Factory Boost Jets ??? - Yes and No.


and Controlled Boost Scenarios

Enough posts have been made now that everyone pretty much knows the "FACTORY BOOST JETS" exist. Well, just to close the case, here you go. On each side of the car, there are the factory boost solenoids. A short hose from the intake/inlet pipe (to the turbo compressor inlet) connects to the "top" (seen on bottom in these pics) port of the solenoid. This is a vacuum signal and always a vacuum signal. The factory boost signal hose (comes from front of engine bay from TB pipe nipple) connects to one other port on the 'T' and the other port has a hose that goes down to the turbo actuator. The bottom port of the solenoid has a small section of hose that then connects to a metal 'T'. On this hose you will find a bulge. In the bulde is a small restrictor. Note that this restrictor is not directly inline with the boost signal line, so you could argue that it is NOT a boost jet, but rather a solenoid signal/vacuum restrictor.

bulge.jpg cut.jpg restrictor.jpg



The factory setup is described in the diagram above. Each side of the engine has this setup. The wastegate is PRE-SET to open when seeing a boost signal of about 6-7 psi, while the setup is "calibrated" to run the engine at about 9-10 psi of boost. When the factory boost solenoid is OPEN (12v signal) the actual boost signal is bled off through the solenoid into the intake tract...

So, the wastegate actuator sees less than actual boost pressure (about 3 lbs less).

If the hole in the restrictor were smaller, the bleed off would be less...if bigger, more. If you were to plug the line (or remove the solenoid) and have a direct boost signal, the boost would be limited to 7 psi. When the factory solenoid is closed (12v signal cut [safety boost, cold engine, or startup]), you see about 7 psi max boost because the actuator is receiving the actual boost signal pressure instead of the "calibrated" signal.

One problem with this setup is that the wastegate actuator sees a gradual rise is pressure. While boost is being introduced, the signal bleeds off incrementally. So, when you are at 8 lbs of boost, the actuator is seeing 5 lbs and can begin to open. Instead of opening only when the exact preset level is matched, the operation can be slightly inconsistent, creating a bit of lag in the spooling of the turbo. This problem can be solved by using an EBC, as discussed below.





Now then -- you decide to install boost jets into the boost signal line while keeping the stock solenoids installed. You put in .045 jets and are now running 14 psi. The boost jet is restricting the actual boost signal, "tricking" the system and showing the actuator less pressure (boost) than actual boost. The small hole in the boost jet allows less pressure through the signal hose (about 4-5 psi in this case). The factory solenoid further bleeds this signal (about 3 psi). The result is that the actuator sees 6 psi of pressure when in actually you are running 14 psi into the manifold. Another issue here, the signal is bleeding to the actuator as boost increases. So, when you're at 10 psi, the actuator sees 2 psi. When at 12, actuator sees 4 psi. The actuator may actually start moving once pressure is getting close the operating opening pressure, as in the stock setup (discussed above).

Another issue with boost jets, also seen in the stock setup (but not as serious), is boost spike. The boost level seen in the engine can spike for brief moments before the actuator operation opens the wastegate and holds boost at max. This can be dangerous for the engine, leading to pre-detonation.


Now, you remove the factory solenoid and connect the boost signal line directly to the wastegate actuator (or disconnect the electrical connector). You still have boost jets in the boost signal hose. What happens now? I was confused about this at first, but then the concept was explained to me on TT.net. If there is no leak, the pressure on both sides of the boost jet remains equal, even though there would appear to be a restriction of flow and less pressure on the other side. So, throwing in boost jets on a dedicated line to the actuators does NOTHING. However, if you remove the line or plug it completely, the actuator can never open and you will hit "infinite boost".


Last, you install an EBC. All the sudden there is a different operation. (not a full or precise explanation, but you can find out all about EBC's on TT.net). The EBC solenoid provides NO SIGNAL to the actuator until the precise moment you reach the maximum set boost. What does this mean? The actuator sees NO PRESSURE and will not start to open until EXACTLY when full boost is reached, at which point it opens immediately. The EBC either opens and closes this signal quickly or will bleed a bit of pressure (like the factory solenoid) to maintain pressure at exactly what is set. This provides better spooling and quicker boost response. Advantage EBC.