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Thread: 6 Speed Conversion vs. 3.91 Swap

  1. #1
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    6 Speed Conversion vs. 3.91 Swap

    I know that folks will have a guttural / emotional reaction to this question. And I know there are pros and cons to both. But, I am trying to make a factual decision on which path to go down...

    Now that I have the mechanical issues, maintenance issues, etc. addressed, I have been getting some great driving time in my '91 4 speed auto. Many discussions have been had around the benefits of the 3.91 swap with the 4 speed. I can certainly appreciate the issue we are trying to solve. Although my car is running really well now, it's just not super exciting off the line. This is a fun around town, nice weather car for me, so looking for more sporty performance.

    The other option is going all out with a 6 speed manual swap. 560s are hard to come by, but looks like the 420 is a legitimate option now. I'm looking for feedback from folks who have driven both the auto with the 3.91 and a 6 speed. The cost and complexity of the 6 speed swap is causing hesitation for me, but the pure sport / fun factor of the 6 speed is a HUGE draw for me especially since this is not my daily driver.

    Another factor to consider is that eventually the 4HP24 will need a rebuild, so that adds cost and complexity to staying with the auto set up as well.

    As you can see, I am all over the place waffling back and forth. I even have an ECS cart already filled up with all the parts for the 6 speed swap, but I have also been on the hunt for the 3.91. Let the debate begin!
    MY SPEEDO ON THE 'BAHN!!! - @155!
    '07 e65|N62 - Alpina B7 - Black Sapphire|Black|Loaded
    '02 e46|M54 - Orient Blue|Beige|'Vert
    '00 e39|M52 - Anthrazit Metallic|Grey (Daughter's)
    '01 e38|M73 - Anthrazit Metallic|Schwarz|Shadow-Line
    '91 e31|M70 - Schwarz|Schwarz|M70

    '13 328i Auto - Black Sapphire|Coral Red|Sport Line|GONE
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    '07 335i Auto - Montego Blue|Grey|GONE
    '05 545i SMG - (Euro Delivery 7/22/05)|Black Sapphire|Auburn|GONE
    '97 528i Auto - GONE

  2. #2
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    ...force inducted auto is the Hot set up fwiw! Since your speaking all out, Now that our cars are appreciating, Not a good idea imo! If a manual is needed as far as 850's are concern, Just buy one instead. Again, My opinion. I could of had one for $2,800 a few years ago until I noticed it had sport seats, Made a deal on those and ran!-) Besides, had no where to park it anyway.


    Schrick Cams, Wokke's and a 3:91 is fun too!
    Last edited by 8eights; 09-12-2017 at 02:34 PM.

  3. #3
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    drove a Euro spec CSi briefly, and really like my WOKKE-chipped, 3.91LSD 4 speed auto much better.
    '93 850Ci - Mineralweiss Metallic
    ALPINA B7 -Alpinweiss III


    ...the price of cool ain't cheap!

  4. #4
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    Do the 3.91 - well worth the money and you will not be disappointed.
    Taylor in Cardiff

    P=(p x v3 x Cd x A)/2

  5. #5
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    Quote Originally Posted by TxGR8White View Post
    drove a Euro spec CSi briefly, and really like my WOKKE-chipped, 3.91LSD 4 speed auto much better.
    That is a very interesting comparison. Was it the actual responsiveness? WOKKE chips already in!

    - - - Updated - - -

    Quote Originally Posted by taylorpatterson View Post
    Do the 3.91 - well worth the money and you will not be disappointed.
    Ok, 2 for the 3.91. Now, where is dragon... and wonder if he still has one.
    MY SPEEDO ON THE 'BAHN!!! - @155!
    '07 e65|N62 - Alpina B7 - Black Sapphire|Black|Loaded
    '02 e46|M54 - Orient Blue|Beige|'Vert
    '00 e39|M52 - Anthrazit Metallic|Grey (Daughter's)
    '01 e38|M73 - Anthrazit Metallic|Schwarz|Shadow-Line
    '91 e31|M70 - Schwarz|Schwarz|M70

    '13 328i Auto - Black Sapphire|Coral Red|Sport Line|GONE
    '05 545i SMG - Jet Black|Black|Bamboo Anthracite|GONE
    '07 335i Auto - Montego Blue|Grey|GONE
    '05 545i SMG - (Euro Delivery 7/22/05)|Black Sapphire|Auburn|GONE
    '97 528i Auto - GONE

  6. #6
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    Quote Originally Posted by TxGR8White View Post
    drove a Euro spec CSi briefly, and really like my WOKKE-chipped, 3.91LSD 4 speed auto much better.
    Quote Originally Posted by taylorpatterson View Post
    Do the 3.91 - well worth the money and you will not be disappointed.
    What they said!
    1993 850Ci.....159,600 miles and counting!

