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Is a 3.91 diff actually desirable in a 6MT?
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05-12-2021, 09:48 PM | #1 |
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Is a 3.91 diff actually desirable in a 6MT?
I recently built a little model in Excel to help me diagnose what I think might be a driveshaft vibration, which naturally led me to comparing engine RPM at various road speeds for the 6MT vs. AT. I don't know why, but I was shocked at how big of a difference there is!
I had never given it much thought before, but it's now obvious to me why BMW felt it necessary to have four final drive ratios for the N52 E90s. There's a solid 25% difference in engine speed in sixth between the AT and 6MT. I've seen a number of posts where putting a 3.91 diff in is described as an upgrade, but seeing what kind of engine speeds I'm going to have at cruising speeds has got me a little concerned. For anyone running the 3.91 with the 6MT: How have you found it? Worthwhile? Easy enough to live with? I don't know why I'm asking considering I just installed a near-new 3.91 rear diff and have a replacement 3.91 front diff sitting on my bench waiting to go in, but hey... more information is always good, right?
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05-12-2021, 10:03 PM | #2 |
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My 330i 6MT came stock with 3.15 diff. I upgraded to 3.64 from a RWD automatic and I feel that's about perfect for the MT. It's a few hundred RPS higher on the highway, not buzzy or annoying, it feels like it can easily pass most traffic without downshifting and the torque multiplication in the low gears feels like extra HP, which it is not just some borrowed against your top speed. IDK how the 3.91 would be, a few more revs over the 3.64. What kind of driving do you do? Unless you're doing a lot of high speed highway cruising where it might feel a bit frantic after a while I think it'll be fine.
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05-12-2021, 10:39 PM | #3 | |
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Sorry—what are you saying about torque multiplication in low gears not being something against your top speed? Didn't quite follow that. The gap between the AT and MT ratios averages 7% from first to fourth, but then climbs to 18% in fifth and 27%(!) in sixth. Any idea why this is? Is there some logical explanation for this?
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05-12-2021, 11:07 PM | #4 |
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It'll be great for city driving and 70 isn't really "fast" I think it'll be fine. Like you said you'll know soon enough.
I meant the lower ratio diff delivers more torque in the lower gears, but you sacrifice top ultimate speed for it. Since I'll probably never go 150MPH it's a good tradeoff for me. |
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05-13-2021, 09:36 AM | #5 |
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I wouldn't want a 3.91 in my 330i, to be honest. I have 4.10s in the E30 - that's enough short gearing in my life.
With the 3-stage, the low end torque is good enough that I never felt like I needed shorter gears, even in city driving, but some people like it, there's nothing wrong with that. |
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05-13-2021, 10:43 AM | #6 | |
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But, torque multiplication via gear reduction has the impact of reducing the output speed, hence the name gear "reduction." A shorter final drive will reduce your top speed in theory - I say in theory because it's probably something like 10-20 mph taken off the top end which is probably above 150 mph, so you never really feel that effect unless you're on a straight at the track or risking some serious legal consequences on the road. The comment is that this torque "feels like" extra HP, because you feel the multiplied torque at the wheels. However, it's not extra HP, the engine is unchanged, you're simply increasing the RPM for a given vehicle speed and gear scenario. My guess is that in order to achieve certain fuel economy and/or emissions targets on the highway, the automatics from this generation - which can't be compared to the latest 8-speed generations - needed to run at lower RPM to overcome their inherent lack of mechanical efficiency. |
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05-13-2021, 11:26 AM | #7 |
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Thanks S4NoMore for the detailed info.
After re-reading Jake’s post I think I just got hung up on a few things due to lack of comma between “not” and “some” . Thanks hassmaschine. I think what I’ll do is manual-swap before I go to replace my front diff and rack. That way if I just can’t live with the gearing I can always put in lower final drive. Not stoked considering I have now purchased both a new (used) rear AND front diff in 3.91, and later realized my rear diff was fine all along, but oh well.
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05-13-2021, 04:48 PM | #8 |
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I’ve been running a 3.91 in my manual 328i for a few years and love love love it! I was 3.23 stock and then ran a 3.73 for a year before going up again to the 3.91. I cruise the highway every day to go to work at 5:00am and am doing ~100mph the whole way engine is very happy and I’m very happy lol
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05-13-2021, 05:17 PM | #9 | |
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My transmission arrived today! Just gotta pick it up from the depot. And then install it… it’s still a few months away with work and life but I’m pretty stoked. Waiting to see if some exhaust MILVS drop in the hope there might be a package deal with both sets haha. Modding is a slippery slope, and I’m learning that now.
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05-13-2021, 05:29 PM | #11 |
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05-13-2021, 09:16 PM | #12 | |
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05-14-2021, 02:19 PM | #13 | |
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Stock AT:0.691*3.64=2.515 total reduction Stock MT:0.851*3.15=2.681 total reduction That means, stock, MT cruises at just 6.6% higher engine RPMs for the same road speed. Unless you're referring to the transmission itself. That's the whole reason they came with different final drives. |
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05-14-2021, 04:57 PM | #14 | |
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The reason I'm looking at it from this POV is that the transmission is the only variable in my case; the final drive ratio will stay fixed at 3.91.
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03-02-2023, 08:42 AM | #15 |
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FYI There are some diff differences between cars, manuals anyway.
If you have a 2006 330i 6-speed like me, you have to get a 3.64 out of a 330i automatic. Something about the way the "guibo flange" connects to the driveshaft is slightly different between the 330i and the 328i/325i/other models. I'm unsure about the automatic/AWD cars though. You can’t use the 3.73 or 3.91 on a 330i, as far as I know. I think you'd have to modify your driveshaft/ possibly something with the rear axles. I can’t remember exactly what the differences were, but there’s a few good threads about it. I believe there’s driveshaft differences between auto and manual too. I believe the N52 6-speed manual transmission(RWD) is approximately 3 inches longer than the auto(maybe it was the N54 trans can’t remember). AWD/Auto transmission setups may be different. Also, kind of unrelated, but the 6-Speed from the N52 is different from the N54. It's smaller and less stout. Last edited by VehementCrom; 03-02-2023 at 01:17 PM.. Reason: Clarity. |
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