3.91 gears from DiffsOnline

blender · 09-25-2009, 10:14 AM

I wanted to post about a good product/service experience with DiffsOnline. I turned to them after two Dinan diffs failed to meet my expectations for noise levels (they whined badly). See my separate post in this board about that saga. As a company, Dinan was fair about the whole thing and finally discovered the root cause, but they have not indicated that they believe it to be a issue worth correcting. Basically, they don't polish the gears before assembly.

So, I turned to DiffsOnline with a request for a 3.91 rear Diff for an '06 Z4M. Dan at DO mentioned he had a set of "factory" 3.91 gears from an M5 that should be factory-quiet. I requested the REM Ring & Pinion polishing which they state lowers operating temps by reducing friction in the gear-set.

The rebuilt diff works flawlessly, and critically for me, it's no louder than the original factory diff. Under spirited acceleration the car will chirp first through third reliably.

The rear gear change makes a visceral difference, and I agree fully with the opinion that it's the single best performance mod you can for a Z4M.

I recently walked away from a well driven Z3M roadster at a light, and that's a much lighter car with basically the same engine. His expression was memorable

FYI, Total outlay was ~$2500 after the return credit on the removed factory diff.

epbrown · 09-25-2009, 10:57 AM

Glad this worked out for you. I'm still very happy with the 3.91 gear set in mine, and remain convinced this is how the Z4M should have come. The only downside is everyone insisting you should go with the 4.10s.

hammerhead · 09-25-2009, 04:57 PM

Could one theoretically buy a "new" M5 diff from the dealer's parts dept and have it installed by Dinan?

hammerhead · 09-26-2009, 09:45 AM

Rcleme,
Which M5?

blender · 09-26-2009, 11:13 AM

Hammerhead - you may know this already, and if so please ignore me. The Diff from an M5 won't mount on a Z4M, which uses the same diff as the E46M3 [ie the S54 powertrain]. The forged housing, drivetrain linkage, the axle fittings etc are all car specific.

What I and others purchased was a Z4M Diff with different gears installed, and then swapped our factory diff back to the vendor. This is by far the easiest way to do it.

BMW Motorsport does sell replacement gears in different ratios; that's what DiffsOnline, DINAN and others use to rebuild diffs. Take a look at my other long thread about the differences we discovered between the factory parts and the Motorsport parts.

Best guess, the gears I got were from a salvaged 1991 M5 with about 40K miles on them.

regards,

RC

davesz4mc · 03-30-2012, 07:36 PM

What rpm do you run at 80 on the highway? Do you daily drive it? This would seem to be the downside for me...

wow I pulled this one outta the archives lol

I was searching for threads about replacement axles =-(

Finnegan · 03-30-2012, 08:03 PM

Quote:

Originally Posted by davesz4mc

What rpm do you run at 80 on the highway? Do you daily drive it? This would seem to be the downside for me...

wow I pulled this one outta the archives lol

I was searching for threads about replacement axles =-(

Zombie thread resurrection.

Here's are two (I and II) of many discussions on this topic. If you're not going FI in the future, I'd go with the 4.1 gears not the 3.91. Looks like you're talking about ~200 RPM increase for the 3.91, and ~300 for the 4.10 @freeway speeds, extrapolating from the grid Ron Stygar built (which only goes up to 5th gear).

blender · 03-31-2012, 04:41 PM

To answer your question:

I turn about 3000 RPMs at 70mph, with 265/35/18s at the back.

If I had to do it over, I'd probably buy the 4.11s. I rarely get the roadster high into triple digits, so having more torque/acceleration between 10 and 90 is where it's at for me in the canyons.
RC.

beta · 03-31-2012, 06:04 PM

For stock tires in 6th gear at 80mph:
3.62 = 3177 rpm
3.91 = 3432 rpm
4.11 = 3607 rpm

Then it's +255 rpm for 3.91 and +430 for 4.11.

I started one of the threads linked by Finn and I'm still not sure what ratio to go for.

P.S: you can figure out the increase in rpm by multiplying by the change in gearing. 3.91 is tiny bit over 8% increase, so you multiply by 1.08. 4.11 is a 13.5% increase so you multiply by 1.135.

gtMc · 03-31-2012, 06:11 PM

The forum must have been reading my mind as I just sat down to start researching gears, and this is right here at the top. Must be a sign....

Slippery slope....

