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E46 Xi Forum
The E46 XI was produced from 01-05 in sedan and touring body styles. Powered by either a 2.5L inline 6 in the 325xi or a 3.0L inline 6 330xi. Discuss all thing about BMW AWD E46 'Xi' here.

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Old 02-02-2011, 06:12 PM   #21
RogueStatus37
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Originally Posted by Kubica View Post
Sell your xi then.

It's an amazing machine for a talented driver, which you most certainly are not with an opinion like that.
The kid couldnt be any more right... A proper AWD (Audi, Subaru, Porsche and so on...) system have a center locking diff, and even better, an electronically controlled torque sensing diff meaning that the car will actually detect slip in one wheel and send power the other wheels. In the XI's, its like having 2 rwd drivetrains stuck together. One wheel will spin forever in the front, and one in the rear will spin forever and the other side of the car wont do anything. It will never send power to the wheels that need it. Its a poorly designed AWD system.

For anyone interested in more info: http://everything2.com/title/Torque+...g+Differential
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Last edited by RogueStatus37; 02-02-2011 at 06:25 PM.
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Old 02-02-2011, 06:52 PM   #22
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^+1 2 open diffs=fail... still beats my rwd though, even though i have lsd...

its about a good driver and good tires (mostly) the newer xi's seem to be pretty decent in the snow, maybe someone should retrofit that into an e46

btw if i ever get an xi it probably will get the 3.07 lsd out of my zhp if/when i upgrade to 3.38 lsd
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Old 02-02-2011, 07:14 PM   #23
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Originally Posted by bigdog2003_99 View Post
^+1 2 open diffs=fail... still beats my rwd though, even though i have lsd...

its about a good driver and good tires (mostly) the newer xi's seem to be pretty decent in the snow, maybe someone should retrofit that into an e46

btw if i ever get an xi it probably will get the 3.07 lsd out of my zhp if/when i upgrade to 3.38 lsd
Well obviously in the front, they have no choice but to make it an open diff. Just think about it, the car could only go straight because the wheels couldnt turn at different speeds. But in the rear, there is no reason they couldnt have put an LSD. Our suburban even has an LSD in the rear The reason the XI is so much better than the coupes is that 2 wheels HAVE to be spinning at all times instead of just 1 wheel with RWD. Plus the front wheel has the weight of the engine directly over it.

I believe the newer AWD BMWs have a torque sensing diff, which is a HUGE improvement to any AWD system.

Just remember your front and rear ratios have to match. It would annihilate your transfer case if you didnt.
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Old 02-02-2011, 07:57 PM   #24
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Well obviously in the front, they have no choice but to make it an open diff. Just think about it, the car could only go straight because the wheels couldnt turn at different speeds.

Uhhh.... not true

the key is limited slip. Not a locker or a spool.
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Old 02-02-2011, 08:25 PM   #25
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Sell your xi then.

It's an amazing machine for a talented driver, which you most certainly are not with an opinion like that.
Sorry I dont swing from BMW nutts.

I love my car, it does handle very well compared to my other vehicals. That doesnt change the fact the awd system is lacking compared to most awd systems. Its better then rwd in snow, but its slow to react and doesnt have the grip that it would if it had a limited slip center and rear.
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Old 02-02-2011, 08:44 PM   #26
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Uhhh.... not true

the key is limited slip. Not a locker or a spool.
That is true. However, lsds get worn out over time (the more use, the quicker they wear) which is why you have to get them rebuilt over time. If you had the diff trying to rotate the outside wheel every single time you turned the wheel even a hair, it would wear out and lose its limited slip feature in a matter of days. Plus it would make turning feel like you had no power steering all the time.
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Old 02-03-2011, 02:22 AM   #27
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Quote:
Originally Posted by RogueStatus37 View Post
That is true. However, lsds get worn out over time (the more use, the quicker they wear) which is why you have to get them rebuilt over time. If you had the diff trying to rotate the outside wheel every single time you turned the wheel even a hair, it would wear out and lose its limited slip feature in a matter of days. Plus it would make turning feel like you had no power steering all the time.
i've got a helical limited slip in the front end of my turbo sentra from the factory. no clutches there, so it is doable.

