F80 3.46 LSD

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doublespaces

Administrator
Oct 18, 2016
8,009
AZ
A thought that came to my mind recently is the momentum the driveshaft has when shifting and how the momentum and the stresses exerted differ between differential ratios. I've read (on multiple occasions) the stock diff's input shaft snaps with the DCT cars even at lower power levels, due to the 'shock' supposedly. The DCT ratio is 2.56 for N54 vehicles, so that means the drive shaft is spinning faster at 7000 engine RPM than it would be on the same car with a 3.15 ratio diff, right? I realize transmission gearing ratios affect the drive shaft speed at a specific ground speed when comparing actual cars themselves but I have not worked that out yet.

So in a theoretical scenario with a driveshaft spinning faster, carrying a higher momentum, what does that do to the amount of force being applied to the input shaft compared to the same car with a 3.15 or 3.46 diff ratio? Perhaps I'm stating the obvious, however the F80 comes with a 3.46 rear end(Gear ratios are also identical to 6AT) and a carbon drive shaft. They are making what... over 1000 and nobody has broken a differential? The structural design looks the same as the ones paired with N54 cars, they have an LSD built in and cost peanuts:
30557


I would really like to get the gears out of an F80 to swap into my DCT and keep my 3.46 MFactory and see what happens.
 
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Bnks334

Sergeant
Dec 1, 2016
402
New York
With a driveshaft spinning faster, carrying a higher momentum, what does that do to the amount of force being applied to the input shaft compared to the same car with a 3.15 or 3.46 diff ratio? Perhaps I'm stating the obvious, however the F80 comes with a 3.46 rear end(Gear ratios are also identical to 6AT) and a carbon drive shaft. They are making what... over 1000 and nobody has broken a differential? The structural design looks the same as the ones paired with N54 cars, they have an LSD built in and cost peanuts:

I would really like to get the gears out of an F80 to swap into my DCT and keep my 3.46 MFactory and see what happens.
I was looking into that diff recently. Better than the M3 viscolok. Problem is I couldn't verify fitment. Looks like physically it would fit but is there a hurdle electronically? Doesn't it operate off input from the DME and transmission?
 
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doublespaces

Administrator
Oct 18, 2016
8,009
AZ
Made a new thread.


Based on this, a varying amount of current changes between open and gradually becomes a full lock by allowing the discs inside to slip.
30558
 

aus335iguy

Captain
Nov 18, 2017
1,036
It’s been said on the internet that the f80 diff does fit but because of the electrical adaptation required it’s not common. A diff that was open in normal mode and fully locked when in sport would be effing cool.
 
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