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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > NA Engine (non-turbo) / Drivetrain / Exhaust Modifications > 3.46 Rear Diff from AT on 6MT



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      08-06-2012, 12:43 PM   #1
RupertPupkin
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3.46 Rear Diff from AT on 6MT

I have a 6MT 330i e90. Seems my rear is 3.15. Searching shiows people attempted to swap pumpkins but no one seems to have a definite answer for our platform. Yes, the 335 has had some swaps and LSD etc. Im looking to see what is necessary to swap a non LSD pumpkin from an Automatic to my car. My goal is a budget boost in low end acceleration. I dont track so LSD is both unnecessary, and out of my price I want to spend.


I have seen them go for $300 on Ebay. Curious if it is a direct swap, or a huge pain, etc.

I could care less if my car can hit 155. 99% of my driving is sub 100 and that is where I want to see the benefit.


Thanks.
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      08-06-2012, 01:04 PM   #2
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Not sure on your question sorry But I can tell you that my auto diff can hit 155 easily, so I'm guessing you won't have to sacrifice top end speed

Also wouldn't the manual one be better? I mean the manual cars tend to be like .3 seconds or so faster to 60
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      08-06-2012, 01:24 PM   #3
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Putting the AT rear diff in the MT allows for even quicker acceleration. It sacrifices top end speed. It all has to do with the gear ratios. Our gears are completely different. Thanks Though!



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Not sure on your question sorry But I can tell you that my auto diff can hit 155 easily, so I'm guessing you won't have to sacrifice top end speed

Also wouldn't the manual one be better? I mean the manual cars tend to be like .3 seconds or so faster to 60
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      08-06-2012, 01:54 PM   #4
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I remember reading two members did this or a similar swap. One was RichP and the other was ksfrogman or something. See if you can find them to PM them for more info.
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      08-06-2012, 01:58 PM   #5
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Great! Thanks for the heads up.


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Originally Posted by jntar View Post
I remember reading two members did this or a similar swap. One was RichP and the other was ksfrogman or something. See if you can find them to PM them for more info.
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      08-06-2012, 08:46 PM   #6
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the diffs work diff in the automatic and manual cars because the ratios in the transmissions are also different
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      08-06-2012, 08:47 PM   #7
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the diffs work diff in the automatic and manual cars because the ratios in the transmissions are also different
in response to virtuous wolf
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      08-06-2012, 11:07 PM   #8
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Oh wow I learnt something new, so I can go over 155 in my auto, but if you put the same diff in a manual it will go slower?
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      08-06-2012, 11:17 PM   #9
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No, the top speed is governed electronically (149mph for ZSP cars). The diff just changes the final drive ratio. All that means is top speed in each gear will be slower but acceleration faster. You reach the vehicle's ultimate top speed before top gear in either case. Also the rpm for a given speed will higher in all gears. So cruising at 65mph will be around 3300rpm (I think) vs 2700rpm with the standard diff for 6mt cars. Not sure where the auto cruises at.
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      08-07-2012, 02:13 AM   #10
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Quote:
Originally Posted by e90pilot View Post
No, the top speed is governed electronically (149mph for ZSP cars). The diff just changes the final drive ratio. All that means is top speed in each gear will be slower but acceleration faster. You reach the vehicle's ultimate top speed before top gear in either case. Also the rpm for a given speed will higher in all gears. So cruising at 65mph will be around 3300rpm (I think) vs 2700rpm with the standard diff for 6mt cars. Not sure where the auto cruises at.
great to know, thanks!

So sounds like for fuel economy, not so great. I would love the faster acceleration too but I drive a lot for work so I value fuel economy.
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      08-07-2012, 09:46 AM   #11
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With zsp the limit is 155, in fact it goes past 155 because of "error" and goes faster than that if you delimit it.
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      08-07-2012, 12:39 PM   #12
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I did the swap and I like it. I used 335i diff and I had to use 335i driveshaft because of the different coupling. At first in top gear BC showed to shift up. After a while it adopted and it doesnít do it anymore. My gas mileage is slightly worst.
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      08-07-2012, 12:45 PM   #13
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Thanks for the info! What did it end up costing you all said and done? Worth it for the improved acceleration?

What ratio did you end up with from the 335?






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I did the swap and I like it. I used 335i diff and I had to use 335i driveshaft because of the different coupling. At first in top gear BC showed to shift up. After a while it adopted and it doesn’t do it anymore. My gas mileage is slightly worst.
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      08-07-2012, 02:43 PM   #14
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The ratio is 3.46 and the acceleration is better. I didnít do any measurement before and after. I got the diff and driveshaft from a wrecker in Germany. I think I paid less than $500.
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      08-07-2012, 04:45 PM   #15
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Very cool Thank you!




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Originally Posted by BudVlad View Post
The ratio is 3.46 and the acceleration is better. I didnít do any measurement before and after. I got the diff and driveshaft from a wrecker in Germany. I think I paid less than $500.
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      08-08-2012, 08:20 PM   #16
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The lowest ratio is from a 328xi (E90) auto @ 3.91. The whole damn thing can be had for about $500.

Going from 3.15 to 3.91 will be a significant kick down low, BUT just remember that the final drive and the gear ratios are fine tuned, so there may be gear or two that seem to be oddly matched.
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      08-10-2012, 12:22 PM   #17
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bump
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      08-11-2012, 01:00 AM   #18
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Thanks for starting this thread.

Changing the diff ratio is the best way to increase performance of our NA E9x, yet there aren't many threads on it.

We can significant increase performance without hurting reliability or increasing complexity. The whole drivetrain runs exactly as it was designed to except the diff sends more torque to the drive wheels.

The only downsides I can think of are cost, worse gas mileage and more engine noise while cruising on the highway.

I look forward to hearing from more enthusiasts that have done it.
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      08-16-2012, 04:23 PM   #19
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If you want to really make the car dance and get rid of the one wheel peel, check out Performance Gearing. For $2500 you can swap your oem unit with an LSD. I really like the 3.64 in mine. Plus the LSD helps with hallucinations... I mean makes autocross and driving in the snow a lot more fun
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      08-16-2012, 07:48 PM   #20
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Are you 6MT? That ratio seems wild. What rpm in 6th 80?


Edit: Just found your thread. Seems like maybe 3.64 is what I want....hmmmm


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If you want to really make the car dance and get rid of the one wheel peel, check out Performance Gearing. For $2500 you can swap your oem unit with an LSD. I really like the 3.64 in mine. Plus the LSD helps with hallucinations... I mean makes autocross and driving in the snow a lot more fun

Last edited by RupertPupkin; 08-16-2012 at 08:34 PM.
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      08-16-2012, 11:13 PM   #21
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I'm not 6mt but my car is

It's pretty sweet, why say no when it feels so good to say yes?
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      08-16-2012, 11:32 PM   #22
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Haha cool. Sounds like a good diff to get. Appreciate the feedback.


My mind is telling me no.....but my body.....my body is telling me yeeeessss




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I'm not 6mt but my car is

It's pretty sweet, why say no when it feels so good to say yes?
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