Teaser: 3.91 Ring and Pinion shot peened and REM polished

[inTgr8r]

A bunch of part #s in this thread...
most same as Ron's post

copy & paste from various posts
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BMW diff seals- 33107510289
BMW Pinion seal- 33121213949
BMW crush sleeve- 33121204657
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From BMW
33121205138 - securing plate for pinion, im not sure if its necessary but just in case
33121204657 - crush sleeve
From summit
TMK-1012N - seal x2 <-- wrong part do not buy
TMK-223520 - seal for pinion <-- wrong part do not buy
TMK-HM803110 bearing race
TMK-HM803149 inner pinion bearing
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output seals straight from BMW pn: 33107510289 quantity 2.
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......ordering these parts:
BMW diff seals- 33107510289 (x2) $44.10
BMW Pinion seal- 33121213949 $34.05
BMW crush sleeve- 33121204657 $6.26

Timken HM803110 (race) $14.95
Timken HM803149 (bearing) $23.95
BMW secure plate 33121205138 $2.35

Total $125.66
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I purchased turner 3.91 gears. Below is what you need for the install.

33121205138 securing plate for pinion
33121204657 crush sleeve
33107510289 output seals X2

TMK-HM803110 bearing race - ( From summit racing )
TMK-HM803149 inner pinion bearing - (From summit racing )

5 Bottles BMW Differential fluid.
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[inTgr8r]

interesting comparo post...

Originally Posted by M&M

Ok so I have over 20 runs recorded on the VBox at a private testing facility. After installing the 4.1 diff, I went back there. I record EVERYTHING whenever I do a run. Things like:

- Ambient temperature
- Barometric pressure
- Humidity
- Type of fuel
- Amount of fuel in the tank
- Tyres
- RPM at launch & type of launch

So I called up a run from with the stock diff under similar conditions. Here are the results.

Speed over time:



I deliberately called up the fastest run with the stock diff. The stock diff accelerates harder in 1st gear from up until 2 seconds into the run where the speeds even out. This may be due to better traction on the stock run.

At about 2.5 seconds the 4.1 car is forced to go into 2nd gear, while the stock car holds on until after 3 seconds. However the 4.1 car is already pulling hard in 2nd gear at that stage & accelerates to higher speeds which the stock car never recovers. The lines go get closer every time the 4.1 car shifts up, but then they separate again when the stock car has to shift.

Speed vs distance:



Similar trend to the top graph, where there are phases where it is close, but the 4.1 car gets higher speeds as the distance goes on. It seems the most damage is done IMMEDIATELY after the stock car has to shift.

Distance over time:



Well this is the most important one as it shows who will actually win in a race. In this particular race the stock car had better traction & was ahead initially. At around 3.5 seconds the cars are level. But from then on the 4.1 car slowly puts distance on the stock car & after 15.38 seconds it is 8m ahead, which is 2 carlengths.

On the 1/4 mile times, the 4.1 car was 0.2 faster & 1.4 mph faster depsite having a slightly poorer 60ft time.

[beta]

Great info, thanks for helping verify the parts that finnegan posted intgr8r.

Finnegan already mentioned this, but another thing to consider is the shift points of your favorite track(s). See this post as an example.

[Finnegan]

Thanks Ian!

Those are two very helpful posts!

I can now complete my parts ordering in time for Santa to deliver!

[beta]

Gears are going in on Thursday! Proper review will have to wait until after break-in.

The installation cost for a reputable shop is a little higher than we estimated, so total cost is looking closer to 1750 all inclusive for: new bmw motorsport ring and pinion, shot peen and REM polish, all consumables (including 2 sets of fluid for break-in) and install.

Still cheaper than the typical 2k-2.5k + shipping + install that a full diff would've cost.

[seank]

Originally Posted by inTgr8r

interesting comparo post...

Was this done on a dyno or real world situation?

[beta]

Originally Posted by seank

Was this done on a dyno or real world situation?

Says Vbox and private testing facility right in the first line. Here is the full post if you want to read through the thread.

His 60ft times were worse as he only did 2 launches with the new gears, so it's possible the results would've been better with better traction/better tires/better launch. It doesn't look like a big difference, but if you look at page two he compares +35hp and you can see how what appears to be a small difference equates to 2-3 car lenghts in the real world.

Anyway it's a lot of good real-world info so it's worth a read.

