[E46] 4 speed auto upgrade

Ok gentlemen this is not really an E46 board but I need some advanced knowledge. Here's the story:

My mother has an E46 Touring with M43 1.8 engine and 4 speed GM autobox. This is the 4L40 box.

I really like the car and even as touring, it has a very acceptable daily driver character. I am thinking of buying an E46 Sedan M43 Auto in good condition as company car. You would ask me why I would buy a company car which is more than 15 years old (even if in mint condition). This is because in the Netherlands, there are tax benefits for company cars older than 15 years.

So, that 4L40 box. Basically, GM made a 4L40 and a 5L40 for BMW usage. Regarding these boxes, I have found the following claims to be true:
- Both autoboxes use the exact same housing, the 5 speed has some extra drums and a more complex gear set for that 5th gear
- The front of the bellhousing can be swapped
- The valvebody is very similar but not identical (3 or 4 accumulators depending on 4 or 5 speed)
- The number of solenoids, solenoid position, wiring harness and connector, are identical
- The EGS are identical in size, shape and connectors but not identical in programming
- PROBABLY, the wiring from the EGS up to the gearbox itself, is identical

Since I am always looking to improve stuff. I have the idea of getting an E46 318i Sedan with M43 engine and 4 speed autobox. I will then take a 5L40 box, rebuild it completely, strengthen it if possible and fit this to the car. I will then repogram the gearbox computer by adding all the M43 variables to the 5 speed shift program.

Since the final gear is 0.7 on the 4 and 0.75 on the 5, you basically get much faster acceleration. I can either keep that faster acceleration or I can install a different differential to reduce Highway RPM to like 2800.

Either way, the main challenge here, is to get the 5 speed computer working on M43 variables. I have researched how these autobox computers work and they basically need to know basic engine info to be able to determine:
- When to shift
- With how much pressure to shift (more torque from engine = more pressure has to be applied)


Do you think reprogramming that 5 speed computer is doable on a regular "chip tuning" type of budget? I am not afraid of all the rest. I can rebuild a 5L40 easily and fitting it to the car would also be easy as these boxes are just about identical.

Chip tuning is way short. I read a similar thread of someone that spent 8+ hours and had an advanced idea of what he was doing. Going to need to swap the engine and trand harness along with both computers and then reset them to zero. After that you can program them and it might work.

I have a 2000 E46 330i sport and my wife has the 320i (2.2)SE, both 5 speed autos. If you look at the official mpg figures, there is very little difference between the 3.0, 2.5 and 2.2 engines. My sport is obviously faster but if you have back trouble I'd suggest you might be better with the SE as the suspension is very firm on the sport. The year difference also brings up and amusing change. On mine, you push the lever forward to shift up in manual mode. On my wife's, you move it backwards. You have to remember which one you are driving :) Not as technically interesting or challenging but you might consider one of the above fits your requirements off the peg

Basically, I will:

Swap a 4L40 with a 5L40 (direct fit)
Swap a 4L40 computer with a 5L40 computer (direct fit)

Now, that 5L40 with that computer, will be taken from an E46 and probably a 320d as there's alot of these on wrecker yards. This 5L40 will shift different. It will:
- Shift at lower RPMs as it handles a diesel engine
- Have a different throttle position to RPM to load conversion calculation

I know 100% that the 4L40 and 5L40 are wired IDENTICALLY and that both the M43 and the 2.0d have an identical Gearbox computer + wiring. The only difference, is the PROGRAMMING.

So, I THINK my only challenge is to:
- Modify the 5L40 computer to shift at 4 cyl petrol RPM figures (use the raw data from the 4L40 computer to know when it wants to shift)
- Make the proper throttle position to RPM to load conversion calculation so it has an accurate estimation of the expected torque on the gearbox (This is an SAE formula)

And all other variables should already be read from the IDENTICAL wiring harness. Engine temp, outside temp, gearbox temp etc. etc..

Since both transmission computers are wired identically and have the exact same pin configuration (ATSG manuals tell me this), my challenge is:
How hard is it to reprogram that 5L40 computer as described above?