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Discussion Starter · #1 · (Edited)
I will be doing an Evolve tune and I am also strongly considering doing a diff rebuild with 3:91 gears. My beast has 70k miles so it might be about time to do a rebuild regardless.


I live in central Florida and spend most of my time on the interestate under 50mph becasue traffic is terrible. I have only had the beast over 160mph a few times so the majority of my spirited driving is in the 60- 140 range.


From my research on this forum the major downside to the gears is losing some top end acceleration. But what is "top end"? If by top end people mean above 160mph I don't care but if they mean above 110mph then I care. I don't track my car so I am not concerned about shift points or wheel spin when trying to launch.


Wolverene did a great job documenting his finds back when this topic first came up but it appears his research stops at 110mph. (see below)


Does anybody have knowledge to share on this topic? Specifically regarding what happens from 110 - 160mph?


Thanks for the insight guys!




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"OK, I've finished my Gtech runs with the Dinan 3.91 differential, and have some accurate comparisons with the stock setup. I calibrated the Gtech at our local dragstrip with the exact setup I used for these runs, so it should be pretty accurate. The conditions were very similar for all the runs, temperature varied from 58 deg to 62 deg. I did 9 runs, 2 aborted with the stock car, and 7 runs, 1 aborted with the 3.91 differential (I used the 60' and 0-60 times on the aborted runs).

There is a substantial difference in the acelleration. Here are the results, using average numbers:

------------------------Stock -----------------------Dinan ----------------------Difference

60 ft. time ---------2.229 sec------------------2.225 sec----------------0.004 sec

0-60 mph ---------4.56 sec---------------------4.40 sec------------------0.16 sec

Quarter Mile -----12.78 @ 115.59 ---------12.54 @ 118.24 -------0.24 sec, 2.65 mph

50 - 110 mph-----7.86 sec---------------------7.31sec-------------------0.55 sec


Interestingly, my 60' times were really not much better with the new diff - the traction was just not great, and with more torque, it's not really any easier to modulate the throttle. Also, with the Dinan I got a fair amount of wheelspin in second gear, and had to modulate the throttle slightly. With the stock car, I could just mash it once I hit second - a good chirp, but it didn't break traction.

These numbers don't really show how much harder the Dinan is pulling in gear. Some of my Dinan 0-60 times included a 2-3 shift at 59 mph if I didn't quite hit redline. All of the quarter mile times included a 4-5 shift at about 117 mph. If the rev limit was raised slightly (listening Dinan?) the car would have easily trapped over 120, and the times would come down .15 sec.

Note - I just edited the table to include the 50-110 mph times. These give a good indication of how hard the car is pulling in third and fourth gears. As a reference, Road and Track has the M5 running 50-110 mph in 8.0 sec, and the M6 at 7.6 sec."
 

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You won't lose acceleration even in the top end. You gain acceleration throughout (less noticeable at top end aka higher speeds). What you lose is top end speed aka you won't be able to hit 200mph anymore.

You also have to shift earlier/more often compared to the stock 3.62.

I would recommend it though. I have a 3.91 LSD on my car and it's a great "bang for the buck" mod. It's nicknamed "the poor man's supercharger" for a reason.

Also, it's hard to judge by 0-60 times because the driver skills comes into play.
 

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Discussion Starter · #3 ·
Randy - That is what I thought was logical. Obviously your top speed is reduced but I am not going to go over 200mph so I am not too concerned with it. But some people were suggesting that stock gearing would actually pull harder in the top end, there was even a video of a stock vs a 3:91 and the stock car started to pull away. This is what had me confused...


Any suggestions on where to purchase one? I have mostly looked at Turner Motorsports.
 

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I think this will be my next mod along with an OSG diff. I was going to go SC but the CAD v USD is going to be horrible for a while longer.
 

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Read these two threads from 2007 on head to head races between similar modded e60's with one having the 3.91. The videos are gone but the information is still available. I think Darren deleted all the videos after he sold his car. The first thread was against my car with the 3.91. This is the first modd I did to my M5. It felt faster due to the extra torque and I liked it but we found it did not translate to being faster. These results have been proven over and over in head to head races. The 3.91 may have a better result from a dig.

http://www.m5board.com/vbulletin/e6...on/99796-another-m5-challenge-new-videos.html
http://www.m5board.com/vbulletin/e6...105950-videos-modified-m5-vs-modified-m5.html
 

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IB DinanS2++ e60m5 6MT, Dogleg s38b38 e28m5 TT/ST3, e30 TT/ST4
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OP~ the 3.91 is worth it. I have a custom 3.91 from Performance Gearing (Jim Blanton). He makes each one to your specs and applications. When he built mine, I had it at ****fS3xtor two days after installation, with only 200 miles on my new 3.91. Zero break-in was required, just a flush after the first 1000 miles. I drove it like I stole it -- that was about 20,000 miles ago and a dozen days at the track. Today, my 3.91 is still bombproof.

Meanwhile this was my experience with my Dinan 3.91 exploding:
http://www.m5board.com/vbulletin/e60-m5-e61-m5-touring-discussion/402657-boom-goes-my-3-91-a.html


Real world on an airstrip...the 6MT "should" be slower.

6MT, 3.91, catless headers, Dinan base tune (not for headers) v. SMG, 3.62, catless headers, Evolve tune (for headers) Both s85s are on pump gas. Video shot from my 6MT.
https://vimeo.com/110043439



Shot from the Evolve tuned for catless headers, 3.62 SMG
http://youtu.be/wY1SJ-AzFDg



Real world on a track 3.91 6MT and 3.62 SMG...Watch the SMG e60 disappear, once I clear traffic. Video shot from my 6MT. And watch the e46 m3 on R-Comps smoke us both.
https://vimeo.com/127415426#t=148s
 

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Discussion Starter · #7 ·
Looks like BMW engineers knew what they were doing when they designed our cars... who would have known. Maybe stock is optimal.


I guess one further question I have is during these runs did you have your rev limit raised. Much of the covnersation on this brings up the fact that you have to shift more which takes more time so in certain ranges it could hurt you becasue you might require on more shift. Does it make a differnece if the rpm limit is raised?


At this point it looks like my 3k would be better spent elsewhere.
 

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All the 3.91 gears do is move the power band down so you get better lower end response., it does not add hp but provides trq sooner which is where most people complain that the car needs it.


I would not bother with raising the rev limiter a few hundred rpms as we are taking minimal pickup.
 
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