  7. #7
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    '92 BMW 850i 6 speed
    Since this is not your daily driver, you have to ask yourself what you really want out of your car. My wife and I have automatics for our daily drivers so my toy car has always had 3-pedals just because I want something different. Unlike shows such as Top Gear, I don't get too caught up in HP numbers, 0-60 times, or how the car drifts since I don't drive my cars that way. I prefer to find twisty roads and row through the gears. But that's just me.

    Sean

  8. #8
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    Quote Originally Posted by adwebinc View Post
    That is a very interesting comparison. Was it the actual responsiveness? WOKKE chips.
    I had a Z06 6-speed at the time (that's the car you want in a manual), the CSI is just too heavy for the horsepower - it should be closer to 500 if you really want to rip it. The WOKKE chips really wake the auto up. Although people will say that you want the manual because it's rare, I say it isn't that much better to make me want it - the E31 is already "rare" in the world of cars. Something else to consider is that the manuals are running out of spare parts - making it difficult when it comes time to rebuild.
    '93 850Ci - Mineralweiss Metallic
    ALPINA B7 -Alpinweiss III


    ...the price of cool ain't cheap!

  9. #9
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    The automatic transmission in my 97 is great. I bought it to be a cruiser but when I want to play the manual shifting mode is quite responsive. I wouldn't compromise the originality of your car to do the conversion. You'll be investing at least $8,000 and god only knows how many hours. One 6 speed conversion on eBay looked like a very nice car but it sold for less than the conversion cost the seller- sub $8,000. I was shocked. Many buyers apparently don't appreciate the conversion. If you're going to do it, definitely don't do it for resale value...

    Also, I know a guy who has a 3.91 available. Check the Parts for Sale page on this forum. Viadotas just bumped his thread...

    Edit: He only has a 3.45 remaining. My bad...

  10. #10
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    Quote Originally Posted by adwebinc View Post
    That is a very interesting comparison. Was it the actual responsiveness? WOKKE chips already in!

    - - - Updated - - -


    Ok, 2 for the 3.91. Now, where is dragon... and wonder if he still has one.
    Here I am, and indeed I do!
    Last edited by dragon850; 09-12-2017 at 11:29 PM.

  11. #11
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    IMO biggest buck for the bang is a differential for sure, with the 3.91 LSD and chips slam it into S3, turn of the traction and have some fun! Or just cruise easy in D.

    Don't understand how Vaidotas still has that 3.45 lsd remaining, that's and absolute perfect ratio for 840s and M73 850s.

  12. #12
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    Quote Originally Posted by dragon850 View Post
    Here I am, and indeed I do!
    I thought you might! You want to PM me price?
    MY SPEEDO ON THE 'BAHN!!! - @155!
    '07 e65|N62 - Alpina B7 - Black Sapphire|Black|Loaded
    '02 e46|M54 - Orient Blue|Beige|'Vert
    '00 e39|M52 - Anthrazit Metallic|Grey (Daughter's)
    '01 e38|M73 - Anthrazit Metallic|Schwarz|Shadow-Line
    '91 e31|M70 - Schwarz|Schwarz|M70

    '13 328i Auto - Black Sapphire|Coral Red|Sport Line|GONE
    '05 545i SMG - Jet Black|Black|Bamboo Anthracite|GONE
    '07 335i Auto - Montego Blue|Grey|GONE
    '05 545i SMG - (Euro Delivery 7/22/05)|Black Sapphire|Auburn|GONE
    '97 528i Auto - GONE

  13. #13
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    I would never go back to auto. The car is a blast to drive now and not to mention when I took it to Romania.
    That was something

  14. #14
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    Quote Originally Posted by dragon850 View Post

    Don't understand how Vaidotas still has that 3.45 lsd remaining, that's and absolute perfect ratio for 840s and M73 850s.
    Any point to bumping my 3.15 to a 3.45?

