Wastegate boost source, discuss

Discussion in 'Technical' started by MagicMike, Oct 4, 2016.

  1. MagicMike

    MagicMike Moderator Staff Member

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    Just been reading up on dual solenoid boost controllers, and came across this thread:
    http://www.sr20-forum.com/informati...ost-control-wastegates-how-turn-up-boost.html

    Where should I get my wastegate boost source from?

    This is a common question so it is time to go over the options.

    First off, you only want the wastegate to see positive pressure (boost). Under no circumstances should the wastegate ever see negative pressure or vacuum unless you're a mad scientist doing something crazy in which case you don't need my help.

    There are two reasons for this.

    1) Allowing a diaphragm-type wastegate to see boost and vacuum in quick succession or vacuum at all will wreak havoc on the diaphragm. The diaphragm is not designed to see vacuum or cycle between vacuum and boost. I'm not sure how much this shortens the life of the diaphragm, but the wear is significant.

    2) Even if you don't have a diaphragm-type wastegate, there is a more important reason not to let the wastegate see a vacuum signal. If you use a combination boost/vacuum source then there will be situations where you're trying to limit boost to the engine with the throttle plate but the turbo will be forced to create more and more boost.

    Think about what happens during a WOT shift.
    You have boost to the wastegate and it is fully open as you approach the redline. Then you lift off of the throttle which instantly pressurizes the system from the compressor to the throttle body. Hopefully your BOV or recirculation valve is now opening to prevent compressor surge. It is at this time that you want the turbo to stop creating boost. As the pressure in the intake system drops from the spike it experienced at throttle closing the wastegate should be open to reduce the chance of adding additional pressure to the charge pipes. If you have the wastegate plumbed to the intake manifold it will see vacuum this entire time and stay securely closed, promoting more boost in the charge system at the very moment you're trying to get rid of it.
    On some systems this can cause minor drivability issues and on other systems this can cause severe on/off throttle behavior and reduce your partial throttle control to almost nil.
    The worst situations happen during partial throttle closing situations. You're closing the throttle trying to reduce power output and creating a vacuum in the intake manifold, meanwhile the wastegate forced closed by the vacuum signal it is getting from the manifold so it tries to force more boost into the system. Transient throttle response gets quite messed up.



    I don't think many (if any) on here have ever questioned taking the reference for a boost controller install off the plenum.

    Discuss.
     
  2. black baz

    black baz black 'n blue Bazemy

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    Okay ... I'll be the bunny ..... I is discuss-ted ... !!!!!
     
  3. MagicMike

    MagicMike Moderator Staff Member

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    Prob wrong section. On my setup, i have no pre throttle body nipples (hehe).

    So what happens inside the varying solenoids between gate nipple and plenum source under vac? What about mbc's? Are they all one way valves as such?

    The statement that you never want the diaphragm to see vac, under what circumstances would this happen to warrant that guy saying it?

    And is there any merrit in his comments around transient throttle?
     
  4. rob260

    rob260 Moderator Staff Member

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    I haven't read the article but most (all?) manufacturers specify a boost source beteeen the turbo and the throttle body. Running it off the balance tube (mine was setup that way too; border piping kit) I think is just lazy. It's not hard to weld a fitting or even tap the compressor.
     
  5. BLACK BEAST

    BLACK BEAST SLICKTOP TT R-SPEC

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    I'm guessing mine is running from balance tube as well ..

    Is it going to make any noticable difference if I change it?


    .
     
  6. Fists

    Fists Well-Known Member

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    Some manual boost tees have check ball which will prevent the actuator seeing anything negative, used to improve spool a bit. Others are just simple bleed valves the same as our stock set up minus the solenoids. In either case the gate will slam shut if you close the TBs so your BOVs will be working overtime, maybe good for keeping on boost though? I would be surprised if the diaphragm in any modern actuator wasn't strong enough to handle manifold vacuum.

    My best guess with an EBC at transient throttle it will probably make setting the gain/solenoid profile a bit more confusing/annoying, since you're using the TBs to control airflow into the engine they're obviously a significant choke point, which means there's a pressure drop from one side to the other. With a standard (pre-tb) signal line when you've still got you 25psi in the intercooler but the restriction of the TBs means your controller is 'seeing' 20psi it will move to the 20psi point on it's map. The solenoid duty map controls how much boost is bled off and is a function of target boost and the 'gain' setting (good modern controllers have more options to give a more intelligent map). If you set the gain really aggressively for maximum spool the controller would ramp up at this point until you have, say, 30psi in the cooler and manifold rises to the set 25psi, counteracting your attempt to reduce power. This is part of why you have to 'calibrate' the gain so the gate stays open enough at partial throttle that you don't hit the 'critical mass' of airflow and get boost creep and boost spiking (like boost creep with split dumps it's only a problem once there's enough air to saturate the gate's flow capacity).
    If the signal line pressure also drops then the controller gain will have to be even lower because you've got less force to work on the actuator so it will close more, increasing turbine speed, you will probably see this effect less the higher above spring pressure you're running. Lower gain means slower spool on WOT as well though so you want to run it as high as possible without causing spike or creep.

    As to the magnitude of the impact all that has on your car you'll probably have to ask someone with relevant experience with your set up.
     
  7. ProckyZ89

    ProckyZ89 Senior Member

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    I'm going to look tomorrow
     
  8. SRB-2NV

    SRB-2NV #TEAMROB

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    My eboost2 is tapped into the cooler piping pre TB. Never heard of using the balance tube for the ebc boost reading. Only thing that should be taken from the balance tube is bov and boost gauge signal.
     
  9. Fists

    Fists Well-Known Member

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    When you say boost reading for the eboost do you mean the line that goes in the back of the controller? Turbosmart actually specify that should be from the manifold, not sure why exactly.
     
  10. Cheddle

    Cheddle New Member

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    It should be between TB and turbo.

    I ran a CA18DET from the manifold and it would overboost something chronic if I held 1/4-1/2 throttle.
     
  11. Martin Williams

    Martin Williams Well-Known Member

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    My race car was set-up exactly the same.
     
  12. SRB-2NV

    SRB-2NV #TEAMROB

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    No i mean for the solenoids boost reading. The solenoid is connected pre tb and to the wastegates.
     
  13. BLACK BEAST

    BLACK BEAST SLICKTOP TT R-SPEC

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    Yeah mine is at the plenum

    Going to look at putting it just after compressor outlet on piping


    After some quick reading seems to be the best place

    .
     

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