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Rear diff question... testing the waters...

2291 Views 36 Replies 13 Participants Last post by  PropellerHead
What kind of acceleration gains could I get by modding my rear diff? What if I put in a 3.73 or a 4.10? How much would this cost me?

I just got talked into test driving an '06 beautiful black/black M3 (with the competition package, too) at crevier BMW and I want it. :( Obviously, I can't afford a 62K car as a 21 year old student (although the salesperson swore I could :tsk: ) but I was wondering if I could get that "seat of the pants" feeling of shear acceleration out of my 540i with a new diff...?

Also, I don't really care about fuel economy or top end (I chicken out over 140ish anyhow) so, is a 3.73 or 4.10 LSD (or even open) right for me? Where could I get one? Thoughts people, thanks! :)
21 - 37 of 37 Posts
How do you come up with this number, 18.5% !?!?!?
AirFrcd said:
The ratio of the 2 gear ratios.

4.10 / 3.45 = 1.1884 = 18.84%

I'm just showing where he got it from. I'm not supporting it.
Ok, this is where the crazy number is coming from :)

Diff ratio changes do not increase horsepower.

Diff ratio changes do however change torque, you get higher torque multiplication BUT (and this is a bit BUT) for _shorter_ amounts of time because the shorter gears.

The result is an increase in the "butt dyno" because of the strong but short torque bursts but you are not getting a big increase in acceleration.

This has been discussed alot on m5board, see this thread for one: http://www.m5board.com/vbulletin/showthread.php?t=67150
franka said:
The ratio of the 2 gear ratios.

4.10 / 3.45 = 1.1884 = 18.84%

I'm just showing where he got it from. I'm not supporting it.
The result is an increase in the "butt dyno" because of the strong but short torque bursts but you are not getting a big increase in acceleration.[/url][/QUOTE]

A higher numerical diff ratio is going to improve acceleration.

An extreme example to make the point obvious would be to imagine starting a drag race in 2nd or 3rd gear. Dropping to 1st gear would yield the same type of improvement as going to a numerically higher diff ratio. It would improve acceleration.

Most drag racers run a 4.10 or 4.56 diff because it works.
A higher numerical diff ratio is going to improve acceleration.

An extreme example to make the point obvious would be to imagine starting a drag race in 2nd or 3rd gear. Dropping to 1st gear would yield the same type of improvement as going to a numerically higher diff ratio. It would improve acceleration.

Most drag racers run a 4.10 or 4.56 diff because it works.[/QUOTE]

I assume you are not talking about BMW 540/M5 and keep in mind that you cannot compare diff ratios between different cars because their gearbox ratios may be way different.

My point is this. I have not been able to get ANY hard data on performance improvement in 0-60 og 1/4 mile times with the change from the stock 3.15 diff to 3.45 or 3.64 in the M5. I have repeatedly asked people to show me that this mod does result in better times - nobody has been able to. And in the thread I pointed to earlier it is suggested that the cars with stock gearing 3.15 do better 1/4 mile times than the 3.45 ones.
franka said:
s62 said:
The result is an increase in the "butt dyno" because of the strong but short torque bursts but you are not getting a big increase in acceleration.[/url]
I never stated that a change in diff ratio increases HP :dunno:
However, and I quote Jim Blanton from Performance Gearing "... and changing your diff from a 3.45 to a LSD 4.10 in a 530 will result in a net gain in accel. of approx. 18.5%."
I have not gone to the track / strip (yet) to test my times - however from a daily driver perspective (yes, I do drive my car hard), I can tell you that the increase in accel. is SIGNIFICANTLY noticeable and an absolute blast to drive (especially when combined w. the s/c) :thumbup:
s62 said:
Ok, this is where the crazy number is coming from :)

Diff ratio changes do not increase horsepower.

Diff ratio changes do however change torque, you get higher torque multiplication BUT (and this is a bit BUT) for _shorter_ amounts of time because the shorter gears.

The result is an increase in the "butt dyno" because of the strong but short torque bursts but you are not getting a big increase in acceleration.

This has been discussed alot on m5board, see this thread for one: http://www.m5board.com/vbulletin/showthread.php?t=67150
Final drive changes only affects the torque multipication factor to the rear wheels. The engine still puts out the same amount of torque. So 3rd gear feels like 2nd and 2nd feels like 1st.

It may or may not be faster on the drag strip, but a higher final drive is definitely more fun on the streets.
Sorry, I did not mean to imply you said it increased HP - I was just stating that diff ratio changes have no effect on power output, just torque to the wheels.

