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  1. #1
    william540i
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    Yes | No

    Rear swap and ASC...

    Good day all!!

    Happy holidays everyone!! I have a weird question for ya's... more of an observation, but I need a little help putting this together in my mind... I had the original 2.81 gears until last month... Whenever I took off fast enough to break traction and kick in the ASC, it would cut the power to such an extent that it seemed overkill to keep the rear tires from spinning... All power would be gone far longer than you would think it should and I was very unhappy with the way the ASC worked when I got on the gas hard in first gear... Okay, a month ago, my son and I installed the extremely heavy 3.15 gears... I noticed immediately that this mod should have been done a long time ago!! I am running noticably higher RPM's in every gear, but the speedo still goes all the way to the speed limiter (guessing as I haven't actually gotten it to 155)...

    I said all that to inquire this: Why does the ASC intervention, when loss of traction, seems more like it should be with the 3.15 gears??? I can be at WOT in first gear and still be pulling away from the line, even with ASC hitting constantly the whole time... It seems like ASC doesn't get in the way of power to the wheels nearly at all compared to when I had the 2.81 gears... The power would be very abrupt, like I slammed on the brakes with the 2.81 gears... With the 3.15 gears, it seems like the ASC takes just enough power from the drive line to keep the wheels from breaking free, but absolutely nothing more... It is VERY balanced and highly desired!! I love this upgrade, but I'm annoyed why I can't figure this out!

    Thanks all!! Have a great holdiay everyone!!!

    Cheers!!!


  2. #2
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    LSD or non LSD?






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  3. #3
    william540i
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    went the cheaper "non-LSD" route...


  4. #4
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    Yes | No

    That's weird because I switced to 3.15 as well,

    but I never experienced what you describe with 2.81. You're right, 3.15 is best bang for the buck.



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  5. #5
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    Yes | No

    transmission shifting. with 2.81 u were in the

    rpm range where the torque converter wasn't locking up. I had the 2.81 and went with 3.15 LSD. Only downside is the shifting between 2nd and 3rd is a bit more confused. The transmission in normal city driving switching back and forth with 2-3 while with the 2.81 it was a sluggish 3rd gear only. This is between the city driving speeds 25-35mph.

    other then that small issue, the 3.15 is the best. I went with the LSD and it makes the car better in every corner.

    E31 840 is the car I'm talking about which is very similar to 540's.

    On a short track I was able to keep up with the E36 M3's and E46 330i's easily.

    The 5spd auto transmission should have always came with the 3.15 rear end and a good ol' LSD as well.



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  6. #6
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    I'll take a wild guess at it (m)

    Lets assume that the tranny is a 5th gear, 1:1 so we don't need to calculate tranny gear ratios.
    If the engine is spinning at 1000 RPM so is the driveshaft, which means the rear axles/wheels are turning at ~355 RPM with a 2.81 diff.

    Now with the same engine speed and a 3.15 diff the DS and wheels are turning at ~317 RPM.

    When the rear wheels break free the ASC reduces power, probably the amount that gets reduced is dependant on the difference of wheel speed between the front and rear wheels.
    With a 2.81 diff, once the rear tire(s) break free they will wind up (go faster) quicker with a 2.81 diff than with a 3.15 diff.

    When the rear wheel break free.....
    At 2000 RPM with a 2.81 diff the wheels will spin at 710 RPM.
    At 2000 RPM with a 3.15 diff the wheels will spin at 634 RPM.

    So I'm guessing the ASC comes into play more with the 2.81 because the difference in wheel speed between the front and rear is greater.
    Does that make any sense?


  7. #7
    william540i
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    I like it!! About what I was thinking too!!

    Hmmm... Sounds logical...

    I'm trying to picture this in my mind and this is what I come up with... Which is about the same thought you have Jim...

    With the 2.81's, the ASC compensation is actually initiated "late" so it has to make a more aggressive adjustment because the wheels are spinning faster than they would be with the 3.15's...

    For example, it takes .07 seconds (pulling numbers out of the air here) for the ASC to notice a difference and engauge... In that time, the wheels manage to get to a difference of 12% with the 2.81's, so the ASC has to adjust more...

    In that same .07 seconds, the difference only gets to 8% with the 3.15's, thus the ASC only has to "blip" to get things back within specs... However, since it is so easy to get to that 8%, the ASC has to "blip" 3 times as much for the same time span...

    And it actually feels like that too... Which would also explain why the momentary loss of power today caused the joker in front of me to actually hit me... I got it in reverse in time, but since I took off backwards so fast, the ASC cut power to such an extent, the truck closed the distance when the ASC engauged... He only cracked my grilles and slightly dented the hood... It could have been seriously worse, but I doubt he would have hit me if I had ASC off! People should really learn how to look backwards before actually moving backwards... Oh well!!

    Thanks for the thoughts all!!

    Have a great one!!

    Cheers!!


  8. #8
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    too much time on your hands!

    Jim,

    You must have nothing else to do tonite. I wasted my time watching my sabres get beat..

    I have one issue with your argument - the front tires are spinning at the same RPM as the rear tires, no matter the diff ratio, until the rears brake loose and activate ASC.

    Since I don't have the 3.15, can't comment directly on the feel with the 2.81. My observation is the 2003 540 ASC/DSC is much less obtrusive then my prior 2000 528iA. I don't know if it's the tires or the stick(or improvement to algorithm), but I very rarely activate the ASC, even in spirited driving.

    With the 528, I got into several situations where ASC kicked-in (cut throttle) and left me hanging in an intersection, while trying to dart across a street. The 540 has never put me in that position.

    For all of us winter tire guys, snow gives plenty of opportunity to test all the ASC/DSC modes, including the infamous "half-mode" where yuo hold the button for 10 secs. I'm not a believer..

    Safe driving, the new plumbing looks fantastic!

    dave
    03 540i6

  9. #9
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    Yes | No

    Yes I do, co-workers flight was cancelled (m

    so I'm just hanging at the hotel waiting and watching TV.
    It's friggin snowing, but doesn't seem to be accumlating. Glad I have my car cover.
    Yes, the front and rear tires are turning the same until the rears start to break loose, that's when the ASC starts to function.
    I have driven a few I6's, but never to the point of activating the ASC/DSC. Didn't all of the pre-drive by wire I6's have a secondary throttle that the ASC controlled to cut air to the engine?
    A friend that had an Dinan M-Z3 removed this sec throttle to prevent it from interfering with his "fun".
    I don't think mine has the "1/2 mode", it's either on or off. Holding the button for 10 sec didn't seem to make any difference.

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