[Runnin'Rich]

TLDR;
E36 slave cylinder with a 2mm smaller bore makes the clutch release point & biting point closer to the driver and also closer together in the pedal stroke. i.e. you can change gear with only pushing the clutch in a couple of inches.
The pedal will be stiffer to push because of the changed hydraulic ratio, so good to add a clutch helper spring. I preferred the OEM plastic pedal to the ‘Ultimate clutch pedal’ when using the E36 slave.

The Journey:
In the four years I've owned my e91 I've never got along well with the clutch pedal, I think mostly due to its lack of 'feel'. I would often jerk the car due to miss-judging the biting point. (owned MT cars for over 20 years)
I much preferred the old cable operated clutches of my previous vehicles.

I find the action of the e90 clutch is stretched out over more than half of the massive pedal stroke travel. Add a CDV to that system and it's clear the design was to make sure no clutch warranties ever came back to the dealer, due to drivers dumping the pedal/inexperienced shifters.

1) Hammered out the CDV oriface from the CDV housing - didn't notice that much difference. Maybe slightly more predictable for hill starting? Possibly makes poor shifts even more 'shunty' (jerky).

2) Clutch stop. I added some repair washers to bulk it out until it was too large to allow the car to start due to the clutch switch. I then took a washer out so I had the largest pedal stop and therefore smallest stroke possible. This was a welcome change to not have to extend my leg so deep into the dash.

3) Ultimate Clutch Pedal, this helps a little bit to shrink the clutch stroke and in theory bring the biting point closer to the driver. This is done by the pedal pivot point and the Master attachment point being a little bit further away from each other (<10mm).
The pedal was a little bit harder to push due to the changed ratio
The resting pedal position of the UCP is set in-line/slightly behind the brake pedal. This meant that even though the total pedal stroke was reduced, the biting point and clutch stop point was actually just as deep into the footwell.
The release point / biting point were still 'somewhere' in the middle of the travel. i.e nothing happens for the first 2 inches of pedal push. I didn't notice a big increase in feel - even with all those tight tolerances and brass bushings. I was just happy to have the biting point a little bit closer to me and the stroke reduced further.

4) Clutch helper spring (plastic two-spring type). This is an over centre spring which assists the driver in holding the pedal down in traffic.
This was a surprisingly great addition making the most difference yet, the pedal was much easier to push and hold down in traffic. The lighter pedal feel made it easier to play with the biting point for hill starts. At first it felt like the pedal was slow to return, but once you're used to it, it's amazing for reducing the 'pop out' feeling and makes the pedal feel more gradual when finding the biting point.
It also made the car easier to drive for my partner.

I experimented with the 3 different positions of the clutch helper spring on the UCP. It was interesting to swap positions of the spring and observe what happens.
a) Hole closest to driver - helper spring is compressed a lot during first inches of travel, spring then goes over centre point and then helps to keep pedal down.
This means the first inches of pedal stroke is harder to push and then you reach the over centre point and the spring helps the pedal stay down.
This will give you the real 'pop-out' feeling when releasing the pedal; making it harder to control the pedal modulation for hill starting.
Maybe the idea with this position is to simulate a biting point? If the centre position matches the biting point in the pedal stroke then you can feel the change in force and know where the biting point is?
b) Middle hole is same as 1) but reduced slightly - over centre point not as deep into the pedal stroke.
c) OEM position (furtherst from the driver) gives the most 'help' throughout the stroke and is therefore the easiest to modulate for hill start. The spring still compresses slightly in the early part of the stroke. Over centre point is ~1" into the pedal stroke.
The UCP pedal also has less curve to the arm. I was finding some shoes I own size 13 - were contacting the arm at the tip of my shoe, so I just can't drive in those shoes.

5) Replace the E90 22mm bore (7/8") with E36 20mm bore (13/16") slave

This 2mm reduction in the piston bore diameter results in almost a 20% reduction in pedal stroke but with the detriment of 20% more effort required to push the pedal.

Install:
The E36 slave is metal and requires the hydraulic line to be attached with a screw fitting instead of the plastic slaves 'quick' push in and clip system like what is used on the CDV.

To adapt the E90 to a metal slave with threaded connection:
Option 1: Cut off the quick connector at the slave and then add a threaded fitting to the line using a flaring tool.

Option 2 (What I did): Swap the section of E90 hard line with a used one from an early E8X or E9X that came with a metal slave cylinder. One end of this pipe will have the 'quick' connector for the CDV and the other a threaded fitting for the metal slave.

Results:
The pedal is stiffer to press due to the changed hydraulic ratio.

With the smaller bore slave everything happens much earlier in the pedal stroke; you only have to press the pedal a few inches to change gears. I am playing with shifting by keeping my heel planted on the floor mat.

The pedal position at which the drive is de-clutched when pressing the pedal, is now closer in position to the biting point when releasing the pedal. This makes the pedal seem more intuitive to use.

I don't notice any more 'feel' through the pedal, I guess this is just the nature of a hydraulic clutch system.

Using the UCP Without the helper spring the pedal is pretty heavy to push so I put the spring back in.

6) Back to the Plastic pedal.
I swapped back to the original plastic pedal to see if preferred the lighter effort compared to the differing ratio of the UCP.
I noticed that the OEM pedal rests closer to the driver so you don't need to push your foot as deep into the dash before changing gear. There is a slight increase in stroke to get to the clutch stop in comparison to the UCP but it's not noticeable.
The pedal feels easier to push but the biting point feels just as close to the driver as it did with the UCP, I guess because of the improved closer resting position.

Observing the helper spring with the plastic pedal - there is no compression of the helper spring in the first inch of pedal push. So the 'OEM position' of the UCP is a little bit too close to the driver. The helper springs works as intended with the plastic pedal; it doesn't add any extra force at the beginning of the stroke - it just helps you keep the pedal down.

I honestly can't feel the plastic pedal 'wobble' in the pedal with the helper spring fitted. So I think i'm gonna sell my UCP soon. I

[Runnin'Rich]

This is my favorite method to bleed the slave:

This video too is helpful if your struggling getting the air out:

The clutch master is self bleeding by just pushing the pedal 100 times.

[NotSoJDMGC4]

Nice. What's funny is that I was actually looking to perform a similar mod, but in reverse (non BMW vehicle).

The clutch pedal travel was long but the bite and release point was very small. I was trying to make it feel more like my 330i 🙃.

I actually like the stock setup, but might give this a go on my 330i and see how it feels.

[Runnin'Rich]

Harsh bite point Is ok as long as it’s not deep into the travel I think. Adding a larger bore slave will make it more progressive of a bite but will put the point deeper into the travel unfortunately.
Unless on your other car you can adjust the clutch pedal set up like on most Japanese sports cars