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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > NA Engine (non-turbo) / Drivetrain / Exhaust Modifications > Differential Swap. Change Gearing.



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      11-08-2005, 02:32 AM   #1
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Differential Swap. Change Gearing.

The e90 325 has incredibly long gears, and as per my calculations, the gears are good for 280-290km/h, but I am sure the vehicle is drag limited.


The current gears are 3:23 in a 6MT, with a swap from a 330I automatic, to 3:64 would change the rpm range by 1500rpm.

meaning the top speed would be dropped to 260km/h, but the acceleration time and in the long runt he 1/4 mile would be increased significantly.

Any thoughts? or part numbers on the differential to see if its the same as a e46?
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      11-15-2005, 02:32 PM   #2
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sounds like a good idea if it can be done..
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      11-16-2005, 08:59 AM   #3
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Originally Posted by crex
sounds like a good idea if it can be done..

Especially if you want to void your warranty ! :rocks:
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      11-16-2005, 09:43 AM   #4
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E90Fleet posted somewhere that the E90's have different differentials than the E46. This has to do with a new type of bearing design. It would probably be easist to find a wrecked automatic. My question is where does BMW pick up the speedo reading?
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      11-16-2005, 07:03 PM   #5
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Quote:
Originally Posted by dmooreco
E90Fleet posted somewhere that the E90's have different differentials than the E46. This has to do with a new type of bearing design. It would probably be easist to find a wrecked automatic. My question is where does BMW pick up the speedo reading?

At the Differential, There is a speed sensor at the back of it.

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Originally Posted by Socom
Especially if you want to void your warranty ! :rocks:
Why would you even post this? It wouldnt void your warranty in any aspect, as what does it change besides final output? Nothing! They may only attempt to deny warranty claims on the rear driveshafts, assuming they realize there is a differently geared differential in their.
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      11-19-2005, 12:05 AM   #6
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I have been so busy that I havent posted, but I have performed a differential swap in my 325. I purchased a new (ouch) 3.63 from a 330 automatic. (Figured the 3.72 from the 325 auto was too much). It required some modifaction to the input shaft on the differential, namely, cutting part of it off.

E46 differential is a completely different shape. The new E60 5 series may share the same diff (it looks almost identical, would need the E90 diff cover though for the rear mounting point; which is cheap)

And there is NO speed sensor in the differential, or any other sort of sensor. Car reads speed from the ABS/DSC/etc. sensors.

How does it drive? Pretty damn short! It pulls like all hell, and makes 6th gear useful. I like it, however, if you disengage DSC and spin the rear wheels a little on lauch, you run wheel speeds out of the tire monitor specs and you get a warning light for no tire pressure.
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      11-19-2005, 01:07 AM   #7
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Quote:
Originally Posted by RichP
I have been so busy that I havent posted, but I have performed a differential swap in my 325. I purchased a new (ouch) 3.63 from a 330 automatic. (Figured the 3.72 from the 325 auto was too much). It required some modifaction to the input shaft on the differential, namely, cutting part of it off.

E46 differential is a completely different shape. The new E60 5 series may share the same diff (it looks almost identical, would need the E90 diff cover though for the rear mounting point; which is cheap)

And there is NO speed sensor in the differential, or any other sort of sensor. Car reads speed from the ABS/DSC/etc. sensors.

How does it drive? Pretty damn short! It pulls like all hell, and makes 6th gear useful. I like it, however, if you disengage DSC and spin the rear wheels a little on lauch, you run wheel speeds out of the tire monitor specs and you get a warning light for no tire pressure.
That's awesome Rich! First diff swap I've heard of. Maybe I can get a ride in that thing to see the difference come summer when it's warm out and there's actual traction.
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      11-19-2005, 05:18 AM   #8
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Rich, how much did the differential end up costing you all in all? any 1/4 mile or dyno numbers?
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      11-19-2005, 06:57 AM   #9
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Originally Posted by Guess
Rich, how much did the differential end up costing you all in all? any 1/4 mile or dyno numbers?
I hope your not expecting max hp gains from a diff swap. :
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      11-19-2005, 08:49 AM   #10
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Quote:
Originally Posted by Tierfreund
I hope your not expecting max hp gains from a diff swap. :

A diff regear will affect the torque at the wheels, but will have a minimal affect on whp, but still should (2-3whp difference as the engine is under less strain with lower gearing)

Torque at the wheels is equal to engine torque, minus drivetrain losses, times gearing, therefore the wtq will be greatly affected by a differential swap.
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      11-19-2005, 04:07 PM   #11
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It shouldnt make any more horsepower whatsoever. However, it makes 12.3% more torque at the ground. (I dont feel like doing the math for how many 1000 ft/lbs it is in first, second, etc)

I got mine for about 1000.
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      11-19-2005, 04:55 PM   #12
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Quote:
Originally Posted by RichP
It shouldnt make any more horsepower whatsoever. However, it makes 12.3% more torque at the ground. (I dont feel like doing the math for how many 1000 ft/lbs it is in first, second, etc)

I got mine for about 1000.
It should free up horsepower that is already there, but is lost in the driveline. With shorter gearing, the engine is doing less work, as it has more to work with, therefore a few extra whp should be there simply because there is less driveline loss.
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      11-20-2005, 07:37 AM   #13
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Quote:
Originally Posted by Guess
It should free up horsepower that is already there, but is lost in the driveline. With shorter gearing, the engine is doing less work, as it has more to work with, therefore a few extra whp should be there simply because there is less driveline loss.
That is nonsense.
A different ratio in the diff will not change the driveline loss if both diffs have similar resistance (similar efficiency). A shorter diff does not have less resistance per se.

