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.