Hi, I know this thread if old but if your still having problems.
The BCU+ unit's start boost function works like this.
Lets say you set boost at 14psi, and start boost at 10psi, what it means is the wastegate will start to open 4 psi.
If boost is 10 and start is 10, it will open straight away.
Its quite a tough unit to tune, have been using it more than a year and the soilenoid stopped working on me today. Took it apart, gave it a nice clean and now its working beautifully again.
I find setting the gain from low then slowly increasing it till it holds boost till redline better, although many people go from up to down. And its sensitive as well, 1-2% gain makes alot of difference. Good luck.