09-25-2009, 10:14 AM	#1
blender Custom User Title 145 Rep 661 Posts Drives: black Join Date: Feb 2009 Location: n iTrader: (0)	3.91 gears from DiffsOnline I wanted to post about a good product/service experience with DiffsOnline. I turned to them after two Dinan diffs failed to meet my expectations for noise levels (they whined badly). See my separate post in this board about that saga. As a company, Dinan was fair about the whole thing and finally discovered the root cause, but they have not indicated that they believe it to be a issue worth correcting. Basically, they don't polish the gears before assembly. So, I turned to DiffsOnline with a request for a 3.91 rear Diff for an '06 Z4M. Dan at DO mentioned he had a set of "factory" 3.91 gears from an M5 that should be factory-quiet. I requested the REM Ring & Pinion polishing which they state lowers operating temps by reducing friction in the gear-set. The rebuilt diff works flawlessly, and critically for me, it's no louder than the original factory diff. Under spirited acceleration the car will chirp first through third reliably. The rear gear change makes a visceral difference, and I agree fully with the opinion that it's the single best performance mod you can for a Z4M. I recently walked away from a well driven Z3M roadster at a light, and that's a much lighter car with basically the same engine. His expression was memorable FYI, Total outlay was ~$2500 after the return credit on the removed factory diff. Last edited by blender; 04-08-2012 at 03:40 AM.. Reason: update.
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09-25-2009, 10:57 AM	#2
epbrown Colonel 60 Rep 2,128 Posts Drives: BMW M Coupe, Porsche Boxster S Join Date: May 2007 Location: Chicago, IL iTrader: (0)	Glad this worked out for you. I'm still very happy with the 3.91 gear set in mine, and remain convinced this is how the Z4M should have come. The only downside is everyone insisting you should go with the 4.10s.
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09-25-2009, 04:57 PM	#3
hammerhead Private 2 Rep 67 Posts Drives: Z4MR Join Date: Aug 2009 Location: Midwest iTrader: (0)	Could one theoretically buy a "new" M5 diff from the dealer's parts dept and have it installed by Dinan? Last edited by hammerhead; 09-25-2009 at 10:22 PM..
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09-26-2009, 09:45 AM	#4
hammerhead Private 2 Rep 67 Posts Drives: Z4MR Join Date: Aug 2009 Location: Midwest iTrader: (0)	Rcleme, Which M5?
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09-26-2009, 11:13 AM	#5
blender Custom User Title 145 Rep 661 Posts Drives: black Join Date: Feb 2009 Location: n iTrader: (0)	Hammerhead - you may know this already, and if so please ignore me. The Diff from an M5 won't mount on a Z4M, which uses the same diff as the E46M3 [ie the S54 powertrain]. The forged housing, drivetrain linkage, the axle fittings etc are all car specific. What I and others purchased was a Z4M Diff with different gears installed, and then swapped our factory diff back to the vendor. This is by far the easiest way to do it. BMW Motorsport does sell replacement gears in different ratios; that's what DiffsOnline, DINAN and others use to rebuild diffs. Take a look at my other long thread about the differences we discovered between the factory parts and the Motorsport parts. Best guess, the gears I got were from a salvaged 1991 M5 with about 40K miles on them. regards, RC
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03-30-2012, 07:36 PM	#6
davesz4mc Second Lieutenant 25 Rep 296 Posts Drives: 2007 z4mc Join Date: Feb 2012 Location: San Antonio, Tx iTrader: (0)	What rpm do you run at 80 on the highway? Do you daily drive it? This would seem to be the downside for me... wow I pulled this one outta the archives lol I was searching for threads about replacement axles =-(
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03-31-2012, 04:41 PM	#8
blender Custom User Title 145 Rep 661 Posts Drives: black Join Date: Feb 2009 Location: n iTrader: (0)	since you asked To answer your question: I turn about 3000 RPMs at 70mph, with 265/35/18s at the back. If I had to do it over, I'd probably buy the 4.11s. I rarely get the roadster high into triple digits, so having more torque/acceleration between 10 and 90 is where it's at for me in the canyons. RC.
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03-31-2012, 06:04 PM	#9
beta Lieutenant Colonel 80 Rep 1,663 Posts Drives: 2006 M Roadster Join Date: Mar 2011 Location: CA iTrader: (2)	For stock tires in 6th gear at 80mph: 3.62 = 3177 rpm 3.91 = 3432 rpm 4.11 = 3607 rpm Then it's +255 rpm for 3.91 and +430 for 4.11. I started one of the threads linked by Finn and I'm still not sure what ratio to go for. P.S: you can figure out the increase in rpm by multiplying by the change in gearing. 3.91 is tiny bit over 8% increase, so you multiply by 1.08. 4.11 is a 13.5% increase so you multiply by 1.135. __________________ modification journal Last edited by beta; 03-31-2012 at 06:10 PM..
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03-31-2012, 06:11 PM	#10
gtMc Captain 21 Rep 704 Posts Drives: '08 MC Join Date: Nov 2011 Location: NNJ iTrader: (1) Garage List '03 e39 m sport [0.00] '08 mc [0.00]	The forum must have been reading my mind as I just sat down to start researching gears, and this is right here at the top. Must be a sign.... Slippery slope.... __________________ ZHP/Zeck CDV/VT engine mounts/RE trans mounts;clutch stop/Stubby
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