Remember the restrictor in the y-hose? It goes closest to the compressor
outlet, and the solenoid and wastegate actuator are on the other two legs of
the tee. When the solenoid is closed (and the actuator isn't leaking

)
there is no flow once the tubing is pressurized and thus no pressure drop
and so the actuator gets the full compressor outlet pressure. When the
solenoid opens it lets air bleed out, and now there is flow and a pressure
drop across the restrictor so the actuator gets less pressure and closes up
some. Another way to look at is that it takes more compressor outlet
pressure to get the same actuator pressure as before. This is how the
solenoid raises the boost over the mechanical setting of the wastegate. The
ecm doesn't measure boost, it just has a table of duty cycles for the
solenoid valve (the higher the duty cycle the more air the valve bleeds off
and so the greater the pressure drop across the restrictor) vs. rpm, another
table vs. tps that is all 1's in the stock chip, and then a multiplier to
reduce the duty cycle for 3rd gear and another multiplier for 4th gear. The
ecm leaves the solenoid turned off until the maf pass a threshold, 76 gps
stock.
If you put the restrictor in the y-hose in wrong, so the restrictor is
closest to the solenoid, then the actuator is getting its pressure before
the restrictor (directly from the compressor outlet) and so the boost is
about what the mechanical setting gives. That's why you see 12-14 psi boost
if you get the hose on wrong. The other way to get the restrictor in wrong
is to put it closest to the actuator. In this case there is no flow through
the restrictor and so no pressure drop there, and even when the solenoid
valve is open since there is no restriction in the supply line from the
compressor outlet there is basically no pressure drop and so again, the
actuator sees the full compressor outlet pressure and the boost is low.
Finally, you might say, well, why not make the restrictor hole very, very
small. When I said that there was no flow to the actuator earlier, I meant
in the steady state once the actuator was holding the flap valve open at
some position. To get to that position the diaphragm has to move and some
air has to flow to make that happen. The smaller the restrictor the slower
this will be, and eventually you will see a boost spike (the boost will
overshoot the setpoint by a few psi then come back down to the setpoint)
because the actuator isn't able to bleed off exhaust flow fast enough. Flow
is also required to pressurize all the tubing leading to the solenoid valve,
which is pretty close, but if you replace it with a manual bleed valve and
put that inside the car you now have many more feet of tubing to pressurize
and sometimes this will also cause a boost spike. Going bigger than
necessary on the restrictor makes the controls work really fast but wastes
some of your boost. Also, the internal orifice in the solenoid valve is
calibrated for the size of the orifice in the restrictor, and when GM had
that batch several years ago with smaller holes in them the solenoid was
bleeding off more air that expected and the final boost was too high.
__________________
Regards,
Carl Ijames