3.0 Si rear diff/ LSD swap ?

[Bimmerbri]

Any 3.0sI owners out there perform or consider performing the rear differential upgrade to a 3.46 w/ the limited slip differential. I have read on a couple of threads that this would increase the performance dramatically and bring the 0-60 and 1/4 mile times closer to, if not faster than the M/// version.
3.46 Rear differential

Limited slip diff.


I would realy be interested to see if anyone has done this swap and what kind of performance differance it has made to your car.

Regards,

[WLH]

It's a modification that I have interest in...

[inTgr8r]

Any 3.0sI owners out there perform or consider performing the rear differential upgrade to a 3.46 w/ the limited slip differential. I have read on a couple of threads that this would increase the performance dramatically and bring the 0-60 and 1/4 mile times closer to, if not faster than the M/// version.
3.46 Rear differential

I would realy be interested to see if anyone has done this swap and what kind of performance differance it has made to your car.

Regards,

IIRC, the M is using 3.62 final.
How is 3.46 going to make you faster? (I know the 3.0si wheels are smaller)

That said a diff change can be one of the best performance mods you could do on a 3.0si
I've done it on other cars and it can be a very dramatic change.

What sources & $$ have you found for diffs?

[ay8306]

Slightly off topic - what is the stock 3.0si gear ratio?

[Bimmerbri]

Any 3.0sI owners out there perform or consider performing the rear differential upgrade to a 3.46 w/ the limited slip differential. I have read on a couple of threads that this would increase the performance dramatically and bring the 0-60 and 1/4 mile times closer to, if not faster than the M/// version.
3.46 Rear differential

I would realy be interested to see if anyone has done this swap and what kind of performance differance it has made to your car.

Regards,

IIRC, the M is using 3.62 final.
How is 3.46 going to make you faster? (I know the 3.0si wheels are smaller)

That said a diff change can be one of the best performance mods you could do on a 3.0si
I've done it on other cars and it can be a very dramatic change.

What sources & $$ have you found for diffs?

I am just going on the theory that BMW gave the 3.0i the 3.38 rear diff. and the 3.0si a 3.07;I think the si got the lesser diff because with a 3.38 (or even better, 3.46) its 0-60 numbers would be too close to the Z4M. Make any sense?

As for pricing, Turner motor sports offers the LSD for $2149 and the 3.46 rear Diff for $1295. IMHO, not bad for the rear diff if you can afford to do the work yourself

[ay8306]

I am just going on the theory that BMW gave the 3.0i the 3.38 rear diff. and the 3.0si a 3.07;I think the si got the lesser diff because with a 3.38 (or even better, 3.46) its 0-60 numbers would be too close to the Z4M. Make any sense?

I have a hard time believing that. I am also somewhat skeptical of gear ration change making up for the 80+ HP that the ///M has over the si.

While it is generally true that higher final drive ratio’s tend to give you better acceleration, the selection of the optimum final drive ration is a more complex balancing act involving torque curve/power band, transmission gear ratios, shift points, fuel economy, drivability, etc. In some cases, going to a higher ratio can even hurt 0-60 and 1/4-mile times by pushing another shift point down into your run.

[ay8306]

BTW, the bigger benefit from this swap is the limited slip behavior – not the final drive ratio. The LSD lets you get the power to the ground and can dramatically improve performance in aggressive cornering situations.

[inTgr8r]

I am just going on the theory that BMW gave the 3.0i the 3.38 rear diff. and the 3.0si a 3.07;I think the si got the lesser diff because with a 3.38 (or even better, 3.46) its 0-60 numbers would be too close to the Z4M. Make any sense?

I have a hard time believing that. I am also somewhat skeptical of gear ration change making up for the 80+ HP that the ///M has over the si.

While it is generally true that higher final drive ratio’s tend to give you better acceleration, the selection of the optimum final drive ration is a more complex balancing act involving torque curve/power band, transmission gear ratios, shift points, fuel economy, drivability, etc. In some cases, going to a higher ratio can even hurt 0-60 and 1/4-mile times by pushing another shift point down into your run.

^ very true.
You will definitely feel faster at launch, but it's very complicated process.

Check out this thread on Bimmerfest.
There's a spreadsheet that you can download to do the math for you. (you'll need all your ratios to fill in)

[Bimmerbri]

I was just throwing a theory out there. That is why I was curious to see if anyone has done the swap and to see what type of performance gains it brought. I am not trying to ruffle the feathers of you M/// folks.

[plowy]

messing with ratios is a very technical subject and best you know what it's all about proper before spending big bucks, on a similar note I messed with the gearing on my CBR600RR by changing the front cog going down 1/2/3 teeth, in the extreme going down 3 teeth meant the bike accalerated like a bas turd but at 110mph it was reving it's nuts off flat out and to that effect not really road rideable, great for the drag strip, all this diff thing is the same, the trick is to find the happy medium if your using the car on the road...

[ay8306]

I am not trying to ruffle the feathers of you M/// folks.

No feathers ruffled here ... I drive a 3.0si myeslf

[ay8306]

BTW ... gear ratios aside, I think that the LSD itself is a very worthwhile mod to consider (if I only had the spare cash)! I was very surprised to learn that the non ///M Z4 did not have LSD stock from the factory.

[azroadie]

I was considering a gear change/rear diff. change in my 2.5i. I wouldn't do it until I had the supercharger installed. So I too would love to hear some pro/cons on this topic.

[rc'Z]

I don't believe there is any such thing as a dramatic acceleration change achieved by a single bolt on change, other than nitrous oxygen. The most noticed dramatic change is through your bill fold and your butt dyno. Road and Track Sept 06, Z4 M Coupe: 0-60 4.9, 1/4 mile 13.5. Motor Trend reports same 0-60 and 13.4 1/4 for the M Coupe. Motor Trend 2007 Z4 Si: 0 - 60 5.1, 1/4 mile 13.8. Same figures for the 06. One bolt on change is not going to make up the technical goodies of the M from the factory. It only works in a dreamers world. No offense to anyone.

[Bimmerbri]

I am not trying to ruffle the feathers of you M/// folks.

No feathers ruffled here ... I drive a 3.0si myeslf

I should of gussed by the avatar. I know what you mean about the LSD, I think they stopped using them on amything but the M/// models back in 85 from what i understand. It really would be nice to have both wheels bite.