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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! :)
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Look in your Roundel and Bimmer magazine ads.

Metric Mechanic also has them I believe. They are a good source.
and the authority in BMW gearing [/QUOTE]

How's that?

There are 3 or 4 ratios to choose from. That's the end of it.

Where does expertise come in?
I recall reading they are a direct swap but expensive. Maybe someone here knows for sure.
Demetrios540i said:
could you use an m5 diff. ?
No problem. Same as a manual box.

Car would go thru the gears normally but top speed in each gear would be reduced.
Demetrios540i said:
also...how would it effect an automatic trans?
That would take 1.5 seconds off your 0-60 time.
AirFrcd 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.
MJTSUB said:
No the automatic is a different length than a manual transmission therefor you will need a custom driveshaft made to fit a M5 diff as there was never an automatic offered for the M5 hence there is no OEM part.
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.
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.
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.
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