Diff for 6 speed and m20 stroker

[raudonis]

Guys, please help me choose a diff for a 6 speed and m20 stroker.

In my plans I have a NA m20 stroker with peak flywheel torque of no more than 300Nm (220lbf) and peak power no more than 240 flywheel horse and redline of no more than 7000rpm (optimistic figures). Think m54b30 which by the way had either 2.93 or 3.07 final drive in e46, z3 and z4.

I want to mate it with a 6 speed gearbox.

The box is GS6-37DZ from a 2L turbodiesel (e46, e90, etc...).
Since it is for a low revving diesel, the gears are spaced quite far apart.
Here are the ratios compared to Getrag 260:


Since the gears are this much far apart, I can't decide on my final drive. I either get ridiculous tall 6th with top speeds in range of 300km/h (190 mph) which will be impossible to achieve or a ridiculous short 1st which will be of no use except when parking the car. Tire size for calculation 225/45R16.

I've narrowed my selection to 2.79 2.93 3.07 3.15 3.23 and 3.25.

I've made a table where I present a closest final drive equivalent with a G260 for every gear in GS6-37DZ with a certain final drive from the selected 6 options.

For example you have the six speed with 3.15 final drive. The 1st gear will drive similar as 1st in g260 with 4.27 and the 6th gear will drive similar as 5th in g260 with 3.23 final drive.

Anyone with a built eta and a stock 2.79 diff?
Or running 4.45 with g260?
Please post your experiences and opinions.

[digger]

for what purpose do you want a 6sp? i think 2.79 or 2.93 are the only sane choices otherwise 1st is too short and even more useless than it is the the standard G260 and stock diff

[TobyB]

Good God, that 1st is lower than most pickups!
Why would they do that? The 5 speed starts to make a lot more sense here...

Honestly, I think I'd put something like a 3.25 in it and just never use first.
Otherwise, the jumps between gears don't gain you much- the 2-3 gap on the 6 speed is
WORSE than on the 260, for crying out loud...

Trying to shift that into a 2.7 diff is going to be a real bummer...

Thanks for talking me out of a 6- speed!

t

[raudonis]

Well there is GS6-37BZ which is a petrol box with petrol ratios, but the casing does not mach the tilt of the m20. Maybe the ratios could be swapped between the casings, but that requires more cash thrown at it.

Actually the gap between 2-3 is the same in the diesel box as in g260, but the other jumps are greater

[The Dark Side of Will]

What's your highway cruising speed? Get the tallest diff that will let you pull that with your M20's stump pulling low end. You'll hit top speed in 5th instead, but so what?

I assume you're running a 3.0 stroker.

And you can probably swap the DZ case onto the BZ transmission... much simpler idea than swapping BZ gears into a DZ case.

[The Dark Side of Will]

.

[raudonis]

My maximum highway cruising speed would be 140km/h (90mph).
Yeah, swapping a DZ belhousing onto BZ transmission much better idea (if they are swapable).

[raudonis]

Snagged a used Valeo kit 835101 with 3000km on it. it is a single mass conversion kit for m54 engine with a 6 speed box. It is still as heavy as the factory assembly, but it can be lightened and balanced :)

[raudonis]

Valeo (835101) single-mass flywheel weighed in @ massive 12.4kg (27lbs)(when factory double-mass is specified @11.3kg (25lbs) in the m54b30 parts catalog).
Got it conservatively lightened to about 9.1kg (20lbs).

[digger]

raudonis what did you end up with? i have been looking into doing this and for me the 3.07 seems right for a good balance.

Also how did you resolve the other issues?

[raudonis]

digger There has been many hinderances and distractions over the past years I've just brought the car and all the parts to where I live 10 months ago, to be fair washed it for the first time since 2016.

I've decided to start with 3.07 as well because if I build a fresh LSD it can be switched to higher ratios later and with the other offset casing I can only go 3.15 and lower.

Almost got the shifter/linkage sorted out.

[raudonis]

digger Got another option for a driveshaft ready e36 rear half + e46 front half: http://bmwfans.info/parts-catalog/26117518375/.
Got some stainless cut, bent and welded to start with the gearbox mount according to this (yet to confirm the fittment): https://hobby.farit.ru/custom-bracke...on-in-bmw-e30/
Right before Christmas got this from Australia: https://racehead.com.au/product/bmw-...e-molly-steel/
Need to measure things out, I'd love to fit clutch/pressure plate/release bearing from the Valeo kit I got in 2017. When this is confirmed and I find some spare time I'll rip the old gearbox out (need to change the rear main seal carrier as well).

[varg]

Man BMW sucks with their transmission gear ratios. Good to know that the 3.07 torsen I have in my garage is something I should hold on to in case I ever decide on a GS6-37DZ for some reason

[raudonis]

Man BMW sucks with their transmission gear ratios. Good to know that the 3.07 torsen I have in my garage is something I should hold on to in case I ever decide on a GS6-37DZ for some reason

It's not that it sucks, but simply diesel gearboxes have gears further apart vs gasoline. DZ boxes are much easier to find and cheaper to buy than BZ here in Europe.

[varg]

Yeah I've had a bone to pick over it ever since I realized I'd need a 2.93 on my daily because a ZF320 has a 1:1 fifth gear.