If all you are chagning is the ratio of the diff, then all that changes is the torque to the wheel at the expense of a shorter gearing (the higher torque coming in at lower wheel revs). Max hp stays unaffected. Since it is a linear equation, the power (torque times revs, in this case both at the wheel) stays the same.

"Freeing up horsepower that is allready there" is the kind of esoteric voodoo stament that wannabe performance modders talk about with useless chilidsh modifications such as so called CAIs. Its the kind of language people slip into when they have no idea of the mathematics involved.
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      11-20-2005, 11:57 AM   #14
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Quote:
Originally Posted by Tierfreund
That is nonsense.
A different ratio in the diff will not change the driveline loss if both diffs have similar resistance (similar efficiency). A shorter diff does not have less resistance per se.

If all you are chagning is the ratio of the diff, then all that changes is the torque to the wheel at the expense of a shorter gearing (the higher torque coming in at lower wheel revs). Max hp stays unaffected. Since it is a linear equation, the power (torque times revs, in this case both at the wheel) stays the same.

"Freeing up horsepower that is allready there" is the kind of esoteric voodoo stament that wannabe performance modders talk about with useless chilidsh modifications such as so called CAIs. Its the kind of language people slip into when they have no idea of the mathematics involved.

You are a tool for calling my childish, how many useful posts have I seen come out of you? All you do is be an asshole towards anyone on this board trying to ask questions and actually explore further possiblities. Note how I am the only one that has done an independant wheel dyno? I am the only one on this board that is actually attempting to provide the other readers with factual information by providing them with dyno's.

I have a 92 YJ with a regear from 3:07 to 4:10, and a buddy of mine has a 318i with a regear from 3:42 to 4:44, both vehicles saw the dyno, and the 318i actually had an increase of 5.5whp, and 33wtq.

So I suggest you re-check your math champ. With shorter gears, the engine has to do less work to actually get the engine going, in a 4th gear pull, the engine has to do less work to get to 7000rpm.
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      11-20-2005, 12:26 PM   #15
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Well tierfreund is right, the engine doesnt produce anymore power with a change in the diff, all its doing is changing where in the power the torque is being fed. So with the different gearing, your friend will see a 33ft-lbs increase in torque at a different place in the rev-band but will see a 33ft-lbs decrease somewhere else in the rev-band. a 5hp increase in HP is not statistically significant, its considered normal varience.

All in all, changing the diff is basicly a shift of torque from one place to another. So if you want a car that will get up and jump off the line, then it would be worth it to change the gearing and sacrifice high speed abilities for off the line speed. But if you are someone who needs power up top then shorter gearing would be useless for you.

There is no extra power or "driveline loss" being gained at all.... You just feel like the car is alot faster because its accelerating faster off the line. But try accelerating from 100-->120 and ure car will be a slug. Its all a question of what you want the car to do.

For example, back when the McLarenF1 came out and they ran the car up to 241mph for a top speed record, the wanted to then show the car had a really awesome 0-60 time, well with the long gearing needed to achieve 241mph, it was impossible to get any kind of shattering 0-60 time. So they went and pulled out the diff, changed it for a shorter one and ran it again to get the really low 0-60 time. No extra power is being created by the engine, the diff is just redistributing the torque along the powerband.
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      11-20-2005, 06:06 PM   #16
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If gearing affected horsepower output, then dynoing a car in different gears would show dramatically different power outputs. But it doesnt. (as we all know)

That being said, if you want some serious g forces at the expense of more shifts, diff is very nice. I enjoy mine, especially in NYC traffic, where it really shines.
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      11-20-2005, 06:46 PM   #17
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Quote:
Originally Posted by Guess
You are a tool for calling my childish, how many useful posts have I seen come out of you? All you do is be an asshole towards anyone on this board trying to ask questions and actually explore further possiblities. Note how I am the only one that has done an independant wheel dyno? I am the only one on this board that is actually attempting to provide the other readers with factual information by providing them with dyno's.

I have a 92 YJ with a regear from 3:07 to 4:10, and a buddy of mine has a 318i with a regear from 3:42 to 4:44, both vehicles saw the dyno, and the 318i actually had an increase of 5.5whp, and 33wtq.

