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 8)

 

[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. :wink:

 

[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. :wink:

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. :wink:

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.

 

[Curtis]

My original diff ratio was 3.23 with no LSD. I installed a complete E46 M3 rear axle with 3.62 ratio and obviously an LSD. The extra grip is superb and the general accleration is slightly improved however my 0-60 time has been reduced because I now have to change up to 3rd before reaching 60 when before I could get there in 2nd. I've no doubt that my 0-70 time is reduced because I would have had to change to 3rd to reach 70 anyway.

 

[Jacobnbarber]

Will the z4m diff be a direct replacement for the standard one or does it require sub frame and drives shafts etc.

 

[pokeybritches]

It requires subframe, driveshaft, etc.

 

[Mr Whippy]

power = mass x speed x acceleration

So

power / ( mass x speed ) = acceleration


You can improve power figure by being near peak power longer which can mean closer gear ratios.

But then time during shifts is 0 power too, so add in another shift into an acceleration interval and you can lose your gains.

Also too low a first and second (and maybe higher) gear can mean loss of traction which is wasted power too.


For a track car you get ratios (not just FD) set to your corners, top speed etc.

For a road car it's better to choose a cruising gear and a 1st gear that just loses traction in good conditions.
Then stack the rest evenly for optimal acceleration.

I'd only ever lengthen a FD while adding more power on a road car.

 

[Ed Doe]

1. Simple mathematics dictates that reducing the final drive multiplies the torque to the road by the same factor, so yes it is literally a bolt on power adder - available torque at the wheels is higher. HOWEVER, as has already been said, a shorter final drive will result in very short gears, and ultimately higher rpms for the same speed in 6th gear. So there is room to modify it a bit, and you'll get a noticeable benefit in the small additional torque at the wheels, and a further benefit that the gears will be more closely stacked so you'll be in the powerband all the time, but if you go too mad you'll end up buzzing about at 5k rpm in 6th doing 60mph (I exaggerate but you get the point).

2. For the avoidance of doubt the MC/MR Final drive ratio is 3.62, the 3.0si Manual final drive is 3.46, the 3.0si Auto final drive is 3.64, the 3.0si Roadster is 3.23, and the 3.0i Roadster Manual is 3.07 and 3.0i Auto is 3.46. So hypothetically (without knowing the inner workings of the 3.0si auto boxes) there's plenty of scope to tear down an auto box as a donor unit and make up an oem shorter box with the auto final drive, if that's what you're shooting for?

3. Remember the M has another 80hp, but also has another ~1,200rpm to play with at the top end (redline 8,000rpm-ish vs Si soft-limiter at 6,700rpm-ish), so yes it has a shorter final drive but it's revving harder, so wheelspeeds are higher at the top of each gear too. Given the 3.0si with a standard rev limiter wont hit 60 in second until you're into the realms of the soft limiter, you're already using 3 gears to reach 60, to the M only using 2, so if you're fussed about that sort of thing the 0-60 comparison really is a non-starter....