Did some more research and talked to some more people.

Removing material from the steels to add another clutch carries the risk of overheating clutch packs. Extra coolers won't help as this heat appears for a very short duration. My gearbox specialist does not know what the minimum required thickness of the steels are. He said the only reliable way would be to increase the pressure a little bit. This would maybe up the 310 limit to 330/340. He was not sure if the 4-5 clutch variant of the 5HP18 could take more power but the theory behind adding clutches (if the steels allow it) is correct.

Since a M60B35 with performance exhaust, high flow cats and modified intake (whatever way I go on that intake) can probably do 330-340Nm, the 5HP18 is a risk factor. A reliable and known path would be to fit a 5HP30 + wiring from a 540 or 740. Sockets may be a bit different but the wiring diagrams would show me exactly what to solder.

However, if I want to fit another type of box, I want a 6HP26 (600Nm max.). Funny thing is both the specialist and I proposed just that at the exact same time. I said the risk with a box from an OBDII car is that it won't run properly but the specialist can write custom software that would run on the signals my 5HP18 uses as well.

This makes the 3.5 project obsolete as a M60B30 with a 6HP26 would already have better performance figures. The 6HP26 is lighter and has a shorter 1st gear with a similar ratio build up towards the 0.69 6th gear. That 0.69 6th gear is longer than the 0.71 I have now so that 2.x% difference can be compensated with another differential or smaller tires.

In short, I would gain acceleration.. get quicker shift times and keep highway cruising RPM. In addition, this gives me engine tuning space up to 600Nm (lets say 550 to be safe).

I will talk with my guy some more and think this out. We think we can make it work so I will probably go for the 6HP26 path, get that to work properly and then see what I would want with my engine.


More soon.