" Doubt its anything to do with backpressure. It can happen though when true MAF isn't reported as mentioned earlier even on upgraded stock frame turbos.
Don't use Cobb, use the BB flash for instance and run a strong tune with stock load targets on the flash and 20-21psi on the jb with timing at mbt for pump+meth. You may see something very similar unless you add fuel scalar or richen it up like Terry mentioned. "
" I suspect it's due to the backpressure of mounting the sensors pre-turbo. Bosch reports that backpressure can significantly skew readings on wideband O2 sensors. I'm not sure how susceptible the sensors on the N54 (LSU4.9?) are to this, but I would imagine that backpressure has some effect. I think the fact that the N54 adjusts A/F ratio in real time may help compensate for this this, though. Being 1/2 a point or so off at over 20 psi really isn't too bad as long as you can compensate with the flash.
EDIT: I just saw that you are noticing a .75 point difference. Are the sensors reading .75 richer than actual or leaner than actual? "
Because of complexity, cost, and maintenance. The gains, if any, won't be realized over a quality ATA IC. Would you use the already saturated coolant system? What would cool the coolant, radiator or separate cooler? The whole idea for an N54 isn't feasible IMO.
But there's so much room in these engine bays for additional rads, dedicated coolers, and plumbing "