now, the awd system can be pretty terrible cause of the ops diffs... maybe i should start figuring out how that abs unit and pump work electronically... start working on my own traction control for low speeds.
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Old 02-03-2011, 07:32 AM   #28
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Originally Posted by RogueStatus37 View Post
That is true. However, lsds get worn out over time (the more use, the quicker they wear) which is why you have to get them rebuilt over time. If you had the diff trying to rotate the outside wheel every single time you turned the wheel even a hair, it would wear out and lose its limited slip feature in a matter of days. Plus it would make turning feel like you had no power steering all the time.
Really? Hmmm... lol, I think you need to get a little more experience with limited slip before you make statements like "it would wear out and lose its limited slip feature in a matter of days". Because it's just not true. Plenty of cars and trucks on the road to prove you wrong. And it would do the same thing to the rear as well.
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Old 02-03-2011, 07:41 AM   #29
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...If you had the diff trying to rotate the outside wheel every single time you turned the wheel even a hair, it would wear out and lose its limited slip feature in a matter of days. Plus it would make turning feel like you had no power steering all the time.


completely not true.
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Old 02-03-2011, 09:23 AM   #30
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completely not true.
^That.

The limited slip feature only engages after a certain torque difference. If you're under that, it's an open diff and nothing is getting worn out.
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Old 02-03-2011, 10:10 AM   #31
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^That.

The limited slip feature only engages after a certain torque difference. If you're under that, it's an open diff and nothing is getting worn out.
I didn't know that. That's good to know
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Old 02-03-2011, 11:53 AM   #32
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Originally Posted by scottz View Post
Really? Hmmm... lol, I think you need to get a little more experience with limited slip before you make statements like "it would wear out and lose its limited slip feature in a matter of days". Because it's just not true. Plenty of cars and trucks on the road to prove you wrong. And it would do the same thing to the rear as well.
And it all depends on the type.

The only kind that would need to be rebuilt is a clutch type. Helical and Viscous do not wear out.

And they all have their advantages and disadvantages.

Quote:
Originally Posted by SamDoe1 View Post
^That.

The limited slip feature only engages after a certain torque difference. If you're under that, it's an open diff and nothing is getting worn out.
I have no idea where you got that from.

Basics on some types of lsds:

A Helical limited splip will always attempt the trasnfer torque to the slower turning wheel. If you completely loose traction on a side, then it because a open diff and will just spin the tire that has no resistance.

Clutch type lsd always attempt to maintain the same speed between each wheel. They turn different speeds when the clutch friction is overcome. These can be tuned with different plutch plates and springs, and are sensative to fluid.

Viscous lsd has a ton of thin steel plates with a fluid in it (fluid being a grease type fluid). The plates have a bunch of holes in them. When the plates attempt to rotate against each other the visous fluid resist the movement and makes it hard for the plates to want to turn. A viscous is always resisting any speed difference between wheels, even if one tire is completely off the ground.

Then you have the bmw system. All open diffs. Ours uses the wheel speed sensors to look for speed differences. When it detects a wheel with excess speed compared to the others, it applys the abs system to that wheel which then slows the wheel and shifts the torque to the opposing wheel. Problem with that is you have to have enough speed difference before it detects the lose of traction, there is a very noticable delay in this. You basically have already spun the tire and lost all traction on that wheel before it does anything.

With any of the mechanical LSDs, it is a instanteous reponse, its already reacting before you even know a wheel has a speed difference.
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Old 02-03-2011, 12:43 PM   #33
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And it all depends on the type.

The only kind that would need to be rebuilt is a clutch type. Helical and Viscous do not wear out.

And they all have their advantages and disadvantages.

The point is they don't wear out in a matter of days.
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Old 02-03-2011, 01:53 PM   #34
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Am I wrong ?

I've been reading this thread and I'm wondering if I get it right. I'm driving a 2005 325xi. I got my 2 rear wheels stucked in a mass of snow yesterday and the car wouldn't move. The front wheels were not turning. I thought that the AWD system would have help me but only the rear wheels were running free with no contact on the ground.

If I understood correctly what you are discussing, this is normal ?!
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Old 02-03-2011, 02:11 PM   #35
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From reading about the DSC system, no that shouldn't happen. If DSC was off, yes you could have the front wheels stuck while the rear wheels continued to spin.

