I know this seems like a really dumbass question coming from a turbo guy; but, let's think about this for a second.

Let's say we have a wastegate spring that cracks open at 10 psi and it would be fully open at around 17 psi (this I know from bench testing a few wastegates). Suppose we operate in single port mode, for a moment, and we use a manual controller to achieve a desired 16 psi boost. So, all is well, accelerating along at 16 psi boost in the mid RPMs, keeping your foot into it and approaching the higher RPMs. We would assume, then, that the wastegate valve is almost fully open (because there's 16 psi of pressure pushing on the wastegate diaphragm)
But, as we do approach the higher RPMs, there's alot more exhaust gases that have to go somewhere; either through the turbo or out the wastegate. One could argue that the extra exhaust pressure pushes the valve open even farther. But from the boost signal in the charged system the valve is almost fully open already, in the example above. I can't seem to wrap my head around how a wastegate keeps the system in equilibrium.

I'm having some issues lately, and answering the above might give me some ideas.

 

There is a difference between the gate being open enough to regulate boost and being all the way opened. On my car I had the waste gate valve come apart resulting in it being 'all the way open' all the time. The car would make about 3lb of boost with the gate fully opened all the time.

 

Most likely the wastegate is not fully open when you first hit 17psi. Instead, it (like the turbo) works off of pressure differential. It will only open the minimum amount necessary to equalize the system. Additionally, there is pressure both post-wastegate and post-turbo, which, essentially, reduces the pressure across both.

What problems are you having?

 

At the risk of also sounding dumb, now that I've got a bigger turbo in the car, I've noticed something... the stock (930) recirc valve "flutters" while I'm at max boost (set by MBC at 17PSI). Does this fluttering somehow help to modulate that maximum boost, or is that simply an effect of the valve not being strong enough to stay closed at 17PSI?

I've also struggled to understand how the wastegate and recirc valve operate together to modulate boost. Or maybe they don't? Does the recirc valve only come into the picture when I lift off the throttle and give the charge air someplace to go, and the wastegate is doing ALL of the work to modulate boost under acceleration? If this is the case, then I'm with Thomas... how does the wastegate modulate pressure at a certain point (like 17PSI) if it's already wide open?

 

Recirc valve does not modulate or control boost. If yours is fluttering, there is a problem. Should only open when the intake manifold goes to vacuum (when you close the throttle). It then allows the charge air to recirculate back to the inlet of the compressor, so the turbo keeps spinning - otherwise the turbo would stall out when the throttle is closed, which would create significant lag when getting back on the throttle (like for upshifts).

 

It performs a ballancing act.. the more it opens the less exhaust gas is spinning the turbo, less turbo spool results in lower boost pressure slightly closing the wg, resulting in more gas to the turbo resulting in higher pressure WG opens more resulting in less gas.. and on and on and on.. this is WG flutter, and is most accurately controlled by an EBC imho. true, accurate WG control needs to be modulated to acurately manage the boost pressure being generated by any given turbo.

The opening pressure (back pressure of the exhaust gas) is offset by the spring. and the standard spring "pressures" given are an estimate more than an absolute.. the more efficient the turbo the higher pressure resulting from the standard WG spring.

At the end of the day, the exhaust gas will follow the path of least resistance, as long as the WG is sufficiently sized to allow enough gas to bypass the turbo.. a large enough WG, fully open would bypass enough gas to keep the turbo from spooling at all at maximum RPM. this allows the WG to trully and fully control the boost pressure, all the way to zero.

Yes your recirc valve preload spring is too small to hold the valve closed at your maximum pressure.. likely you have a 1 bar preload spring, and at more than 1 bar boost pressure the boost pressure is pushing the valve open!

 

Thats how i thought it worked,

 

A new recirc valve is on the way!

 

you dont need one

just block it of and get a cool flutter sound!

 

Lol and potentially a new turbo... Unless you run a Holset these things can take some sever abuse!!

 

Thanks guys I too was having this fluttering issue...this thread has given me a better understanding...

 

The thing is that when you bench test the wastegate, it's cut and dry that 17 psi dictates that the wastegate valve has travelled the entire way off its' seat. So if you get 17 psi boost, even early on in the rev range, how could it be that it will only open a minimum necessary to equalize the system?

How does the pressure differential across the wastegate affect its' performance? We might be on to something there.

I'm getting uncontrollable boost creep with a separate exhausted Tial 46mm.

 

Get a forge valve.

 

Getting a Forge knock-off from eBay...

 

You seem to forget to take the function of the manual boost controller (MBC) into account.

An MBC typically works as a spring loaded valve, which subtracts a fixed amount of psi (spring pressure) from the pressure between the compressor and throttle body. So if your manual controller is set to a 9psi drop and your boost at WOT is 16 psi then the WG sees 7 psi, not enough to crack it open.

It is true that the exhaust pressure increasingly helps opening the WG as a function of rpm, which is usually the reason that the boost (and torque) drop with rpm. If the valve area is a fifth of the diaphragm area, then from the example above the WG will start opening at a 5*(10-7)psi=15psi exhaust pressure. If the actual exhaust pressure for the 15psi of boost is 20 psi at a given rpm, then the exhaust will open the WG valve with a force equivalent to (20psi-15psi) / 5 + 10psi= 11psi in the control port on the bench.
The valve lift due to the exhaust pressure is highly non-linear though.

Hopefully this helps a little.

Laust