ShapeGSX

The ECU DOESN'T measure boost. It doesn't have to. There is no need when you have a MAF.

The ECU does have a barometric pressure sensor that comes before the turbo, in the MAF.

It also has an MDP sensor (manifold differential pressure). That measures vacuum, but not boost. It is used to tell whether or not the EGR valve is working.

QuantumEVO

iTrader: (12)

Yes, almost all turbo cars require the factory solenoid to be plugged in. If not, many cars will peform some very aggressive anti-boost behaviour. The MDP is indeed there for emissions purposes. It uses the array of sensors in the MAF housing to "estimate" boost, IIRC. I thought the MDP might be used for this but I don't have the service manual CD in front of me at the moment to verify. I have no doubt Shape nailed it though.

Perhaps I might try to explain boost control a bit. All turbos require a wastegate of some sort. They don't require a boost controller, though.
The wastegate is nothing more than a bypass to get exhaust around the turbo. If you didn't have this, the turbo would spin faster and faster until something broke. The wastegate is physically different depending on whether it's external or internal. It is essentially a mechanism similar to the BOV. The spring holds it shut and is countered by a diaphragm pressurized by the manifold pressure/vacuum line. When you hit a certain boost pressure, the boost pressure applied to the diaphragm equalizes that of the spring so it starts to open and therefore you can't run more boost (in reality you can if your turbo is powerful enough, but you get the idea...). The boost pressure that a wastegate opens at is referred to the spring pressure. When you turn your EBC off, you are running wastegate spring pressure. At that boost level, you aren't using boost control. That is the minimum boost level you can run. To raise boost on this design model (there are several with multiple ports for the wastegate, etc) you simply bleed some of the pressure off before it gets to the diaphragm. This makes the wastegate "see" less boost; the spring doesn't have as much boost pressure trying to push it open.
There are several types of boost control which essentially interfere with the pressure signal on it's way to the wastegate. You can either have the bleeder type as I mentioned. This is how the stock EVO system works and thus the "tee" the solenoid opens on that third leg and it bleeds the pressure off. This works on cheaper solenoids that don't have very good control or flow. Another method commonly used by aftermarket EBC's is the direct method. The solenoid has to shut to close off the signal and raise boost. It is essentially a shut off valve.
I hope I've explained myself; it's 3AM and my brain is shutting down for the day.
Either way you look at it, be careful when messing with your boost control. If the lines are left open, there is no pressure signal to push the wastegate open and you will be back to no wastegate. It will fuel cut (if you are lucky). If the spring tension is no longer there (like the actuator arm), it will have a hard time making boost or holding much boost.


Mark