Differential Time, 3.91 vs 4.10 on 6 speed ZCP

[SandeepM3]

If longevity is the goal, Wavetrac is the best option. No clutches to wear out puts it in a class of 2 (the other of which can't limit slip if you lift a tire).

That was my primary deciding factor on getting the Wavetrac.

As a perk, at least on the street, it feels great (and is completely silent). In 2.5 weeks I look forward to seeing what it's like on track.

Interested to get your thoughts on how it compares to the stock unit on heavy brake zones and decel when you get it on track.

[EricSMG]

Here's the thing with gears after running 4.10s for 25k miles and then back to stock gears. This is one of few (maybe the only) mod where there is a literal, mathematical, practical tradeoff. Yes, you get 13% more shove (for a given RPM in a given gear) but you also lose 13% road speed (for that same given RPM and gear).

It's the former that you FEEL right away and boy do you ever. 4.10s are wicked fun (at first) and make the car feel like it has a small V8 (for real). HOWEVER.... as time passes, the tradeoff I mentioned starts to make itself known and over time, it becomes (at least for) more of a con than the mentioned pro. The gears' reach, particularly out on the road, becomes very hamstrung and the car can actually feel slower/less effective in many scenarios. 3rd gear, in particular, is a monster with stock R&P..... it's so broad and can take you all the way to 100mph at redline. with 4.10s.... that's now only 87mph and while that may not sound like much on paper... it's an immense difference out in the real world. Note - it's not that you MUST actually hit 100mph, but rather the 'overhead' you have available whilst being able to stay in the lower trans gear.... if that makes sense. This is a tricky concept to explain and must be experienced.

In summary:
- everyone should experience gears in these cars
- having had that experience, I prefer stock gears

Edit - not passing judgement at all here, but $5,000 for a diff is a LOT of cash and I question the ROI there. You should really look at this as objectively as you can.

[bigjae46]

Here's the thing with gears after running 4.10s for 25k miles and then back to stock gears. This is one of few (maybe the only) mod where there is a literal, mathematical, practical tradeoff. Yes, you get 13% more shove (for a given RPM in a given gear) but you also lose 13% road speed (for that same given RPM and gear).

It's the former that you FEEL right away and boy do you ever. 4.10s are wicked fun (at first) and make the car feel like it has a small V8 (for real). HOWEVER.... as time passes, the tradeoff I mentioned starts to make itself known and over time, it becomes (at least for) more of a con than the mentioned pro. The gears' reach, particularly out on the road, becomes very hamstrung and the car can actually feel slower/less effective in many scenarios. 3rd gear, in particular, is a monster with stock R&P..... it's so broad and can take you all the way to 100mph at redline. with 4.10s.... that's now only 87mph and while that may not sound like much on paper... it's an immense difference out in the real world. Note - it's not that you MUST actually hit 100mph, but rather the 'overhead' you have available whilst being able to stay in the lower trans gear.... if that makes sense. This is a tricky concept to explain and must be experienced.

In summary:
- everyone should experience gears in these cars
- having had that experience, I prefer stock gears

Edit - not passing judgement at all here, but $5,000 for a diff is a LOT of cash and I question the ROI there. You should really look at this as objectively as you can.

For sure…it’s not a clear free lunch with gears. For street cars, really doesn’t make sense to change the gearing or LSD unit.

If you are an advanced driver…meaning you’re trying to find 1-2mph in a corner vs 5-10…then you can tune the gearing since you kinda know the speed you’ll be at. For my dedicated track car, a 3.91 keeps me around 5000rpm at min speed at the tracks I drive with an extra 1-2 shifts per lap. I think there is about a .5 second gain. Definitely will be keeping the 3.62 though.

The real upgrade for the diff is a clutch type limited slip unit. That makes a SIGNIFICANT difference if the car has a relatively neutral balance. From what I see, most add understeer - stiff front springs and bigger front sway bar.


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[Obioban]

This is me following a car that is similarly equipped. The only difference is he has a 4.10, I have a 3.62. Same OE M-Lock diff. I was chasing him this whole session. I would catch him throughout the lap but he always ran away coming out of the fast sweeper onto the oval. I think that is a great illustration of what shorter gearing does.

It looks to me like you’re a gear too high coming onto the oval.

also, holy shit is that a poorly paved track, at least from what it looks like.

[Tbonem3]

Eric!

I posted a lot on M3f about it, but I had the exact same experience. 4.10s are fun at first, but it fades and soon all you notice are the negatives.

[bigjae46]

It looks to me like you’re a gear too high coming onto the oval.

also, holy shit is that a poorly paved track, at least from what it looks like.

Can't grab a lower gear then I'm bouncing off the rev limiter coming onto the straight. The transition onto the oval is bumpy and you DO NOT want to upshift there. Need to be on throttle going over the bump.

Yeah...that track was falling apart and has closed. It is now a residential community.

[Hyperboost]

bump an old thread. I am doing a lot of research on diff. Trying to buy something before winter.

Read this thread over a few times and I agree the stock gearing is perfect! Less shifting can save sometimes off your lap.
For one of the tracks I go to, Thompson Speedway, the straight way after the NASCAR turn will require me to shift from 3rd to 4th gear with my 3.91 gearing in the stock diff. One second or two seconds later I would need to brake and then downshift before I turn so I consider that a big waste of time. If I don't upshift I would bounce off the rev limiter which is set at 8500 right now or I coast into that turn; to me that is not ideal.

