Just a little info to maybe simplify the functionality of the the Forced Induction systems. I am sure this is on the boards, but maybe we can really break it down so troubleshooting the set up becomes easier. I keeping the terminology and concept basic so we can get the fundamentals going.
The turbo consists of 2 wheels...compressor and the turbine enclosed in their respective housings. These wheels are linked via a shaft, however they are sealed from one another. The exhaust gasses from the motor spin the turbine, or Hot Side. This, in turn, spins the compressor(cold side) and produces boost. Now we need to control the boost...
Try and think of the Wastegate as a literal 'boost controller' The Gate has a spring in it applying pressure to the piston, or flapper depending on style of gate. This spring will hold closed based on the mechanical rating of the spring. Once the pressure exceeds the spring rating, exhaust gasses are diverted and boost is limited. So, an 8PSI spring should produce 8PSI of boost. There are many things that can effect the actual boost, but for our purposes this example is OK. We will com back to this concept in a few.
The wastegate is ofter controlled by a secondary solenoid. This is known as a Boost Controller(BC). The BC can be in the form of a solenoid(electronic) or manual valve(ball spring/bleed/etc). So literally, the boost controller is a Wastegate Controller.
So back to our controlling of the actual boost...
The WG(wastegate) can be run with a port from the compressor housing of the turbo (or as close to it as possible) connected to the WG(bottom port or only port on the swing type). This orientation will run only the mechanical rating of the spring in the WG.
With the BC(boost controller) in line between the turbo(cold side) and WG. This will allow us to divert compressed air to the side of the WG valve opposite the exhaust gasses. This basically makes our WG spring behave like a far heavier spring, thus increasing boost. This can be done in a few ways, depending on the BC style. EBC(3 port) designs allow for 2 main styles of routing. Typically, one port comes from the Comp housing, a second comes from the bottom port on the WG, and the third port on the EBC is vented toi atmosphere or plumbed to the intake(does not matter). This is a very simplified explanation of the boost control system. An EBC assumes the ECU has a table to control it...we can go over tuning in another thread if anyone is interested.
The one variant that 95% of all Forced Induction systems has is a BOV(blow off valve). These come in many different flavors and styles. The main purpose of the BOV is to give the compressed air a place to go, after the throttle is closed, thus protecting the turbo. The BOV has a diaphragm or a piston, a spring and one(or more) ports. The idea here is a little different than the WG. The port on the WG should not see vacuum, so it is not an issue(because we are using a pressure only port...ie the comp housing) so you can run an 8 psi spring at sea level without the fear of a leak. We want a spring for the BOV that is stiff enough to stay closed at idle, and no more. A spring that is overly stiff, can lead to poor throttle response and decreased performance. In this example, we see the Tial Q. The spring should be selected based off of your ambient barometric pressure. The closer to sea level, the stiffer spring you will require. You may find the Tial Q requires a shim on the stiffest spring(11psi) to preload the spring a bit, keeping it closed at idle.
On a MAF or metered air based system, it is critical to make sure you have no vacuum leaks. Ideally, you want a BOV that recirculates back to the intake, so the ECU does not need to make dramatic fuel corrections at transient throttle conditions. Basically, the ECU sees X Airflow at the MAF and provides Y amount of fuel. If you dump it to atmosphere you have fuel Y and and not the metered X volume of air resulting in a rich condition. This over fueling can lead to stumbling, misfires, and poor driveability. This same concept applies to boost leaks after the MAF sensor.
Now we run VTA(vent to atmosphere) BOVs all the time, and can tune effectively for it, so I am not saying you have to run one or the other, that is up to you and your tuner.
I hope this breaks down the system in a very easily digested manner. There are a number of boosted Zs' so we should have a good understanding of how to set it up.
I hope this did not hijack the OP, I only intended to add to the discussion. Good luck, and if anyone needs help, feel free to drop me a line anytime.
-John
