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

2286 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! :)
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It Jim Blanton in my area? I did a gear calculation and I think I may just go with the 3.73, because the 4.10 cuts too far into my top end (brings the max below 140 I think, which mean really high revs in 5th.) I only have 5 gears, being an auto, so it's an issue.
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 :)
Thanks! :thumbup: That site is exactly what I need.
yadayadayada said:
Check out www.Diffsonline.com , the website explains the different setups and gives you a chart of gearing VS speed ..... Dan's builds are considered some of the best in the racing world....
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.
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