F8X M3/M4 CF Drive shaft mod

[George Hill]

I assume they meant RPM of the driveshaft, not the engine.

I should have been more specific... and yes it is 7600 so the delta isn't nearly as big knowing that. 6th gear at redline is going to be the fastest driveshaft RPM, however realistic it is to get there.

S55 - 7600 RPM with .872 = 8,715rpm
S54 - 8000 RPM with .828 = 9,661rpm
*S54 - 7216 RPM with .828 = 8,715rpm

10% faster... maybe it doesn't matter, just curious

I think it is 7600 and typically manufacturers over engineer parts to go beyond the stated limit. I'd be more concerned about the torque rating and fortunately, the S55 has plenty of torque.

Then again, BMW did recall some of them as they can fail spectacularly, probably due to that 406 ft/lbs of torque and a defect -

My car at 7216rpm is 185mph (not factoring any drag) so probably not something I am going to achieve anytime soon or for an extended period of time, but I do have thoughts of going 200 in it so.... lol

(S55 at 7000 RPM is the same driveshaft speed at 170mph in my car which is certainly attainable)

[Slideways]

In the BMW video, they state the CF diameter is directly related to max RPM, which IIRC is 7k on an S55.

Any thoughts on how this relates to spinning the S54 8k+?

I think it is 7600 and typically manufacturers over engineer parts to go beyond the stated limit. I'd be more concerned about the torque rating and fortunately, the S55 has plenty of torque.

Then again, BMW did recall some of them as they can fail spectacularly, probably due to that 406 ft/lbs of torque and a defect -

[Obioban]

In the BMW video, they state the CF diameter is directly related to max RPM, which IIRC is 7k on an S55.

Any thoughts on how this relates to spinning the S54 8k+?

I assume they meant RPM of the driveshaft, not the engine.

[slavik1988]

how is it on the F8x cars with this driveshaft when it comes to install/removal? do they also have to loosen diff? or is the F8x DS flange adjustable/telescoping to allow install/removal with out touching the diff

The YFCM is telescopic as well. In my opinion it is inherent in its 1 piece design. The factory shaft can be bent out at the CSB.

[George Hill]

In the BMW video, they state the CF diameter is directly related to max RPM, which IIRC is 7k on an S55.

Any thoughts on how this relates to spinning the S54 8k+?

[ZiMMie]

how is it on the F8x cars with this driveshaft when it comes to install/removal? do they also have to loosen diff? or is the F8x DS flange adjustable/telescoping to allow install/removal with out touching the diff

Yes it's telescopic by the diff. It probably can be retrofitted into the e46 if pinon size is the same.

But parts can't be bought separately you most likely need an f8x diff to dismantle.

[nextelbuddy]

I wonder if it might be viable to make the shaft a tiny bit shorter when cutting the flanges, to allow the expanding/contracting portion to cover the install without diff removal. I have no idea how much wiggle room there is in that joint.

how is it on the F8x cars with this driveshaft when it comes to install/removal? do they also have to loosen diff? or is the F8x DS flange adjustable/telescoping to allow install/removal with out touching the diff

[Obioban]

I wonder if it might be viable to make the shaft a tiny bit shorter when cutting the flanges, to allow the expanding/contracting portion to cover the install without diff removal. I have no idea how much wiggle room there is in that joint.

[Epsilon]

Also have the YFCM DS and had to remove the front diff bolt and loosen the rears. I didn't have to lower the subframe though. I really wanted to avoid loosening the diff, but I just couldn't get that extra 2 to 3 mm to slip into the front diff flange.

On a side note, the weight savings of the YFCM DS is where it shines, IMO. As Slavic mentioned above, shifts are "more direct and the clunk is a lot less noticeable". There is also faint whirl sound at high speeds over 100+ mph like an electric motor. lol

[slavik1988]

Drop the diff entirely? Or just remove the front bolt to allow movement of the front of the diff?

I had to remove the front bolt and loosen the rears as well as lower the whole rear subframe.

[bimmerfan08]

I run a YFCM carbon driveshaft. I did a before drive in the mountains and then directly after the install. The shifts definitely feel more direct and the clunk is a lot less noticeable. No increase in NVH whatsoever. Installing a 1-piece driveshaft is a PITA. Had to drop the rear differential to get it in.

Drop the diff entirely? Or just remove the front bolt to allow movement of the front of the diff?

[heinzboehmer]

If you have a driveshaft out I can ultrasonically measure thickness without cutting anything

Hmm, doesn't tlow98 have an extra one?

[slavik1988]

Carbon has the extra benefit of stiffness and more inherent damping. The reasons I’m interested in this are for those two and an inertial reduction to help with the clunk when shifting. Would love feedback from anyone who’s done a back to back on those metrics. Seems worth a shot!

I run a YFCM carbon driveshaft. I did a before drive in the mountains and then directly after the install. The shifts definitely feel more direct and the clunk is a lot less noticeable. No increase in NVH whatsoever. Installing a 1-piece driveshaft is a PITA. Had to drop the rear differential to get it in.

[Bry5on]

Ooh that cutaway picture is great. Can estimate wall thickness pretty well with that. Now we just need someone to cut open a stock driveshaft and measure the wall thickness at all its different diameters

If you have a driveshaft out I can ultrasonically measure thickness without cutting anything

[heinzboehmer]

Found some pretty good info from bmw blog:
http://www.bmwblog.com/2021/04/09/vi...bmw-m3-and-m4/

The guts:


Video from the linked blog post:
https://www.youtube.com/watch?v=gVqHSron50U

At 1:00 in the engineer mentions f8x carbon driveshaft is 90mm heinzboehmer

Ooh that cutaway picture is great. Can estimate wall thickness pretty well with that. Now we just need someone to cut open a stock driveshaft and measure the wall thickness at all its different diameters