Boost Control Essentials

[S2central.net]

Paul, you're on the way to answering a question I had after the previous thread which is if the ECU controls the boost under closed loop control using the pressure sensor for feedback, why does the duty cycle of the WGFV matter? In fact the WGFV doesn't have a duty cycle as such, it's driven by the the duty cycle output from the ECU and as long as it can reach the required pressure within the duty cycle range the ECU can supply then any WGFV would do???

Great stuff, look forward to reading more

John

Yes - the WGFV is nothing more than a solenoid that is driven by the ECU... The duty cycle is provided by ECU software. You are right to say that any WGFV would do the job - as long as it responds quickly to duty cycle modulation and is reliable (doesn't stick) and robust enough in the hot engine bay to keep operating.

AFAIK - the ECU uses the nominal amount of duty cycle set in the software to achieve target boost pressure for the prevailing conditions (load and demand). It has the ability to adapt the duty cycle to cater for aging, thermals and such like... I don't know how much it can adapt by, but there are fault codes raised by the ECU if it detects excessive boost (unable to reduce duty cycle to achieve target) and for a failure to achieve the target boost if duty cycle cannot be increased within adaptation limits.

Something that occurred to me today about these dual port WG configurations is that the upper chamber MUST remain airtight for those to work properly. Note that many a wastegate cap that has been 'frankensteined' with an adjustable screw will not have an airtight upper chamber - even though the membrane to the lower chamber is a perfectly airtight seal.

As time permits I will do some dabbling with different springs and WG topology and make ECU tweaks to seek the best from them. I'll also test that turbosmart solenoid as well for kicks.

[S2central.net]

Thinking more about this Turbosmart eBoost Valve (TSBV) and how it would behave with consideration for the large amounts of air that bleed past the WG valve when control pressure is applied to the lower chamber... Noting how the TSBV behaves differently from the WGFV in the way the air paths are setup in the 'ON' position I will wager that the TSBV provides for improved boost control as there is considerably less of an air leak via the WG escape port and the MAF hose.

With the factory WGFV, regardless of whether the solenoid is ON or OFF - see how pressurised air from Port 3 gets to Port 2 - meaning the turbo has to do more work to sustain target boost levels. This blowby only happens with the TSBV when its in the OFF position - allowing the WG to open for a reduction in boost to keep it in control and so directly reducing the amount of air lost to blowby.

Thats the theory at least... I have only just tonight postulated this theory - so forgive me if it turns out to be a bit pants

[prj]

Going to do some necroing here.

In the "Alternative Layout B" in the PDF, why does it say "When the WGFV is OFF, the upper chamber is at Ambient pressure"?
If you boost, then when you lift off and the WGFV goes OFF (right?), wouldn't the pressure be trapped in the upper chamber? Or am I misunderstanding something?

Both of the other layouts I understand, since the upper chamber is either always at ambient pressure or when Port 1 is closed as it has a physical connection to the intake.

[donce]

Going to do some necroing here.

In the "Alternative Layout B" in the PDF, why does it say "When the WGFV is OFF, the upper chamber is at Ambient pressure"?
If you boost, then when you lift off and the WGFV goes OFF (right?), wouldn't the pressure be trapped in the upper chamber? Or am I misunderstanding something?

Both of the other layouts I understand, since the upper chamber is either always at ambient pressure or when Port 1 is closed as it has a physical connection to the intake.

ye, i am thinking the same, that "the pressure would be trapped in the upper chamber"
any corrections from anybody?

[prj]

It actually will be trapped there.
And the spring will need to be 3-4 psi, because wastegate will be opened by exhaust backpressure.

Just use alternative layout A, that works nicely

[GazzaG60]

good stuff paul.

never used the new gate design. gotta say though i have always routed external gates to have pressurised top chamber. was my first mod on the S2 along with the eboost. i like the idea of equalising pressure both sides of the valve.

i have never seen boost flutter other than on n75 or older designed EBC.

[prj]

Boost jumps 0.1 bar back and forth because N75 has two states. ON and OFF. Boost control is achieved by having the valve oscillate. This is why it is normal for the needle to flutter under boost and it is nothing bad.

[GazzaG60]

sorry for me it should be smooth. the open asn shut should be fast enough to not feel flutter. thats the idea.

my boost dont jump back and forth. 0.1bar is about 20hp.

