Even more mud
:
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.
After drawing it out on paper the right way, I see how pulsing the sol open will in fact, reduce boost.
(couldn't NOT say hello to my mentor!)