[beta]

Diff removed and sent off to the diff rebuilder. He'll work on it tomorrow (4.5 hours) and then it'll go back to get installed. Should be all done tomorrow by end of day.

[Finnegan]

Can't wait to see this job finish!

[beta]

Picked up the car tonight. No noise or issues. Need to break it in before giving a proper review, but in the higher gears the difference is very noticeable. I found myself over-blipping the downshift rev matching because the ratios are just slightly closer now, will have to adapt.

[Finnegan]

But and on break in periods--it's got to be difficult not to just let'er rip. Oh well, price well worth paying!

[inTgr8r]

Sweet

[beta]

The more I drive it the more I like this mod. I do find myself shifting more though, when tooling about going 50mph I tend to reach for 5th now.

I also realize that if you want to go 4.10 you definitely should. This is a small difference, and the difference between 3.91 and 4.10 is only half the difference between stock and 3.91. I'm still happy I went with 3.91 though, it's very conservative and feels just right.

I think that overall it fits really nicely into the theme of stiffening up the car:

- Increased steering effort from 245 front tires.
- Strut brace, didn't feel much change when I installed it.
- Engine mounts, big difference in how tight the car feels.
- AS SSK, also a very big difference in the shifting feel, much heavier than stock.
- Coilovers, stiffer yet smoother than the stock suspension.

Now with this mod the accelerator feels a little heavier too.

[mfanatic325]

Grats! Wait till you get it out on the track. That's when the beast really comes alive

[JeffchaPz]

Alright my peopleZ... so I would love to go with 4.10 - but I also want to upgrade to 19 Inch rims for looks.... so would you say 4.10 and 19s is equal to a 3.91 ??


Help me out please?

[Finnegan]

Originally Posted by JeffchaPz

Alright my peopleZ... so I would love to go with 4.10 - but I also want to upgrade to 19 Inch rims for looks.... so would you say 4.10 and 19s is equal to a 3.91 ??


Help me out please?

Wheel size isn't the determining factor, tire diameter is.

For example, stock front tires are 255/40/18. Diameter of tire is 26.03". Sidewall is 4.01".

On a 19" wheel you'd run a 255/35/19. Diameter of tire is 26.02". Sidewall is 3.51".

Tire diameter is virtually the same. No change in wheel torque.

Now if you plan to run really low profile tires (like a /30, which is close to a 4% difference in diameter from stock, and has a very small sidewall) on that 19 (and I wouldn't, as one pothole and BAM) then you'd have the 3.91 be closer to the 4.10 because you're getting more wheel torque due to the tire diameter. Since you can play with tire sizes easily, don't let this be a deciding factor.

You can play with tire and wheel sizes here.

More here on how diff FD ratio + tires interact. If you think you want the 4.10, get the 4.10.

[marinb]

Rim size doesn't matter. The total wheel diameter rim+tire will effect your relative ratio. But it's very easy to choose a tire size for a 19" rim that will match the total diameter of your 18" setup.

[JeffchaPz]

You guys ROCK!!!! thanks for the info.

[beta]

Originally Posted by JeffchaPz

You guys ROCK!!!! thanks for the info.

Post when you get them. You're going to love the 4.10s.

[RobMCoupe]

Bring this thread back as I ordered 4.10 gears today.

I have a couple of questions for those completed this mod.

1. Do I need all the parts listed? Just wondering if some are not needed.
2. Are there any additional parts needed not on the list?
3. Is there a specific fluid for break in period?

Thanks.

[mrsuper2]

Can someone please post a link as to where 4.10 & 3.91 final drive gear sets can be ordered from. I am interested in the ring & pinion gear set only and not a completed rear diff assy.
Thanks in advance

[RobMCoupe]

Originally Posted by mrsuper2

Can someone please post a link as to where 4.10 & 3.91 final drive gear sets can be ordered from. I am interested in the ring & pinion gear set only and not a completed rear diff assy.
Thanks in advance

The information is in this thread, but I pasted it below. You have to call them to place an order. They will send you a waiver you need to sign.

I bought my 3.91 ring and pinion from Bobby Rahal BMW for the price of $624 shipping included. I spoke to Jim and he was great. You need to have the part number when you call them. Part number for the 3.91 is 33127768525 and the part number for the 4.10 is 33127768526. Double check the 4.10 number because I am not 100% sure on that one.


$804 for the new edition 4.10 (Part #33.10.8.321.899)