have wondered why when you quickly jump on the throttle you can get one wheel spin up and often a bit of wheel hop while other times with more progressive throttle application you can get some decent power slides. Now it makes sense.
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      05-06-2014, 07:46 AM   #13
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Quote:
Originally Posted by SenorFunkyPants View Post
Thanks for that. Its interesting that you say that this LSD is "slow"...I have wondered why when you quickly jump on the throttle you can get one wheel spin up and often a bit of wheel hop while other times with more progressive throttle application you can get some decent power slides. Now it makes sense.

The GKN website has some good technical documents on how it works. It's a speed-sensitive LSD, not torque-sensitive. The locking is based on the speed differential between the two rear wheels.
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      05-06-2014, 09:34 AM   #14
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I had some very loud grinding noise coming from the back when my car was cold and i thought it was my diff. Thats the only reason i switched my diff. fluid to the one with FM booster. But that sound didnt go away. Later, we found the real reason for that noise, unrelated to the rear diff.
Great info in this thread OP
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      05-06-2014, 09:37 PM   #15
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Quote:
Originally Posted by Groundpilot
I had some very loud grinding noise coming from the back when my car was cold and i thought it was my diff. Thats the only reason i switched my diff. fluid to the one with FM booster. But that sound didnt go away. Later, we found the real reason for that noise, unrelated to the rear diff.
Great info in this thread OP
What was the noise?
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      05-06-2014, 11:17 PM   #16
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Quote:
Originally Posted by ec_E92 View Post
What was the noise?
X-pipe was touching the heat shield
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