[prj]

sorry for me it should be smooth. the open asn shut should be fast enough to not feel flutter. thats the idea.

my boost dont jump back and forth. 0.1bar is about 20hp.

I don't feel any "flutter" at WOT due to boost control.
You probably mean other kind of flutter, where not enough air bleeds off towards MAF through N75 and boost surges hard.

For that use "Alternative Layout A" and you will have same setup as eboost and other controllers.

[GazzaG60]

sorry i dont use a maf. im vems.

i mean boost fluctuation due to poor control or an issue.

boost should hit and hold not fluctuate on the gauge 0.1bar.

i cant remember the diagrams. i pipe top and bottom. always have on external gates be it with apexi or eboost.

think we just misunderstand each other like you say

[prj]

sorry i dont use a maf. im vems.

Doesn't matter what you use, the topology of the N75 matters. Air gets bled off from the top wastegate chamber back between the MAF and the turbo. If you don't have a MAF, then before the turbo.

i mean boost fluctuation due to poor control or an issue.

There is no issue with the N75 up to 2.2 bar. I have not tested more.

boost should hit and hold not fluctuate on the gauge 0.1bar.

It's not even 0.1 bar, more like 0.03-0.05... and why shouldn't it?

think we just misunderstand each other like you say

I don't think we misunderstand eachother.
To avoid getting personal and totally offtopic, I'll PM you the rest.

[Jamo]

N75 = bad for big boost

Remove them and replace with solenoid valve.

[prj]

N75 = bad for big boost

Remove them and replace with solenoid valve.

Define big boost? And why is it bad?

[GazzaG60]

its not bad if it works. there are a few who have found, maybe due to age of valve that it wont hold boost properly.

i found this at stock boost.

above you stated boost jumps back and forth 0.1bar. now is 0.03 -0.05.
i only stated i have not seen this with a proper newer valve only N75.
as far as im concerned boost should hit and hold. the solenoid is not doing a good enough job otherwise.

maybe i stepped into this one too late and am missing something.

[prj]

its not bad if it works. there are a few who have found, maybe due to age of valve that it wont hold boost properly.

If you use a broken N75 of course it won't work.

above you stated boost jumps back and forth 0.1bar. now is 0.03 -0.05.

The needle slightly oscillates around the requested boost point. I don't remember exactly how much it changes. But the car accelerates perfectly smooth.

i only stated i have not seen this with a proper newer valve only N75.

And how do you think a newer valve regulates boost? You think it can regulate it like a spring? You think there is some sort of magic involved?
No, it cycles between ON and OFF states exactly the same way. Perhaps newer boost controllers have a higher frequency than motronic. But even with motronic the pull of the engine is PERFECTLY smooth. The only fluctuation I see is slight vibration of the boost needle once the ECU starts toggling the N75 to limit boost.

as far as im concerned boost should hit and hold. the solenoid is not doing a good enough job otherwise.

There is no "hold". It's like a computer monitor. There is 50hz, there is 100hz and there is LCD, which has no refresh frequency because it works differently. In this case LCD is like mechanical boost control, because it is smooth, as the wastegate can be open, partially open etc, it's fluent... and electronic boost is like a CRT with varying refresh rate.

maybe i stepped into this one too late and am missing something.

Maybe try a properly connected non-leaking N75 valve, before you say "N75 is bad" in absolute terms?
I have no issue at stock boost, no issue at 1.7 bar and no issue at 2.2 bar with the stock N75 valve. Of course I am mapping it properly. If your ECU is mapped wrong or the valve is defective then of course all kind of weird things will start happening.

I have tested side by side, N75 and AEM Tru-Boost. Only difference was that Tru-Boost lacked any kind of throttle control, because it would build tonnes of boost at part throttle, making the car annoying to drive.

It's really interesting too, how you guys come and say in every thread "N75 is bad, replace it", without any argumentation to back it up.
I can bet you, that if I install a proper N75 valve in your car, connect it properly and map it properly, your car will have NO issue holding boost smoothly and the car pulling well upto 2 bar.
I can say that because I have done this.

P.S.
This is all written assuming you use Layout A in the PDF, so boost does not escape when N75 has signal, because the wastegate starts tapering...