I think it would be very interesting to have Mr. Blanton back up his claims with some hard empirical data. I think this just goes to teach people not to trust salesmen :)

I think it is just the SC that is giving you the quicker acceleration ;)
AirFrcd said:
I never stated that a change in diff ratio increases HP :dunno:
However, and I quote Jim Blanton from Performance Gearing "... and changing your diff from a 3.45 to a LSD 4.10 in a 530 will result in a net gain in accel. of approx. 18.5%."
I have not gone to the track / strip (yet) to test my times - however from a daily driver perspective (yes, I do drive my car hard), I can tell you that the increase in accel. is SIGNIFICANTLY noticeable and an absolute blast to drive (especially when combined w. the s/c) :thumbup:
Is he running a 4.5 0-60... that would be cool. :)
franka said:
That would take 1.5 seconds off your 0-60 time.
I think the main problem with an increase in 0-60 times with a diff change is that the increase in acceleration is offset by the fact you have to shift into third before you hit 60 mph. That's why the butt dyno registers faster. The car runs through the gears faster, but that extra shift ruins the 0-60. However, I'd bet on an improved 1/4 mile. :dunno: Just my $0.02.
s62 said:
I assume you are not talking about BMW 540/M5 and keep in mind that you cannot compare diff ratios between different cars because their gearbox ratios may be way different.

My point is this. I have not been able to get ANY hard data on performance improvement in 0-60 og 1/4 mile times with the change from the stock 3.15 diff to 3.45 or 3.64 in the M5. I have repeatedly asked people to show me that this mod does result in better times - nobody has been able to. And in the thread I pointed to earlier it is suggested that the cars with stock gearing 3.15 do better 1/4 mile times than the 3.45 ones.
Good point but that's not a good reason to not do a lower gear posi diff. (That's a double negative, well you know what I mean)
fm_illuminatus said:
I think the main problem with an increase in 0-60 times with a diff change is that the increase in acceleration is offset by the fact you have to shift into third before you hit 60 mph. That's why the butt dyno registers faster. The car runs through the gears faster, but that extra shift ruins the 0-60. However, I'd bet on an improved 1/4 mile. :dunno: Just my $0.02.
Keep in mind that Airfrcd's car is a 530, not a 540......our 540/740's come with either 2.93 or 3.15's.

Personally coming from a car that made first gear useless after changing ratios (91 MR2 Turbo) I'd never go anything above 3.64 N/A or 3.46 F/I in the 540/740 if you lookin to still remain streetable without making your car drive rediculously high RPM's on the freeway....just my 02.
AirFrcd said:
Depending on what your final drive is geared at now, let me tell you, upgrading my diff. from a 3.45 to a 4.10 LSD was by FAR the best mod that I ever performed to my 5'er :thumbup:
A net increase in accel of 18.5% is very noticeable, and a match made in heaven w. the forced induction (as the s/c kicks in only past 3K RPM's, and the shorter gears kick in as soon as you touch the skinny right pedal :D ).
Cost was approx. $3,600 from Jim Blanton at Performance Gearing - btw, he is an awesome guy to deal with, and the authority in BMW gearing. I highly recommend his work and services :)
wrong. the auto tranny is longer than the 6 speed tranny in the M5 and you need a custom driveshaft to fit between the auto tranny and the M5 dif. He wouldn't know the length of the driveshaft until after the dif was installed and doing him absolutely no good. Akin to asking what kind of underwear one should buy for a SuperModel.
franka said:
IF ... If the original question that Demetrioss asked was about fitting an M5 diff in his automatic car the answer should be no problem because he is just swaping out diffs. He already has the auto box and the correct length drive shaft.
Blanton's been doing Bimmer gears since before I started driving. He and Jim Rowe USED to be biz partners in Metric Mechanic from whom I bought my '02 tranny... well.. lessee.. 18 years ago now? Yipes. It developed a lil catch into 2nd 4 years after purchase and they replaced it with no questions from my mechanic. The man knows his gears.

Having said THAT, I bought my E39 Quaiffe from Jeff @ Koala. Great guy who also really knows his stuff. Since I wanted a quaiffe and he had 'em, he was my choice.
franka said:
Where does expertise come in?
Yes but he has the auto tranny in the car now. Is only swapping out diffs. Both diffs use the same case so it should be a simple bolt in using the existing drive shaft.

Am I missing the point here or are you?
PropellerHead said:
wrong. the auto tranny is longer than the 6 speed tranny in the M5 and you need a custom driveshaft to fit between the auto tranny and the M5 dif. He wouldn't know the length of the driveshaft until after the dif was installed and doing him absolutely no good. Akin to asking what kind of underwear one should buy for a SuperModel.
You are. Both difs are not the same. The M5 LSD dif is longer.

A longer auto tranny meets up to a longer auto driveshaft to meet up with a shorter auto (or peg leg six speed) dif.

It's been tried and failed by a former moderator here who S/C'ed his auto tranny 540 and wanted an M5 dif. He needed a cutom driveshaft and sold the dif instead.
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