HOW BOOST IS CREATED The turbocharger is driven by waste exhaust gases, forced through an exhaust housing onto a turbine wheel. The turbine wheel is connected by a common shaft to a compressor wheel. As the exhaust gases hit the turbine wheel, both wheels rotate simultaneously. Rotation of the compressor wheel draws air in through a compressor housing, forcing compressed air into the engine cylinder. The increase in air fed into the engine creates more combustion force and power. More air plus more fuel = a bigger explosion. After the air is compressed, it rises in temperature. It exits the compressor discharge nozzle then enters the hotside piping. It then enters the FMIC (front mount intercooler) which is a radiator for air. Here the air is cooled. The cooler the air, the more dense it is which yeilds more HP. After the FMIC, it enters the cold side plumbing and then enters directly into the intake manifold. WASTEGATE The turbo has what is called a waste gate. It's exacty that, a gate that opens and removes the excess exhaust gas which is waste. As the exhaust exits the manifold, all of it enters the turbine housing causing it to spin. Once a boost level is reached, the wastegate opens up bypassing some of the exhaust so only a portion of it passes through the turbine housing where as the rest enters the exhaust system. HOW BOOST IS CONTROLLED The SRT-4 has a 4lb spring (S2 has 7lb)inside the wastgate actuator. If you hook up a vacuum line directly to the wastegate actuator, you will only boost 4psi. Once the turbo spins to 4psi, the wastegate will then sense this pressure and open the wastegate. How is it that we have a stock 11psi then if the WG is only 4lbs? Well take the line that goes to the WG actuator. Imagine if you put a hole in that line. Now you are running 11psi. 4psi is seen at the wastegate but the other 7psi is bleed out into the atmosphere. The bigger you make that opening, the more boost the turbo will put out. Its simular to you blowing through a straw that has a crack in it. YOu have to blow alot harder to get that spitball out being that there is a crack in it. HOW ECU CONTROLS BOOST Stock solenoid The ecu controls boost . The stock solenoid valve which is the hole in the straw, only that it modulates to get that 11-12psi out of the car. So once you remove the stock solenoid and place your own bleeder T or MBC or EBC you now can control the boost. Stock surgevalve. The stock surge valve has a pink line going to it. This is the headpressure that keeps the valve closed during WOT. WHen you get off the throttle, the pink line now has a vacuum which caused the surge valve to open. It opens because the 10lb spring inside the solenoid is not strong enough to keep it closed. The pressure in teh compressor scroll is higher so it opens the surge valve. If you remove the surge valves pink line then you will boost 10-11 psi at the most. YOu will spike, but will only hold 10psi or so because the spring in the surgevalve is being opened by the higher boost inside the compressor area of the turbo. The best thing to do is to remove the surge valve and install a block off plate and use a Real BOV. The MP BOV provides no gains at all, its just a newyears party favor that makes noise. It cant handle the higher volumes of air once you turn up the boost. UPGRADING WGA I recommend upgrading your wga. This will allow you to hold higher boost at redline. The boost tends to drop off because the exess pressure in the manifold forces the actual valve in the manifold to open. With a stiffer spring you dont have this problem. If you put to much spring tension on your wga, you will end up spiking high on boost. If you want to run say 16psi, you will spike 19+ etc. If you use this in conjunction with an electronic boost controller, you can set it to 16psi and if tuned in correctly, you will not spike above that. An EBC also helps you reduce lag. It does this by not allowing the WGA to see any pressure until you hit X amount of boost. ON the greddy units this is tunable to help reduce lag. ON a regular manual unit, the wga alway sees pressure so it tends to crack open causing the turbo to spool slower.