Teaser: 3.91 Ring and Pinion shot peened and REM polished

[beta]

Just got my 3.91 ring and pinion back today from being shot-peened then REM polished. Here are some teaser pictures. Hoping to get it installed after the holidays.

[johanness]

Sweet. Where'd u get it done?

Looking forward to your review, as I've been on the fence w/this for some time.

[papitosabe]

I'm a complete newb, but why does shot-peen and polishing do for them?

[marinb]

What was the cost break down? I'm interested in the REM polish.

[Finnegan]

I just bought my 3.91 ring and pinon as well, and am getting them treated. (Thanks to Beta, who has been a great help with sourcing the gears, and with the treatment and associated legwork and research.) These are "before" shot peen and REM polishing.

I'm shooting for an install early next year (still need to ship for polishing and just got the ring and pinon).

Good description of the shot peen process here. The REM polishing/treatment creates a micro layer of treated material reduces friction (reduces wear and frictional losses). IIRC, the 4.1 gears from BMW are shot peened, while the 3.91 are not.

Beta will have the give the exact cost for the peen and REM. Should be somewhere about $125 based on my discussion with the company (range is lower and higher based on size of parts). Company is Weddle, near Santa Barbara.

[mfanatic325]

You guys are gonna love it!
1st gear becomes practically useless however
And it'll take a tiny bit of getting used to throttle modulation for daily driving, as you'll be seeing more torque earlier on in the rev band; as well as twitchier in-gear acceleration/deceleration.
Overall though, gearing isn't a bad stepping stone to FI

[Rick F.]

Beta,

Those are works of art, for sure!

Rick F.

[beta]

johaness, papitosabe: see finnegan's post. Thanks for chiming in.

marinb: I left my invoice at work with my gears. Everything with shipping was around 150, but I'll get the breakdown for you tomorrow if I remember (working on saturday bleh).

mfan: can't wait, power is addicting. Installing these should push out the temptation of FI another 6 months...

Rick: thanks!

[inTgr8r]

Subscribed

I did a quaiffe plus R&P change on a G35 and it was a huge improvement
This has been on my list for a long time now for the M Coupe.

What was the deciding factor for going this route rather than doing the exchange thing with someone like diffs-online?

Looking forward to the results, need all the info I can get

[Gtfour]

I had a 3.91 with 2-way Carbonetics LSD installed on my car this summer.

The shorter gear ratio compare to OEM 3.62 was a nice upgrade but the LSD was too agressive for me.

I recently installed a 4.10 pinion/ring in my OEM diff with the M-Variable LSD. I love it! It's so rev-happy compare to the 3.91. Even with my ESS VT-445, the 4.10 is a great match. I can't wait to try it next summer on the track.



I still don't understand why people choose a 3.91 over a 4.10???

Especially when NA

[inTgr8r]

^ that is my dilemma
You are the 1st person I know that has had both and can offer a real world comparison.


Everyone that has the 3.91 says they're great & the best way to go.
....those with the 4.10 say the same damn thing.

[Caddyshk]

Originally Posted by inTgr8r

^ that is my dilemma
You are the 1st person I know that has had both and can offer a real world comparison.


Everyone that has the 3.91 says they're great & the best way to go.
....those with the 4.10 say the same damn thing.

Either way; you'll need 5th gear on the Andretti straight which makes a smooth entry into 8 a little more difficult. Added higher revs however should allow for staying in 3rd through 5b and the Moss corner

[Dammmittt]

I think you just need to balance acceleration and top speed...for driving in the states I think you'd go with acceleration over top speed...but I could be wrong....

[inTgr8r]

Originally Posted by Caddyshk

Either way; you'll need 5th gear on the Andretti straight which makes a smooth entry into 8 a little more difficult. Added higher revs however should allow for staying in 3rd through 5b and the Moss corner

I think your absolutely right about the 5 complex
....I wonder where you'd hit the shift point on the Andretti straight, it would suck if it was just before the crest.

--------------

To do this mod I'm not sure who I'd trust to do the job right

When doing the ring & pinion change, are you guys changing Bearings, Seats / Gaskets, Crush Collar etc?

