3.91 rear end questions

Jim M3 · 24th July 2012, 00:29

I was hoping to hear from somebody who has this set up their thoughts on it compared to stock. I see that Turner has the ring gears and it is intriguing to me. It seems like a great bang for the buck upgrade.

Fiftytwoeighty · 24th July 2012, 00:43

Bunch of discussion on the Dinan 3.91. I love my 3.91, courtesy of the previous owner. No problems at 62,xxx miles.

Why Dinan 3.91 Diff Vs. $700 ring and pinion?

DINAN 3.91 Limited Slip Differential initial impressions & review

Owners with Dinan Diff

SSS_M5 · 24th July 2012, 00:54

Any gear change may cause whine. My Dinan rear diff.

mdyaman · 24th July 2012, 01:08

Quote:

Originally Posted by Fiftytwoeighty

Bunch of discussion on the Dinan 3.91. I love my 3.91, courtesy of the previous owner. No problems at 62,xxx miles.

+1

OP, do a search. This has been discussed before.

Jim M3 · 24th July 2012, 04:09

Quote:

Originally Posted by mdyaman

+1

OP, do a search. This has been discussed before.

I had to read three threads that were worthless drivel.

I put a 3.46 into my M3 and it was a significant difference with a supercharger.

This was the only relevant post on the whole thread, the rest were purely anecdotale

Subject to common disbelief, shortening the final drive does not give you more power from the engine.

This will remain a constant.

What changes is the power transmitted to the rear wheel in the same gear.

However, what you gain in power you lose in road speed. Given that maximum attained road speed in any given gear is what we are all after it almost defeats the point under maximum acceleration and this shows from the example given by the above member tjav8b when he raced another car.

So why is it that the members who have changed to a shorter final drive feel that it is an excellent upgrade? Simple. Due to the shorter final drive you are within the power band which produces more power more of the time which naturally gives you more torque plus the effects of the final drive acting as a torque multiplier.

The feeling is great because you are infact accelerating faster at any given point but each individual gear with the same rpm limit (stock 8250rpm) will achieve a lower terminal speed.

The nature of the overall package with the 3.91 becomes way more dramatic and instantaneous. The E34 M5 S38B38 has a similar power band (not as narrow as an S85 but still way narrower than an S62) as the S85. When we used to change the final drive on those the effect was stunning but as tests showed back then, you were not really getting anywhere any faster.

There is no question, the 3.91 will add a huge amount of fun to the daily drive and really add more G per in any given gear just so long as you all understand the above very important fact.... you are not going to go faster!

For 5.7's and other high powered S85's, it is better to stick to the stock final drive otherwise getting that power down will be near enough impossible and you will get tired of it. Gears 1 and 2 will become a little useless.

ykjones · 24th July 2012, 17:27

I love my 3.91 Dinan diff. I noticed an immediate difference in torque and acceleration. So much that in fact no kidding I almost backed through my house when backing into the garage when I came home from the install. I just wasn't used to the extra torque that our beasts have lacked from the get go. In my humble opinion, a Dinan 3.91 diff is a very worthwhile upgrade. I have no drone whatsoever. My car is at 89k mi now.

KenL · 24th July 2012, 19:17

Jim M3-
Maybe this will help put a point on this discussion. I went with a 3.91 ring and pinion and my 1/4 mile time dropped 0.25 seconds. If the Density Altitude was the same as the run w/o the 3.91s the time would have dropped 0.45 seconds. The 3.91s absolutely made my car QUICKER (accelerate faster and cover a given distance faster) and achieve a higher trap speed, at least for 1/8 and 1/4 mile distances since that's all the data I have.
Safe racing,
KenL

mdyaman · 24th July 2012, 22:59

Quote:

Originally Posted by Jim M3

I had to read three threads that were worthless drivel.

I put a 3.46 into my M3 and it was a significant difference with a supercharger.

This was the only relevant post on the whole thread, the rest were purely anecdotale

Subject to common disbelief, shortening the final drive does not give you more power from the engine.

This will remain a constant.

What changes is the power transmitted to the rear wheel in the same gear.

However, what you gain in power you lose in road speed. Given that maximum attained road speed in any given gear is what we are all after it almost defeats the point under maximum acceleration and this shows from the example given by the above member tjav8b when he raced another car.

So why is it that the members who have changed to a shorter final drive feel that it is an excellent upgrade? Simple. Due to the shorter final drive you are within the power band which produces more power more of the time which naturally gives you more torque plus the effects of the final drive acting as a torque multiplier.

The feeling is great because you are infact accelerating faster at any given point but each individual gear with the same rpm limit (stock 8250rpm) will achieve a lower terminal speed.

The nature of the overall package with the 3.91 becomes way more dramatic and instantaneous. The E34 M5 S38B38 has a similar power band (not as narrow as an S85 but still way narrower than an S62) as the S85. When we used to change the final drive on those the effect was stunning but as tests showed back then, you were not really getting anywhere any faster.

There is no question, the 3.91 will add a huge amount of fun to the daily drive and really add more G per in any given gear just so long as you all understand the above very important fact.... you are not going to go faster!

For 5.7's and other high powered S85's, it is better to stick to the stock final drive otherwise getting that power down will be near enough impossible and you will get tired of it. Gears 1 and 2 will become a little useless.

well then it seems you have the answer and I'm confused as to why you even created this thread

I'm not very mechanically inclined, but I've noticed improved acceleration/torque with my Diff. It may not make the car go any faster per say, but it sure seems to accelerate faster. It has its drawbacks too, such as decreased fuel economy, RPMs sitting higher, and top speed decreasing as well. But hey, I enjoy it.

Wulffza · 25th July 2012, 00:45

Since I haven't found a road in the US that allows me to run ungoverned to 200mph, I wouldn't miss the reduction in max speed in each gear, but like many, appreciate the extra grunt in every gear at accessible rpm on the street.

MIGHTY5 · 25th July 2012, 01:56

im interested in this upgrade as well what can i expect my max speed to be? how bad is the whine people mention? total install price and labor time would be what? sorry i just didnt feel like researching for a half hour to find this all out.