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Why would boost PSI be tapering off at a constant WGDC

Boost Control

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Hey Guys,

Recently was working on re-tuning boost after having installed a downpipe and one thing I noticed in a most recent log that seems a bit odd to me is that I'm seeing the manifold relative pressure taper downwards during a pull even though WGDC is sitting constant at its max value. I would've thought that for a given WGDC the boost level would remain more or less constant? Is this an example of the turbo transitioning from a high efficiency area of operation to a lower efficiency area as the flow rate increases? Is it a symptom of a problem? Something else entirely?

Here is a screen cap of the relevant log.



Generally exhaust pressure will increase as exhaust flow increases, which will generally increase as RPM and intake side airflow increase. That exhaust pressure is acting on the wastegate valve, pushing it open, requiring more pressure in the opposite direction. If you can't apply more pressure via boost controller, boost will taper.

Keep in mind that if you have to max a boost controller out, you're likely creating a lot of exhaust pressure and heat, and not running at high efficiency.

Also, it appears boost controller duty is in the 6x% range, so I wouldn't call that maxed out.

Another thing to consider is that manifold pressure is a measurement of restriction.

If your previous downpipe was creating a choke point and you've improved on that then there is less restriction for the engine to breathe, making it able to consume more air, ergo inlet manifold pressure drops off at a constant gate duty cycle

Chris makes a good point. I happen to recognize the application so I'm familiar with what's going on, but in more generic terms yes that could be a factor.

Awesome! Yeah that makes sense now the more I think about it.

WGDC isn't just a direct boost knob you can turn up & down that isn't impacted by anything else going on in the system.

Also re: > "it appears boost controller duty is in the 6x% range, so I wouldn't call that maxed out."

Yes, sorry I meant up to the max value I had limited it to via the max WGDC tables since you can see TD proportional is getting added to no effect for a good portion of the run. I had actually intended for it to be higher than the 60s but I think the IAT comp might be a bit heavy handed.

Follow up question: So far to tune boost I had been observing what WGDC results in what output boost which I guess works well enough in some cases... but as we've just discussed... The relationship between the two can change depending on other factors. Is there a way to calculate or quantify this relationship? A way to characterize the WGDC to resultant boost level for a given car/setup as some curve function?

If you add sensors for all the other factors then everything makes more sense, but if the control strategy can't use them for corrections you still end up adjusting the base duty tables until the boost level desired is achieved.


So I turned up the WGDC some more to try and compensate but it seems like its still just dropping out the back and I think injector duty cycle there is already around 96%. Seems kind of like the turbo is just out of puff and/or its too restricted by the factory airbox? Either way Im guessing I'll just need to accept it as is until I can get some more fuel system in place and open up the intake path more. Thoughts?

I've attached a file log if that easier to view as well.

Attached Files

It looks like you are maxing out a lot of the components there, the injectors are well into the 90% range by 5k.

What is interesting in the second pull on that log is the primary boost duty drops off like a stone at the end but the boost still tapers off at the same rate, do you have the ability to record the final duty cycle?

Yeah in the second pull (not pictured but in the log), the wastegate duty drops like a rock as I have the commanded WGDC taper off in the final cell of the table which is 6400rpm but it is starting to interpolate to that already starting from 6000rpm so the WGDC tapering off would be expected there (screencap of table attached). As for why it tapers off in the same way Im not sure either. Maybe sensor latency or something like that?


Im not 100% sure what the distinction you are making is between "primary boost duty" and "final duty cycle". As far as I'm aware, the parameter that is logged there as "Primary Wastegate Duty Cycle" Is what I assumed to be the commanded WGDC after all compensations are applied but perhaps Im mistaken. I will double check what log params are available next time I log and see if there is another param available that may be different.

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