So I suggest you re-check your math champ. With shorter gears, the engine has to do less work to actually get the engine going, in a 4th gear pull, the engine has to do less work to get to 7000rpm.

You cant provide any serious explanation so you resort to calling names and dyno numbers? 5 hp difference between dyno runs is way within the tolerances of even the best dynos with professional operators.
Not very convincing.

You have a dyno, fine, helpfull. I have an engineering background. For the nature of this discussion I find an understanding of engineering, physics and mathematics more helpfull.

Let me explain something:

Maximum power is defined as torque times revolutions. So an engine maximum power is the product of torque at the crank times the revs.

As for maximum wheel power the formula is the same. Torque times revolutions. So maximum wheel power is the product of wheel torque times revolutions.

Now by changing the diff gearing by any percentage (let say 10% for example) acording to Newton you are increasing the wheel torque by exactly the percentage your gearing is shorter. You are at the same time lowering the revolutions of the wheel though by the very exact same percentage (again 10%)

Now, since Power = torque*rev

with the shorter gearing you get Power = (original torque*1,10)*(original revs/1,10) = original torque*1,10*original revs/1,10

Elementary mathematics allows us to cancel out the 1,10 from the denominator and the enumerator leaving us with

power= original torque*original revs

Sound familiar? Changing the gearing does not change max power in any way. It changes the wheel pwoer at certain wheel speed because the different gearing will put you in a different part of the engines power band at that certain wheel speed. But you dont get any difference in max power at the wheel. Allways under the pretext that the diffs are only different in gearing, not in efficiency. A better diff with better (less resistance) running gear may reduce drivetrain losses. But even then the engines power output does not change but simply the losses on the way to the wheel are minimized.
But with the diffs on the E90 it will be difficult to fine one with better efficiency since the E90 has the latest generation off low loss diffs.


As for usefull posts, Im sure if you bother to check mine youll find that I try the best I can to provide as much usefull information as I can. And in some areas Im sure I can. Im also very carefull not to provide false information. When Im not sure of something Ill stay quiet or state an opinion and mark it as such. Im also not afraid to call when I see it. Im not intimidated by loudness.
Then again, Im mature enough to admit when Im wrong. So provide me with a serious explanation as to why a diff change would yield an increase in overall hp (youre not actually explaining so far, just stating) and Ill listen.

As for your post, while mostly friendly and willing to be helpfull, Im calling you on something you are stating as a fact which I know to be wrong. (Within the paramters I stated, two diffs that only differ in gearing, not in anything else)

To my dismay e90pot.com which started out as a very classy board with lots of usefull information is slowly becoming the E90 version of e46fanatics.com. The good people and the good info is still there, but its becoming clogged with more and more less helpfull posts.
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      12-04-2005, 07:35 PM   #18
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so if you have a 330i 6mt what diff would you put in it ?
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      12-05-2005, 02:59 AM   #19
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Quote:
Originally Posted by crex
so if you have a 330i 6mt what diff would you put in it ?
The one from the step 330. Unless you want a LSD which would make a lot more sense. Then youll have to go aftermarket, but Im not sure if anyone is offering a LSD for the E90 yet.

Besides, please remeber, a shorter diff will not make your car faster overall. Itll only improve ingear acceleration. Youll have to shift more often though while accelerating to a certain speed.
A shorter diff really is only usefull if you derive lots of pleasure from hard acceleration. If youre into really fast and fluid driving or even tracking, a shorter diff may be counterproductive (depending on the track layout)
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      12-05-2005, 01:02 PM   #20
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Quote:
Originally Posted by Tierfreund
... a shorter diff may be counterproductive (depending on the track layout)
I had a fleeting thought to put in a longer gear, taking advantage of the wide torque band for 'normal' driving, with less shifting. I know, it's sacrilege, and I'd never do it. I do occasionally skip 2nd if I'm just getting up to a 30 mph speed limit.
Is this car really drag limited at the top end? I am just thinking that that the ratio is matched to the drag limit fairly closely.

Just talking here, please don't flame me too bad.
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      12-05-2005, 06:18 PM   #21
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Quote:
Originally Posted by Guess
At the Differential, There is a speed sensor at the back of it.



Why would you even post this? It wouldnt void your warranty in any aspect, as what does it change besides final output? Nothing! They may only attempt to deny warranty claims on the rear driveshafts, assuming they realize there is a differently geared differential in their.
I post it because regardless of how innocent a modification you do, it can open you to warranty issues. If BMWNA wants to find a cause and effect they will. You just never know.
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      12-05-2005, 06:20 PM   #22
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If you want to see what a lsd does to a good car:

http://www.e90post.com/forums/showthread.php?t=8748

It's a Drexler lsd (0-100% just like the M3 E46 etc)

Makes the car mor fun to drive. I'm thinking about buying it for my 330i next year.But it has some disadvantages in the connection to the DTC/DSC electronical units when they are normally switched on: The DTC/DSC interference is much faster/heavier than normal. So you have to drive always with those systems turned of or get used to power-loss when flooring it...
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