If your going from an open diff to a LSD its a good time to kill two birds....
There should be a metal tag on the aft diff cover, left side. An "S" = LSD, No "S" = Open diff. in both cases always followed by your ratio.
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http://i18.photobucket.com/albums/b1...eese/Diff4.jpg)
I take it your using a calc like this one.
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Gear Ratio Calculator)
It is doable but your only going to drop 200-300 or so rpm. Don't know what rpm your shooting for. An example:
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http://i171.photobucket.com/albums/u.../DSC_0929.jpeg)
This may help in your source search for a LSD with the right gearing.
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http://i714.photobucket.com/albums/w...erentieels.jpg)
A 3.15 LSD on EBay right now....
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BMW 325i E36 3 15 Limited Slip Differential LSD Diff | eBay)
In my search for a reputable diff shop I'm leaning towards:
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https://www.turnermotorsport.com/BMW...aring-upgrades)
On my wishlist as I have an open diff.