    4.4l M62, 6 speed Getrag 420g. Just for lower speed tooling around town... But I assume still a plenty reasonable 6th gear RPM at highway speeds?
    2001 BMW E38 740i Sport (Sahara Beige Metallic)
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    1994 BMW E31 840Ci 6spd (AVUS Blue Metallic)
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  15. #15
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    Quote Originally Posted by Chamberlin View Post
    Any point to bumping my 3.15 to a 3.45?

    4.4l M62, 6 speed Getrag 420g. Just for lower speed tooling around town... But I assume still a plenty reasonable 6th gear RPM at highway speeds?
    Yes, I could do it, fully reseal and change all bearings, it only make sense when you have it apart.
    That would a sweet upgrade.

  16. #16
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    Well, I can certainly see why you would want a manual given that you already have two vehicles that have an SMG! However, I agree with the others that the diff swap would be the way to go IMO.
    1994 850CSi, 2007 M5, 1990 VW Westy Syncro

  17. #17
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    How can you verify you have a 3.91 diff? My friend has a 91 850...
    155,000 Miles

  18. #18
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    Quote Originally Posted by dragon850 View Post

    Yes, I could do it, fully reseal and change all bearings, it only make sense when you have it apart.
    That would a sweet upgrade.
    Thanks for the input Dragon....
    2001 BMW E38 740i Sport (Sahara Beige Metallic)
    2000 BMW E38 740i Sport (Sahara Beige Metallic)
    1994 BMW E31 840Ci 6spd (AVUS Blue Metallic)
    2000 BMW E38 740i Sport (Titanium Silver Metallic)
    2007 BMW E91 328iT 6spd (Deep Green Metallic)
    2000 BMW E36/7 ///M Roadster (Oxford Green Metallic II)
    1999 BMW E36 ///M3 Coupe 5spd (Fern Green Metallic)
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  19. #19
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    Quote Originally Posted by purplecty View Post
    How can you verify you have a 3.91 diff? My friend has a 91 850...
    Count the turns, input shaft vs output

  20. #20
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    If you can find the 3.91, it will put a smile on your face. I myself was indulgent, and went to Jim Blanton for a custom LSD differential with BMW Motorsport 4.10 gears. Puts a big smile on my face every time I drive my 93 M70 4 speed auto car. Being in CA. and all the traffic which one can get caught up in once too often, a manual transmission would just be a pain. Also parts are getting very hard to come bye for the manual.

  21. #21
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    Quote Originally Posted by Chamberlin View Post

    Any point to bumping my 3.15 to a 3.45?

    4.4l M62, 6 speed Getrag 420g. Just for lower speed tooling around town... But I assume still a plenty reasonable 6th gear RPM at highway speeds?
    I think the 3:45 w/6 speed would make first gear useless and make fwy cruising run rpm higher lowering fuel economy.
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  22. #22
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    Quote Originally Posted by purplecty View Post
    How can you verify you have a 3.91 diff? My friend has a 91 850...
    About 2400rpm when driving 62mph

  23. #23
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    Quote Originally Posted by IJustDrive View Post
    I think the 3:45 w/6 speed would make first gear useless and make fwy cruising run rpm higher lowering fuel economy.
    Kinda what I figured, and probably a good chunk of cash all said and done for very little difference.... I should probably just drive the 3.15 some more too, as I barely get a chance to explore the car as is yet!
    2001 BMW E38 740i Sport (Sahara Beige Metallic)
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  24. #24
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    Quote Originally Posted by IJustDrive View Post
    I think the 3:45 w/6 speed would make first gear useless and make fwy cruising run rpm higher lowering fuel economy.
    I am inclined to agree. Definitely don't try both -- the 6-speed with the 3.91 -- which is what I believe is in my car. First is completely pointless, and highway revs are unnecessarily high with our 75mph speed limits / 80mph travel speeds. I would actually love to go back down to a 3.15 or so, as this is just a cruiser to me. I've made this comment before and one or two guys (dragon, maybe?) sent me a PM about swapping parts. With two little kids at home, the issue is just finding time to get under the car and pull the old diff out. I haven't done nearly as much work on this car as I had intended, so it's still just sitting out in the shop -- I believe I even let the inspection on it lapse last month.

    Cheers, John

  25. #25
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    The largest I would go with a manual would be a 3:23 It drives very nice!

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