WGDC Requirement Higher After Raising WG Spring Pressure

[llxkevinxll]

Hi All,

Over this past winter I swapped the spring set in my Tial-MVR from ~10 psi of spring to ~20 psi of spring. After doing that, I have found that I require a lot more WGDC command to get similar boost levels. Nothing else changed in the system other than this spring change. Same turbo, manifold, exhaust, engine combo, 3 port solenoid, plumbing, etc... I would have assumed a doubling of the spring pressure should have resulted in much less work needing to be done by the solenoid to keep boost where I want it. Am I misunderstanding something about how this system works?

Two log screen shots attached, last fall with the 10 psi spring, this year with the 20 psi spring. You can see when highlighting a point near 6k rpm in 3rd gear, the older logs shows 37.5% WGDC required for ~30 psi. Now I am at ~85% for a pound or two less. Obviously DA, IATs etc. affect the pressure the intake manifold will see but this feels like a large change in required WGDC. I assume it has something to do with the exhaust back pressure and its interaction with the stiffer spring but I am at a bit of a loss. The only reason I care is that I don't want to run the solenoid at 100%, thereby leaving the ECU with no real control authority to modulate the WGDC to meet target. Any thoughts?

10 psi of Spring

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20 psi of Spring

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[curt-s]

On a normally closed solenoid, more duty allows more air (& pressure) to reach the WG before bleeding off when the solenoid closes the IN side. A heavier spring in the WGS requires more air mass/pressure, thus a greater duty cycle. As the WGS duty cycle increases, it allows air to fill the WG chamber for a longer period of time, increasing the pressure when the volume is filled, until you reach your target and the WG valve no longer opens any wider.

Basically, the longer the duty, the more time both sides of the solenoid have to equalize their pressures. With a heavier spring on the right side it's going to take a little longer as the volume is constantly changing when the valve starts to move.

Due to the design of a particular solenoid, there will likely be a non-linear correlation in duty cycle as the spring pressure is increased. For this application, with a heavy spring it may take larger increases in duty to make smaller increases in boost.

At least, this is how I understand it.

 

[llxkevinxll]

A heavier spring in the WGS requires more air mass/pressure, thus a greater duty cycle.

I think you have this in reverse. If we assume this is true and take it to its logical extreme, the lightest spring possible, or ideally no spring at all, would result in the most boost pressure with the least amount of influence (lowest DC) by the boost control solenoid. We know the general rule of thumb is 2x spring pressure is possible with typical boost control methods, so a very light spring would only be capable of very low boost.

 

[curt-s]

I think you're right, as lower duty cycles result in less boost potential. So it's not a normally closed solenoid, it's a normally open and duty cycle on controls how long the solenoid is NOT venting the wastegate side.