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Discussion Starter #1 (Edited)
Been doing a little researching and investigating and there appears to be a ton of confusion and a bit of misinformation going around on how the wastegate actually operates on a 7.3 powerstroke. All of the below is based on my best understanding so there could be errors so feel free to correct me and I will update the post.

First and easiest thing I see is to clarify that the exhaust backpressure valve IS NOT the wastegate valve and has nothing to do with the wastegate. The exhaust backpressure valve can close off to restrict exhuast straight out of the up-pipes BEFORE if even gets to the turbo. This valve is used to warm up the engine faster in cold temperatures and should be open fully and not close once the engine is warm enough.

The wastegate valve is built into the turbo that can open to allow exhaust gasses to bypass the exhaust turbine. The wastegate is NOT a neutrally balanced valve like your typical butterfly valve which pivots in the center so that pressure on the valve does not push the valve open or closed as the pressure on both sides of the pivot point are balanced/equal. The wastegate valve is typically designed with an offset pivot point or sometimes with linear valve like an exhaust valve. I think on the Powerstroke turbo it is just valve with offset pivot point. Because of the offset this allows exhaust pressure to push the valve open. The point at which it opens is dependent on the spring force and/or actuator that resists this force. When the actuator line is disconnected the only thing that will open the waste gate is exhaust back pressure. Does anyone know what exhaust back pressure it takes to crack the valve open with the red boost line disconnected and out of play? I have also seen aftermarket wastegate controllers that are simply an adjustable spring with a lever arm which will perform the same function as a disconnected stock actuator though I have no idea how the forces on the actual wastegate valve and thus turbo exhaust backpressure values ultimately compare.

The stock wastegate actuator as well as several aftermarket actuators also allow boost pressure to assist in opening the wastegate valve dependent on what the PCM controlled solenoid is doing.
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The solenoid is normally closed so the wastegate actuator is connected directly to manifold pressure ONLY until energized. Once the solenoid is energized by the PCM it will instead dump a controlled amount of boost pressure directly back into the intake track where it essentially becomes a very small boost leak. It doesn't block the pressurized air from the manifold it only controls the pressure in the red line by allowing some of the boost to dump to atmosphere instead of pressurizing the actuator. I'm guessing this is the hissing noise I sometimes hear when in high gear high load situations but I do not know for sure. The solenoid can only act to open the wastegate earlier than spring pressure alone.

This is my basic understanding of the system so far after some research online and a bit of testing. I still have some questions and would appreciate input from anyone who has modified their wastegate system.

How do the backpressure limits (resistance force holding wastegate closed) compare between the stock actuator spring and the initial setting on an aftermarket unit like the TurboMaster? Is backpressure control ONLY sufficient to protect the turbo or does this method fall short?

Aftermarket actuator replacements advertise having a larger spring AS WELL as a larger diaphragm. This means that while the actuator will provide greater resistance from exhaust backpressure opening up the valve alone, the larger diaphragm will essentially negate the stiffer spring force when it comes to activating the actuator from boost pressure to an extent dependent on actual increase in spring force vs. increase in diaphragm area. The only reason I can come up for doing this would be to control the actuator more precisely with the PCM fired solenoid while being less affected by exhaust backpressure forces. This is a completely opposite control theory from the TurboMaster spring ONLY unit discussed above.

So which side of the turbo are pressures more critical to control for turbo protection, intake boost or turbine exhaust pressure? If you could safely protect the turbo by limiting exhaust pressure only that would be my preference because it wouldn't required a controlled boost leak into the intake tract which, although small, is still a waste.
 

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The exhaust backpressure valve can close off to restrict exhuast straight out of the up-pipes BEFORE if even gets to the turbo.
Not exactly right. The EBP valve is on the OUTLET of the turbo.
Does anyone know what exhaust back pressure it takes to crack the valve open with the red boost line disconnected and out of play?
The opening pressure is dependent on the adjustment of the actuator rod.
This is from the PCED manual on the wastegate solenoid.
"A wastegated turbo is designed to reach maximum boost sooner then a conventional turbo, but overboosting will cause damage to the turbo. The PCM will control the boost pressure by duty cycle to the solenoid to maximize boosting performance (no more than 16-1/2 psi). When pressure is supplied on the red hose going to the actuator (solenoid NOT energized) the valve will open, dumping boost. When low or no pressure is on the red hose going to the actuator (solenoid is being energized), the valve will stay closed."
The service manual says adjustment is done with 5 psi applied to the actuator and the rod adjusted to just slip over the pin, but does not give any specs. Basically that procedure applies preload to the spring.
 

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Discussion Starter #3
The opening pressure is partially adjustable by adjusting the pre-load on the spring but I wonder if anyone has any idea what back pressure a properly loaded stock spring would allow with the diaphragm out of the equation. Alternatively it would be interesting to compare to the spring-only TurboMaster aftermarket option.
 

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Discussion Starter #4
I've disconnected my diaphragm and it now vents to atmosphere. The green line is plugged of, I put vacuum caps on both ports of the solenoid, and a vacuum cap along with zip tie as clamp on intake plenum nipple. The old rubber on the red and green lines have degraded so this should give me a better seal than plugging them with a ball or something.

I have an EDGE CTS2 to monitor, importantly, boost, EGT, and EBP. I've only driven it to work since I've done this but on the way I accelerated to pass up-hill and I built 20 psi partial throttle and the EGT's appeared to be lower and the boost higher than similar load conditions from before (Running Hydra on PHP 80hp daily tune). I don't use my truck to race and I rarely go full throttle so I'm thinking this mod may be beneficial to my driving style even though it may place a bit more stress on the turbo than the same driving conditions before. I do us my truck primarily to pull trailers and one task is to pull my ~4k lb car and trailer down interstate around 75-80mph and I'm hoping this mod will lower my EGT's going up hills into a head wind.

I'm posting this all here to document results as I couldn't find much anything conclusive when researching personally. If I have a decent understanding of the wastegate function I'm assuming I should be fine with this mod as long as I don't allow the truck to boost over 25 psi?
 
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