Thread: Ask a tuner...
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Old 03-23-2010, 06:30 PM   #7
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2014 P3 Import Auto
AWD Dyno, Mods, etc


Originally Posted by Brando5185 View Post
Surging boost under 3/4-full throttle. Causes?
Before I go into too much detail its important to understand that most turbo charged engines with larger high pressure turbos don't like part throttle boosting and control can be a real pain. I'm not saying it cannot be done but let me throw out some of the factors.

MAIN PROBLEM: The gain or duty cycle of the boost control to reach desired boost is usually drastically different at part throttle vs. wide open.

Lets assume were talking about an IWG (internal wastegate) which is what stock Subarus and a lot of bolt on turbos use. The IWG has a WGA (waste gate actuator). The WGA has a spring that is designed to keep the waste gate shut up to a certain pressure load. What ever the WGA spring is set to is what your min. boost will be. In other words if you told your boost controller that you want zero boost you would still get what we call "Spring pressure" which might be 6-11PSI in the case of a Subaru typical turbo setup. So the first problem is you cannot make zero pressure at part throttle or below min at part throttle.

Now lets look at a lot of guys/gals that run manual or added on electronic boost control. Does the boost controller know the difference between part throttle and wide open or even off throttle? A good boost controller will monitor throttle position for this very reason. Does yours?

Part throttle boost control issues can also stem from you point of boost control signal. Where do you pick up a boost signal to control your WGA, are you getting it pre-throttle body or post. When your at part throttle there can be differences between the manifold pressure and the pressure you might read at the turbo outlet nipple for instance. In fact there may be a pressure drop after you intercooler as well, but thats another discussion. So you really should be getting your boost control signal from the manifold post throttle body.

The ultimate setup would be two boost gauges one from the turbo outlet region and one from the manifold, then you could monitor for leaks, blocks, or large pressure drops due to restriction in the intercooler. Cool huh?

There can also be some other strangness like turbulence, compressor stalling, etc which will cause the boost to spike and dip as the controller tries to keep up with the changes. Remember a boost controller (electronic style anyway) is constantly changing the duty cycle that it pulses the signal to the WGA. It has to look at the boost currently and figure out what changes to make to hit the desired levels. Sounds simple but its not considering every change is not instant due to turbo lag, delay from longer pipes, and other factors like what I mentioned above. The boost controller is also only allowed to make so much change at one time. You controller problably isn't allowed to pull enough duty cycle off the WGA to lower the boost where you want it.

There is tons more to this and I suggest some reading on how boost control works in general. I know your running an EBC so check all the hoses and make sure you have it connected the best way. If it has no idea of throttle position then you may not be able to solve this problem. I would need to know more about the EBC to answer better.

I think the best solution would be to go back to using the stock boost control system. It knows throttle position and can control part throttle boost much better than any MBC or most EBC's. Other wise drive with a heavier or lighter foot and stay away from that sweet spot that it spikes at. Also allowing the car to control the boost gives you another failsafe in case of problems the ECU you set you back to spring pressure.

Oh another thought, with Subarus I have noticed the by pass valve or blow off valve will crack open at part throttle boost some times. This can cause all kinds of issues as well and it is due to where it gets its vacum signal from to open and how much pressure it has to keep it shut.
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