What ratio is my diff ?

[Bodyboarder]

Hi all …. Been looking into putting a lsd into my 2006 3.0si roadster na d would also like to look into a lower final drive to make it a bit more snappy on acceleration.

If anybody has any info on what I have at the moment and what I’d need I’d be grateful.

Thank you

[lucasxdiniz]

Real OEM shows the diff ratio.

[Bodyboarder]

Real OEM shows the diff ratio.

Oh great , thanks . Not got the vin to hand but I’ll check when I do . Looks like that link is for a coupe ( been told they are different?) and automatic.

[TheDan]

Made this when I was looking for an upgrade from my 3.07 stock diff (for LSD install):

Screenshot 2024-05-26 184158.png (60.83 KiB) Viewed 422 times

If yours is manual its 3.23, if yours it automatic its 3.64.

[Bodyboarder]

Made this when I was looking for an upgrade from my 3.07 stock diff (for LSD install):

Screenshot 2024-05-26 184158.png

If yours is manual its 3.23, if yours it automatic its 3.64.

Oh you flipping beauty !!

Guessing you’ve done your homework then on all things diff related !!

Can I pick your brains in what I’ll need ?

Looks like the easiest solution would be to buy a 3.64 diff and have an lsd fitted ? Keep mine to one side . Do you know if the automatic diffs are the same fitment / would work with my car ? Presume the M diff is different ( think I’ve read it’s mounted completely different ?)

Thank you

[TheDan]

Oh you flipping beauty !!

Guessing you’ve done your homework then on all things diff related !!

Can I pick your brains in what I’ll need ?

Looks like the easiest solution would be to buy a 3.64 diff and have an lsd fitted ? Keep mine to one side . Do you know if the automatic diffs are the same fitment / would work with my car ? Presume the M diff is different ( think I’ve read it’s mounted completely different ?)

Thank you

Of course, any time

In terms of fitments, excluding the M as that is a different kettle of fish, all E85/E86 housings will fit on all E85/E86 Z4s, meaning you can take a whole diff from a 2.5 and put it in a 3.0si. The main incompatibilities lie with the crown size and output shaft pattern.

Crown size (168K, 188K and 210/215) will affect the specific LSD fitment. Most major manufacturers (such as Quaife) will do different LSDs to fit different crown sizes, the main thing is to make sure you are buying the right LSD for the right diff.

Output shaft pattern, as far as I know there is only 1 E85/E86 variant that has a different diff output shaft, the E85 2.2i manual (might also be the case for the 2.2i auto, but not sure).

To summarise, yes you could just go ahead and buy any of the 3.64 diffs available (E85 2.5i auto, E85 3.0si auto, E86 3.0si auto). And yes the M is completely different.

[Bodyboarder]

Oh you flipping beauty !!

Guessing you’ve done your homework then on all things diff related !!

Can I pick your brains in what I’ll need ?

Looks like the easiest solution would be to buy a 3.64 diff and have an lsd fitted ? Keep mine to one side . Do you know if the automatic diffs are the same fitment / would work with my car ? Presume the M diff is different ( think I’ve read it’s mounted completely different ?)

Thank you

Of course, any time

In terms of fitments, excluding the M as that is a different kettle of fish, all E85/E86 housings will fit on all E85/E86 Z4s, meaning you can take a whole diff from a 2.5 and put it in a 3.0si. The main incompatibilities lie with the crown size and output shaft pattern.

Crown size (168K, 188K and 210/215) will affect the specific LSD fitment. Most major manufacturers (such as Quaife) will do different LSDs to fit different crown sizes, the main thing is to make sure you are buying the right LSD for the right diff.

Output shaft pattern, as far as I know there is only 1 E85/E86 variant that has a different diff output shaft, the E85 2.2i manual (might also be the case for the 2.2i auto, but not sure).

To summarise, yes you could just go ahead and buy any of the 3.64 diffs available (E85 2.5i auto, E85 3.0si auto, E86 3.0si auto). And yes the M is completely different.

Fantastic info and very much appreciated

I’ll start searching and speak to Birds when I’m back from holiday

[RMB]

@Thedan
So if I understand correctly a 3.0si manual will be pulling the lowest revs for any given speed in 6th gear, then the Z4MR, then closely followed by the Si auto, which will have the highest revs. Is that correct?
Can anyone please tell me what rpm these 3 cars will be pulling at 70mph?

[RMB]

Oh you flipping beauty !!

Guessing you’ve done your homework then on all things diff related !!

Can I pick your brains in what I’ll need ?

Looks like the easiest solution would be to buy a 3.64 diff and have an lsd fitted ? Keep mine to one side . Do you know if the automatic diffs are the same fitment / would work with my car ? Presume the M diff is different ( think I’ve read it’s mounted completely different ?)

Thank you

Of course, any time

In terms of fitments, excluding the M as that is a different kettle of fish, all E85/E86 housings will fit on all E85/E86 Z4s, meaning you can take a whole diff from a 2.5 and put it in a 3.0si. The main incompatibilities lie with the crown size and output shaft pattern.

Crown size (168K, 188K and 210/215) will affect the specific LSD fitment. Most major manufacturers (such as Quaife) will do different LSDs to fit different crown sizes, the main thing is to make sure you are buying the right LSD for the right diff.

Output shaft pattern, as far as I know there is only 1 E85/E86 variant that has a different diff output shaft, the E85 2.2i manual (might also be the case for the 2.2i auto, but not sure).

To summarise, yes you could just go ahead and buy any of the 3.64 diffs available (E85 2.5i auto, E85 3.0si auto, E86 3.0si auto). And yes the M is completely different.

So if I understand correctly a 3.0si manual will be pulling the lowest revs for any given speed in 6th gear, then the Z4MR, then closely followed by the Si auto, which will have the highest revs. Is that correct?
Can anyone please tell me what rpm these 3 cars will be pulling at 70mph?

[TheDan]

No, not quite. It all depends on gear ratios as well as the final drive ratio. The rule is larger ratio number, the higher the revs will be, for a given speed.

For example, my E85 3.0i manual has a 6th gear ratio of 0.851 with a (modded) final drive ratio of 3.46. The stock final drive ratio was 3.07. Using a simple online calculator such as this one: https://purperformance.com/p-29669-rpm-calculator.html, we can find the RPM at 70 MPH using the numbers above:

0.851 ratio 6th gear + 3.07 FD ratio (stock) would sit at 2457 RPM at 70 MPH

0.851 ratio 6th gear + 3.46 FD ratio (modded) would sit at 2770 RPM at 70 MPH

Simply put, its a linear relationship between the ratio and the RPM. If you increase the FD ratio by 10%, the RPM at 70 MPH would increase by 10%.