In that situation, the DSC system should brake the two rear wheels, forcing the front wheels to spin.
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Old 02-03-2011, 02:40 PM   #36
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And it all depends on the type.

The only kind that would need to be rebuilt is a clutch type. Helical and Viscous do not wear out.

And they all have their advantages and disadvantages.

I have no idea where you got that from.

Basics on some types of lsds:

A Helical limited splip will always attempt the trasnfer torque to the slower turning wheel. If you completely loose traction on a side, then it because a open diff and will just spin the tire that has no resistance.

Clutch type lsd always attempt to maintain the same speed between each wheel. They turn different speeds when the clutch friction is overcome. These can be tuned with different plutch plates and springs, and are sensative to fluid.

Viscous lsd has a ton of thin steel plates with a fluid in it (fluid being a grease type fluid). The plates have a bunch of holes in them. When the plates attempt to rotate against each other the visous fluid resist the movement and makes it hard for the plates to want to turn. A viscous is always resisting any speed difference between wheels, even if one tire is completely off the ground.

Then you have the bmw system. All open diffs. Ours uses the wheel speed sensors to look for speed differences. When it detects a wheel with excess speed compared to the others, it applys the abs system to that wheel which then slows the wheel and shifts the torque to the opposing wheel. Problem with that is you have to have enough speed difference before it detects the lose of traction, there is a very noticable delay in this. You basically have already spun the tire and lost all traction on that wheel before it does anything.

With any of the mechanical LSDs, it is a instanteous reponse, its already reacting before you even know a wheel has a speed difference.
A viscous LSD will wear out eventually. The fluid in the diff will eventually lose it's characteristics and not behave as it should.

What you described is essentially what I said with more information. The point of a limited slip differential is to allow it to work as both an open and a locked differential in one. I'm not saying that it's an instant locked to unlocked changeover, but there is a certain wheel speed/torque bias that will cause the wheels to turn at the same speed. Often times, this occurs gradually although it can be tuned to not occur gradually.

Mechanically, yes the LSD must always resist wheel speed differences. However, when that magical torque/speed bias is reached, clutch/gear/fluid is able to actually resist that difference. Below that torque/speed bias, it is not and will function as an open diff.
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Old 02-03-2011, 03:06 PM   #37
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From reading about the DSC system, no that shouldn't happen. If DSC was off, yes you could have the front wheels stuck while the rear wheels continued to spin.

In that situation, the DSC system should brake the two rear wheels, forcing the front wheels to spin.
I tried both with and without DSC and nothing changed. I realy don't think the front wheels were spinning. I'm not 100% sure since I was IN the car but anyway.... I'll try to check it up in the garage with the car raised and front wheels blocked. I bet rear wheels will spin free.
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Old 02-03-2011, 03:10 PM   #38
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I tried both with and without DSC and nothing changed. I realy don't think the front wheels were spinning. I'm not 100% sure since I was IN the car but anyway.... I'll try to check it up in the garage with the car raised and front wheels blocked. I bet rear wheels will spin free.
Next time that happen just apply the brakes at the same time as you are giving it gas. Should help some.
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Old 02-03-2011, 09:07 PM   #39
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The front diff can go into what I call 'TC preservation mode' when the rear wheels are spun too much.

When this happens, the front driveshaft is spinning but the front axles are not.

I just blew all your minds.
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Old 02-03-2011, 11:34 PM   #40
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Ok....here are the simple observations from the test of the NV124 case on my workbench.

- Spinning input shaft by hand.....both output shafts rotate
- Spinning input shaft by hand and holding rear output shaft stationary.....front output shaft rotates.
- Spinning input shaft by hand and holding front output shaft stationary.....rear output shaft rotates.

Caveat....these tests were done with spinning by hand, which does not simulate rear world forces transmitted into the driveline.

Thus, my conclusion, you can NEVER make an XI into a rear wheel drive by simply disconnecting front driveshaft. It will just send all the power to the front output shaft and spin endlessly without putting power to the rear.
Thanks for the test. This makes me sad.... I did a similar test when the TC was taken out of my car.... except I only spun the front and rear output shafts. When I spun one output shaft, the other shaft spun as well. I didn't think to do other tests because I was just screwing around lol.

Can you plz check to see if your TC does the same? Spin one of the output shafts and see if the other one spins too...... Thanks
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