So how does having a suck-through setup change the matching of a compressor wheel to an engine, and when you're near the surge limit?
My uneducated reasoning would be that when you're using a blow-through throttle body in part-throttle boost, from the turbo's point of view you're just making your volumetric efficiency go to shit. The PR will stay the same as it would be at WOT since atmospheric is still the same and the wastegate still keeps manifold pressure the same. However, since the throttle body isn't fully open, you're reducing your airflow. Constant pressure ratio + decreasing air flow rate = moving left along a horizontal line on the pressure map, toward the surge line.
I can't figure out what's happening with a suck-through turbo, though. As you close the throttle, you're incrasing your pressure ratio because the wastegate tries to keep boost, the numerator, constant while the throttle body reduces "atmospheric pressure", the denominator. The turbo has to "suck harder" to keep boost constant. However, as the throttle body closes, air flow will also decrease. Eventually engine demand will overtake air flow and boost will start to decrease. This seems like a complicated balancing act.
Where's the tipping point in that process? I've noticed when driving my suck-through car that there's a point of part-throttle at which the car can develop full boost. Let's call it XXX.
With less throttle than XXX, the boost goes down corresponding with the throttle closing-we're choking off the engine and turbo. However, with more throttle than XXX, the engine still seems to pull harder, despite boost staying constant.
That doesn't make sense because power output corresponds to air flow and air flow corresponds to manifold absolute pressure. Since more throttle than XXX doesn't increase manifold pressure, how can it create more power? I'm stumped.
: HELP!
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