& what kind of deals/pricing are you finding out there.

[Finnegan]

Originally Posted by Gtfour

I had a 3.91 with 2-way Carbonetics LSD installed on my car this summer.

The shorter gear ratio compare to OEM 3.62 was a nice upgrade but the LSD was too agressive for me.

I recently installed a 4.10 pinion/ring in my OEM diff with the M-Variable LSD. I love it! It's so rev-happy compare to the 3.91. Even with my ESS VT-445, the 4.10 is a great match. I can't wait to try it next summer on the track.

I still don't understand why people choose a 3.91 over a 4.10???

Especially when NA

Hmmm....Should I have gone with the 4.10?

If I'm doing this correctly, and assuming stock sized tires:

3.62 -> 3.91 = 8%
3.62 -> 3.91 = 13.25%

3.62 -> 3.91 8% with 265/35 tires (2.81%) = 10.81%
3.62 -> 3.91 8% with 265/35 tires (2.81%) = 16%

I went with 3.91 because FI is a future enhancement I plan at some point; and my rear tires are 265/35, which already gives me close to 3% more wheel torque.

But my math assumptions could be off--but FI is in the future. Like Caddy, I figured off speeds (current and factoring in future improvements in times) and didn't like the shift points req'd for my local track with the 4.1 (ran numbers with and without tire factors).

I went the R&P direction because there's a good local shop (off road / diffs only) that will do the work for under 200. They did a great job on my friend's E46 M3. R&P was 545 shipped (Bobby Rahal BMW). Those costs, plus additional parts, fluid, REM and polish, and install are a better deal than an entire diff swap. Plus I know my diff's history...

[Caddyshk]

Originally Posted by inTgr8r

I think your absolutely right about the 5 complex
....I wonder where you'd hit the shift point on the Andretti straight, it would suck if it was just before the crest.

--------------

To do this mod I'm not sure who I'd trust to do the job right

When doing the ring & pinion change, are you guys changing Bearings, Seats / Gaskets, Crush Collar etc?

& what kind of deals/pricing are you finding out there.

I start bouncing the rev limiter now just before the crest...

[mfanatic325]

I also plan on FI in the future, but I got 3.91 primarily because I came across a decent deal for a set of ring & pinion and also had access to a good diff rebuilder for an acceptable labor price.

[beta]

inTgr8r: as finnegan said: price and history. My diff only has 25k miles, I know its full history. About the price, buying new R&P from bmw motorsport, shot peen + rem + installation is going to be in the neighborhood of 1k. Way cheaper than diff swapping.

As for 3.91 vs 4.10. I went the conservative route because my build is about the ultimate roadster, so I try to balance creature comforts, mileage, performance, etc.

Dammmittt: top speed should not be affected by 4.10. With 4.10 gears in 6th @ 8k rpm you'd be going a theoretical 175mph. There's a discussion here that goes into it, but that should be as fast as the car can go without FI.

marinb: REM was $95. Might be able to get it a bit cheaper if you say we referred you, I'm sure Weddle will appreciate the repeat business.

[marinb]

Thanks Beta. I'll give them a call.

[Dammmittt]

Thanks, I couldn't remember that far back

[Krypptic]

Originally Posted by beta

inTgr8r: as finnegan said: price and history. My diff only has 25k miles, I know its full history. About the price, buying new R&P from bmw motorsport, shot peen + rem + installation is going to be in the neighborhood of 1k. Way cheaper than diff swapping.

A few questions:

What do you mean buying the R&P from bmw Motorsport? Dealership?

How much did they charge for the R&P?

How much for installation?

I only ask because every where I looked just the R&P where a 1k, and you are saying soup to nuts for 1k. Not doubting, but if that's true I am definitely making this my next project.

[inTgr8r]

^ I'm having a little trouble following the math as well.....

I can get an R&P for about $600 (probably Timken)
REM $195
Bearing,shim, crush etc... $200

Rebuild labour..... $6-800

That doesnt include Shot pean

I don't see how you get even close to $1k.