[Hyperboost]

Now onto my questions. I have EmTron standalone in my car and I do not have any traction control as of now. Our car electronics are old and the system cannot keep up with wider track tires anyway; so I always turn off the traction when I hit the track.
When running without any traction control. do I lose the full benefit of the Wavetrac LSD? Or the Wavetrac will do all the work. I don't really care for street driving.

How much fluid does the diff hold? 2L? I put together the things I want on Diffsonline and it comes out to be $3812 before core charge😅

The Quaife LSD is $200 more than Wavetrac and what is the benefit? I researched a lot but can't find solid info about the difference between Wavetrac vs Quaife. I would like to know about this.
So far Wavetrac will be perfect for me due to the design and longevity. I don't really care about street driving because the car is barely street legal. lol.
Drexler or the Race system is something I don't want to spend right now.

[Hyperboost]

https://diffsonline.com/bmw-differential-technical-information.html

[bigjae46]

A good clutch type diff (ZF, drexler, OSG) can't be beat for responsiveness and it will improve rear grip and traction by adding speed, not taking it away like traction control.

I've put a 3.91 in and it makes a difference exiting the corner. Its noticeable. It forces an upshift on 2 straights where I was almost at the redline anyway. I would probably be better off with a 4.10. Keep in mind I have a 5 speed.

[Feffman]

A good clutch type diff (ZF, drexler, OSG) can't be beat for responsiveness and it will improve rear grip and traction by adding speed, not taking it away like traction control.

I've put a 3.91 in and it makes a difference exiting the corner. Its noticeable. It forces an upshift on 2 straights where I was almost at the redline anyway. I would probably be better off with a 4.10. Keep in mind I have a 5 speed.

BIGJAE:

I had the 4.10 in my car before the current 3.91. I ran out of 5th gear at big tracks like Road America, Road Atlanta and COTA with the 4.10. I like the 3.91 as a good compromise.

Feff

[Mr.wReckless]

Now onto my questions. I have EmTron standalone in my car and I do not have any traction control as of now. Our car electronics are old and the system cannot keep up with wider track tires anyway; so I always turn off the traction when I hit the track.
When running without any traction control. do I lose the full benefit of the Wavetrac LSD? Or the Wavetrac will do all the work. I don't really care for street driving.

How much fluid does the diff hold? 2L? I put together the things I want on Diffsonline and it comes out to be $3812 before core charge😅

The Quaife LSD is $200 more than Wavetrac and what is the benefit? I researched a lot but can't find solid info about the difference between Wavetrac vs Quaife. I would like to know about this.
So far Wavetrac will be perfect for me due to the design and longevity. I don't really care about street driving because the car is barely street legal. lol.
Drexler or the Race system is something I don't want to spend right now.

Diff holds I believe about 1.3 liters, so would need 2x1 liter jugs to fill up AFAIK.

Also, I believe both Wavetrac and Quaife are that of that Torsen style LSD, however it's the clever technology of the Wavetrac that still provides torque to a wheel without grip as opposed to a traditional Torsen diff, advantage Wavetrac in that case.

But if you don't care abut street driving then you'd probably be better off with a more advanced clutch type setup where you can get the ramp angles to your liking etc..

As a side note, went from stock to 4.1 Wavetrac in my diff. I wanted a mechanical and involving drive, and I realise there's lots of personal opinions in previous comments about final drives, but personally I love the shorter ratios. Sure the speed limits where I live are a bit lower so I don't really suffer @ cruise. Though I do understand the point above with a specific gears "reach" and so on. Before I would sit in 2nd at the top and find the drive perhaps a bit too mental and violent, with regards to noise especially. With a 4.1 I can now sit in 3rd and enjoy more torque, more "reach" as well as not as much of a savage noise level, which makes it as a street car, more enjoyable IMO.

For involvement I enjoy the increased shifting of 4.1. For street driving I enjoy the new aspects of the different rpm driving. Yet to take it on track, but I guess that could result in different experiences and maybe 3.62 would be more beneficial..?

Regards

[Hyperboost]

Thanks! I rememebered wrong, my gearing is the BMW Motorsports 4.1 gearing. stock LSD

[Flat-Six]

… and your cars weight and power. I can only floor it in first in ideal conditions without lighting them up, with stock gearing, with 350rwhp and <3100 lbs.

do you have a thread that shows how you got to under 3100? Just curious, been debating on direction with my car....

[old///MFanatic]

Less shifting can save sometimes off your lap.
For one of the tracks I go to, Thompson Speedway, the straight way after the NASCAR turn will require me to shift from 3rd to 4th gear with my 4.10 gearing in the stock diff. One second-two seconds later I would need to brake then downshift before turn so I consider that a waste of time. If I don't upshift I would bounce off the rev limiter set at 8500 right now or I coast into that turn.

Oh I totally believe this, but this arguement can easily be made opposite for another track and then added downshifting.
You’re talking about your 8500 rev limit on that track. Each track is different. That’s why it’s not uncommon to have different ratios listed in each track notes for a circuit season.