Diff advice

[Aulday]

Hi folks

I've been reading a lot about diff swaps on here, but need a bit of concrete info so I don't look at buying the wrong thing!

I have a 2004 E85 2.5i manual.

Can someone tell me which diff ratio I have, which diff ratios for faster acceleration are available to me, and on which model/variant vehicles I will find such diffs, so I can search the online breaker yards?

Thank you!

Tom

 

[TheDan]

According to this site (http://www.bokchoys.com/differential/GearRatios.htm) you have a 3.46 diff ratio. However, pinch of salt here, because I have heard of the 3.23 ratio diffs are in the 2.5i Z4. Hopefully someone with a 2.5i will be along shortly to tell you exactly which diff ratio you will have.

Its very common for people with the 3.0i (myself) to switch from 3.07 to 3.46 to effectively shorten each gear. If you were to do the same you can go for the next most common sizes up which would be 3.64, 3.73 or 3.91. The link above tells you what diffs are on which vehicles, but bear in mind it us for US vehicles and might not be the most accurate in the UK. Please check the list, then double check before buying a second hand diff. Ebay tends to be a good place to search.

Furthermore, if you are planning to change internals such as adding an LSD you need to make sure the diff type is the same. I believe they are BMW type 188 (sometimes 188K, 188L).

For example, some time in the future I plan to switch to a 3.38, 3.46 or 3.64 diff ratio meaning I could potentially source from the following: E36 and E46 3-series, X83 X3s and E85/E86 Z4s.

 

[Aulday]

Great info thanks! Think I'd go full beans to the 3.91, I wonder if I can see from looking at the diff if it's printed on the housing

 

[TheDan]

Its on the sticker on the top side of the diff usually. Here is example 3.38 one from an E46:



Adds a little peace of mind when buying.

 

[Aulday]

So do I need to remove it to read the sticker?

 

[Aulday]

This UK listing states 3.23 for a UK 5 spd E85 manual 2.5i

From what I gather, most people go +0.5 for an acceleration upgrade, anymore might be too much for 1st and 5th gear. So maybe the 3.73 is a good choice

https://www.ebay.co.uk/itm/BMW-Z4-Series-E85-2-5i-M54-Roadster-Rear-Differential-Diff-3-23-7514149-WARRANTY-/264137581104?_trksid=p2349526.m4383.l4275.c4

 

[Aulday]

Plenty of 5 series 3.73 diffs going. Next question is are they a direct swap?

 

[Aulday]

Nah they look different!

 

[TheDan]

No I don't think the 5 series diff fits the Z4. I believe it its only the cars in the table on the website I sent. Even then a few of those will not fit the standard Z4 such as the Z4M and the M3. Best bet is E46 (minus M3) as I am pretty sure they share a rear subframe so are a direct swap.

Also if you are struggling to decide what ratio diff to change to I'd suggest opening a couple of tabs of this (https://www.boosttown.com/gearbox_differential/speed_calculator.php), put it your own settings on one tab and the new diff ratio on the other tab and compare max speeds in each gear. It will give you an idea of what effect the diff ratio change will have.

 

[Aulday]

Nice one mate thank you!

 

[z4_fella]

hi all iv got 2.5i non m 5 speed lov it. i want a 3.38 diff as i believe the 3.47diff dont work to well with the 5speed. but can anyone help me point me it the right direction is it a straight swap from the 2.0 non m or do i need 2.2 3.38 diff like rocking horse s**t to find will that be straight swap?