Swapping 540i rear diff

[untitled]

Hi, I have a 2002 540i automatic non-sport, which I believe has a 2.81 open diff. I am contemplating with swapping it out with a different ratio? Would 3.46 be a good ratio? How much difference would I expect? Is a 3.64 too much? Would I be losing a great amount of top speed, or would I still be able to reach the 155mph limiter?

Lastly, should I get an open diff or and LSD (quaife). I've heard some stories about quaifes loosing grip when a tire lifts, but could it be any worse than an open diff? The LSD costs almost twice as much as the open diff so would it be worth the cost?

Any help would be appreciated.

[JAlfredPrufrock]

3.64 would be pretty short... might want to look at a 3.38 to maintain more of a highway feel which the 5er is known for.

As for open diff compared to LSD, how much money do you have and what do you plan on doing with the car? If cost is no issue, get the LSD.

[untitled]

How big of a difference would a 3.38 make compared to my 2.81?

[cleoent]

Probably a huge difference. I went from a 2.81 to an open 3.15 (the same as what comes on the sport models) and the differences in acceleration was NIGHT AND DAY. The car feels soooo much faster now and my gas mileage hasn't even taken a hit. Best mod ever.

[Lscman]

Probably a huge difference. I went from a 2.81 to an open 3.15 (the same as what comes on the sport models) and the differences in acceleration was NIGHT AND DAY. The car feels soooo much faster now and my gas mileage hasn't even taken a hit. Best mod ever.

Gas mileage should not drop unless you're seeing sustained highway use above 50 MPH. At 70 MPH, expect your mileage to drop a noticeable amount...like around 10-15%. It's not quite like disabling 6th gear, but it's a significant change. In the city-suburban situations, gas mileage won't be affected much. You'll simply be upshifting into a higher gear sooner and avg rev's will be about the same.

A deeper gearset definitely helps acceleration on auto tranny cars with modest gear ratios.

The same does not always hold true for 6spd cars. For 6spd cars, the car will feel like it accelerates faster. Well it DOES, but the only fly in the ointment is...you're going slower in the same gear. Changes in the seat of the pants accelerometer does not always equate to gains in 1/4 mile or 0-60. The Getrag already has a very steep first gear ratio (4.00+) that would seem better suited to a turbo or low torque 4 or 6 cyl motor. As a benchmark, many ponycars have a first gear tranny ratio closer to 3.00. For this reason, you do not need a deep diff gear for hard launching.

[Skipper5]

How big of a difference would a 3.38 make compared to my 2.81?

2.81-->3.15 = 12.1%
2.81-->3.38 = 20.2%

[humanoid]

At least a 3.45 for a auto 540.
http://www.dinan540.com/STUF/SPECS.jpg

[untitled]

How did you raise the redline to 6500? Can this be done by a software flash?

[Skipper5]

^^^
the 6500 redline is because of the DINAN software flash. other companies provide this type of off the shelf engine tuning as well.

[bimmer540wsubs]

My 540iA Sport has a 3:15???? Hmmm. here comes a NOOOOOOOOB..... what does the different differencial do?? I kno it has to do w/top speed, quickness and gas mileage....

[humanoid]

My 540iA Sport has a 3:15???? Hmmm. here comes a NOOOOOOOOB..... what does the different differencial do?? I kno it has to do w/top speed, quickness and gas mileage....


Changing differential ratios will increase acceleration. It will also reduce top speed. Your cruising rpm will be higher.


Read all about differentials here... (http://www.koalamotorsport.com/services/bmw_differentials.htm)

[Skipper5]

Changing differential ratios will increase acceleration. It will also reduce top speed. Your cruising rpm will be higher.


Read all about differentials here... (http://www.koalamotorsport.com/services/bmw_differentials.htm)

don't confuse him by telling him his top speed will be reduced! though somewhat true, i don't think it's a concern for anyone doing this diff ratio change. as shown above, the dinan car with the 3.45 final drive "can" still go 176.6 mph at 6500 rpm.

with a 2.81 the same car would have a theoretical top speed of 216 mph at 6500 rpm.

also, iirc i dont think a stock 540 will go 176 mph due to power<drag

so if lowering (theoretical) top speed from 216 to 176 is an actual problem for you, i'd like to meet you and go for a ride :D

[humanoid]

don't confuse him by telling him his top speed will be reduced! though somewhat true, i don't think it's a concern for anyone doing this diff ratio change. as shown above, the dinan car with the 3.45 final drive "can" still go 176.6 mph at 6500 rpm.

:D

:evil2 Just stating the facts is all. It easy to get somewhat confused by the torque multiplication going on with gearing changes.

Depending on what gearing you choose it could be a concern, like a 4:10 diff would greatly, reduce your top speed but excellent for the 1/4 mile guys. Just have to do the math and see what works for you.

I posted the Dinan info because it pretty much spells it out in each gear with the rpm. Hoped to make it less confusing a bit more understandable :buttrock

[Skipper5]

its all good humanoid. i know you know your stuff cause i've been reading your posts for a good 3 years or something, but its the other owners out there i'm concerned with.

the dinan info was very helpful to see the comparison side by side like that. great post there for everyone to read, me included.

hehehe, the 4.10 would be a great 1/4 mile add on for the e39. theoretical top speed would drop to a "low" 148 mph.

1st gear red line: 33 mph
2nd gear red line: 54 mph
3rd gear red line: 78 mph
4th gear red line: 118 mph
*for the dinan car with red line at 6500

:thumbup: