Swapping a 5-speed box with a 6-speed?

[Sidewaze Samm]

Been thinking of the possibility, and plausibility, of replacing the 5-speed gearbox in a 2005 2.5 with a 6-speed box from a 3.0. Has anyone actually done this? An old thread makes promising noises https://z4-forum.com/forum/viewtopic.php?t=41454

There's no necessity for me to do it, no problems at all (other than a seeping selector shaft seal) - it's just that on the motorway the car feels like it needs an overdrive. If it was good enough for the pre-facelift 3.0, then why not the pre-facelift 2.5?

The gear ratios are either the same, or minimally different (looks minimal to me anyway); the diff ratios a little more :idunno:
gear 2.5i _ 3.0i _ 3.0i N52
1st - 4.23 _ 4.35 _ 4.32
2nd - 2.52 _ 2.5 _ 2.46
3rd - 1.66 _ 1.66 _ 1.66
4th - 1.22 _ 1.23 _ 1.23
5th - 1.0 _ 1.0 _ 1.0
6th - n/a _ 0.85 _ 0.85
Diff - 3.46 _ 3.07 _ 3.38
Using a couple of online gear ratio calculators ( http://www.tremec.com/calculadora.php and http://johnmaherracing.com/calculators/gear-ratio-calculator/ ) it seems there could be no real-world difference in swapping to a 3.0 6-speed box with the original 2.5 diff.
But at 70mph, the engine could be doing just 2800rpm rather than 3250rpm. And at 3500rpm, rather than the car's speed being 75mph in 5th, it would be 88mph in 6th. Now that sounds like a more comfortable way to cruise distances.

There's a few questions to ask (as I don't know what I'm talking about )

Where is the speedo drive taken from?
I understand the 5-speed is a Getrag, and the 6-speed is ZF. What are the implications of this for dimensions, bell housing, prop shaft, gear selector etc?
Must the 6-speed box come from a pre-facelift car with M54 engine, or can it be from a later N52 model, either 3.0 or 2.5?
A post in the old thread says the proposhaft and clutch will need swapping as not compatible. Any comments on this?

This isn't a job I'd even dream of doing myself for one minute - it's one for Mr DLV (hi Mike ) I'm just trying to establish whether there's any mileage in it. So if anyone has done this or actually knows how feasible it is, understands what needs to be considered or can point out something I should have thought of, please comment.

cheers

EDIT: to include ratios of box from 3.0i N52 engine

 

[dans6490]

Whilst understanding why you’d want 6 gears, I can’t imagine the cost involved would be anything near to being worth doing.

It will also contradict the log book and may put off potential buyers if you decide to sell in the future.

Surely your best bet would be to sell yours and buy a 6 speed?

 

[Sidewaze Samm]

Surely your best bet would be to sell yours and buy a 6 speed?

But not so easy to replace a car that has decent spec, service history, only 2 previous owners and just 50k miles.

 

[brillomaster]

how about keeping the gearbox, but changing the final drive in the diff? thats easily done, secondhand open diffs are easy to come by too.

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

is a good resource if you didnt already know about it.

 

[Sidewaze Samm]

how about keeping the gearbox, but changing the final drive in the diff? thats easily done, secondhand open diffs are easy to come by too

I thought of that - at 70mph the rpm would be 2900 (compared with 2800 for 6sp box and std diff, and 3250 for the standard setup).
And at 3500rpm, the speed would be 84mph (compared to 88mph for the 6sp box and std diff, and 75mph for the standard car).

So, on the face of it not much difference between swapping the gearbox to a 6-speed and the easier/cheaper(?) option of just swapping the diff.
But - and quite happy to be corrected if I've got this wrong - I think the diff option, keeping the 5-speed box, would mean in-gear performance would suffer and a lot more gear changes would be needed to keep the performance level up.

 

[GuidoK]

I wouldnt change the diff ratio on that car. You'll get long gears which will make the car less exciting to drive. Unless you do something to get more torque in the low/midrange like a twinscrew supercharger so you can go through the gears more easily.
The 6 speed box is the main thing missing on a 2.5i. On the motorway and A road cruising speeds it adds comfort and far better mileage.

 

[Scooba_Steve]

I could understand the later 6 speeds not fitting but would have thought the mounting points were the same for the M54 versions. If it's needs a different clutch surely you'd be changing it while the box was off anyway?

 

[buzyg]

Changing the diff/final drive sounds more sensible to me. The reason you are doing this is for fuel economy and comfort. It will still be a quickish sports car, just a tad more relaxed, If your not concerned with those keep it as is. :?

 

[Sidewaze Samm]

Changing the diff/final drive sounds more sensible to me. The reason you are doing this is for fuel economy and comfort. It will still be a quickish sports car, just a tad more relaxed, If your not concerned with those keep it as is. :?

Fuel economy is just a bonus. The sole reason is for more refined high speed cruising. Still want to keep the performance through the gears and I think changing the diff will blunt this too much