Z4-forum.com

Out of interest, has anyone on here fitted a 6 speed box to a 2.5?

I'm finding my gearbox very short and wondered if a conversation to a 6 speed box was a solve!

Personally I find the 5-speed perfectly matches the torque characteristics of the 2.5 M54. 3k revs at 70 mph - spot on. Happy in 5th at 1500 revs and pulls well right through the remainder of the rev range. BMW put a 5-speed with the 2.5 for good reason, although I'm sure both the 3.0 M54 and the 2.0 models both had 6-speed boxes and would think that maybe the 3.0 box would fit but might not suit the smaller engines outputs?? Someone with more knowledge will be along soon.

My Zed is a 2.5i SMG. The gearbox is the same 6 speed manual fitted in the 3.0i. Perfect ratios for the 2.5i in my opinion although 6th is quite tall and I only really use it on dual carriageways/motorways. I'm not sure if the diff.ratio is the same for the 2.5i and 3.0i....maybe someone could clarify this.

I know what you mean OP.

My E46 is a 325ti so has the same engine and box as your Z4 (assuming it is a pre-facelift) so it also pulls 3,000 rpm at 70 mph.

This seems a bit high to me, but does have the advantage that if you want to pick up speed quickly on a motorway you don't really need to change down.

Having said that my nephew also had a 325ti, but his was a facelift model which got the 6-speed box and it was definitely more relaxed on motorways - more like 2,500 rpm at 70 mph IIRC.

I suppose in part it depends what you use it for - track days or B-roads blasts you're never going to need 6th!

Not sure if it applies to the pre facelift Zed but on the 330i E46 when Bmw went from fitting a 5 speed to a 6 speed as standard the ratio's were basically the same 1-5 with 6th giving a more relaxed motorway cruise and slightly better fuel economy on a run.
Might be worth finding the stats for the zed and putting them into one of the online gear/diff ratio calculators to see what you will actually get from changing.

6th in my coupe is 76mph on the motorway at 3,000rpm.

The ratios are slightly shorter on the coupe than the roadster though for better acceleration, I think it's the final drive that's different.

I always find this site useful:

http://www.bokchoys.com/differential/GearRatios.htm

I think generally gears 1-5 are the same, but then 6th adds an overdrive for cruising. Bear in mind also the 3.0 has a longer diff than the 2.5, which will affect how all the gears feel. and a diff swap is a relatively easy thing to do.

I think the bell housings are identical. And the engine architecture is the same. So yes it will fit no problem. I would imagine that they are the same length also. Worst case scenario you may need a 3.0l prop shaft as well.

^5 box is shorter in lenght, so def. a 3.0 propshaft needed.

Also a new clutch might be needed, the early 5 speeds are 9 spline and all 6 speeds (and later 5 speeds) are 22.
I dont know if that also requires a new dmf.

Steve84N wrote: ↑Fri Dec 15, 2017 7:56 am6th in my coupe is 76mph on the motorway at 3,000rpm.

The ratios are slightly shorter on the coupe than the roadster though for better acceleration, I think it's the final drive that's different.

That's interesting - is 76 mph based on GPS? My Coupe tends to be showing almost 80 mph at 3,000 rpm!

I'm running 245/40/18 which is a larger circumference.

Stock speedo over reads by around 2-3mph but with these tyres it's almost bang on against GPS.

Thanks for all the advice guys, really helpful info there.

I might look out for a six speed box to rebuild and gather the other bits up ready for a conversation.

I've got a couple of other priorities at the mo on my house so as soon as they are done I'll crack on.

Thanks again.

Steve84N wrote: ↑Sat Dec 16, 2017 9:54 am I'm running 245/40/18 which is a larger circumference.

Stock speedo over reads by around 2-3mph but with these tyres it's almost bang on against GPS.

NCS has some options for setting wheel size if the mileage is out.

Good to know, I won't be playing around with it though as the car will no longer be mine by next Wednesday.

Steve84N wrote: ↑Fri Dec 15, 2017 7:56 am 6th in my coupe is 76mph on the motorway at 3,000rpm.

The ratios are slightly shorter on the coupe than the roadster though for better acceleration, I think it's the final drive that's different.

6th in my SMG (3.0i) sits at 2500rpm at 70mph