M52 w/E39 gearbox - what diff ratio?

[E21noob]

For my M52 transplant, I am planning to use the gearbox that comes with the E39 donor (S5D 250G). Apart from the engine and gearbox, everything else will be E21 parts.

I have my eyes on a good condition, reasonably priced (though expensive) E21 LSD, and trying to find out what ratio I will need when using the M52 gearbox.

Can anyone help?

[pandemonium001]

westfield-world.com/__shifts.xls

Google for westfield gear ratio shift point.

[nas80]

as Martin says go online - find your gearbox and its ratios and then play with final drive values on an online gearing calculator

[E21noob]

Thanks guys.

Found the calculator: http://westfield-world.com/gearboxshifts.html

and the S5D 250G ratios: http://content.yudu.com/Library/A1pyv7/ ... es/569.htm

1: 4,23
2: 2,52
3: 1,66
4: 1,22
5: 1,00
Final drive: 3,15

Unfortunately I don't have the skill to make a solid conclusions based on this..

A higher ratio will give more accelleration/quicker shifts, but a lower top speed ("short-legged") whilst a lower number will give "longer legs". Right? Where do I want to be for a well balanced setup?

[Mihajlo]

If you are thinking of leaving standard E21 diff housing, you are limited to 3.45, 3.91 and 4.1 final diff ratios

based on 195/50/15 tyres and max engine speed, my calculation is this

3.45


3.91


4.1


non standard 3.15

[Mihajlo]

If I were you, I would go for 3.45 diff because feeling is something like having E21 dogleg gearbox

this is gearbox ratios


3.45 with e21 sport gearbox (dogleg)


this is standard E21 323i setup

[pandemonium001]

depends on what you want to use the car for????
Road or race?

the westy has shift points on the graph you would prob want these to cross 500rpm before peak torque.

otherwise what are your cruising speeds? plunk that in, find the RPM in top gear and go out and duplicate the RPM on a drive, irrelevant of gear and see what you can tolerate, when 100% of the time the car should be under speed limit. Are you happy with 3600rpm at 65??

remember end goal for wheel tyres as well, this can change things.

I would gear for a compromise of cruising in 5th and peak torque when overtaking i.e 40-60mph.

hope that helps.

[pandemonium001]

I have the E12 as follows.
265 dogleg 3.07 diff
rear tyres 245/40/17
TS 155mph at 6500
2950= 70mph
2500= 60mph
41.9 to 64.8 mph is second gear
64.8 to 88.2 mph is third gear

[E21noob]

Thanks for all your input. I really have no idea so I'll just have to try and learn. Yes, I was thinking I'd stick to the E21 diff housing, so I'll go for the 3,45 og 3,91 - then reopen this chapter down the line.

Cheers