[DOCTOR.Y]

This came up in my mind just out of curiosity.

We all know turbo is driven by exhaust gas and thus a certain flow pressure is required to boost the turbo to spin. BMW claimed that N54 is at 1300, N55 is at 1200 whatsoever, is that under the situation of flooring the gas? When we are not flooring the gas the engine actually is not generating full exhaust pressure. In another words, full throttle must have a greater exhaust pressure than 10% or 30% throttle. If at a certain rpm (1200 for N55) only full throttle exhaust pressure can boost the turbo, that means at that rpm, less than full throttle pressure cannot do it.

So how can BMW ensure the turbo to kick in at assigned rpm, regardless of opening degree of the throttle (exhaust pressure)?

[Manberg]

Don't forget 100% pedal doesn't mean 100% throttle on these cars. Also, the waste gate starts open and then closes before opening slowly again towards max rpm. There are various reasons why this is done. Monitoring this can be done with data logging your boot psi, pulse width modulation of the waste gate, compared to throttle position and RPM.

Good info on that can be found http://www.n54tech.com/forums/showthread.php?t=21269 here. This is jb4 guide, but the info is all the same regardless.