Diverter Valves

[Casius]

I'm trying to figure out how exactly DVs work. I understand their purpose but I'm a little confused about how the top nipple and spring inside work together. Is the little tube on top a vaccuum tube and is that what opens the valve? Is the spring there just to force the piston back down? I'm curious cause I got a forge splitter and it has the adjustable spring rate so I want to know what I'm doing by making it stiffer. Does the spring rate control how much pressure it takes to open?
Anyway, I've googled around a bit but the explanations I've found are all about what it does and not how it goes about doing it.

[diagnosticator]

The spring rate controls when the DV opens, and how fast the DV closes again, balanced by the vacuum or boost pressure applied at the control port, (the small nipple.) When the throttle is closing or closed, there is vacuum applied at the control port that causes the diaphragm or piston to oppose the spring force and opens the DV. When the pressure in the manifold is approaching ambient air pressure or exceeds ambient and crosses into boost pressure, the control port is pressurized by boost pressure forcing the DV to close quickly aided by the spring force. The manifold vacuum or pressure states are the main forces controlling if the DV as open or closed, the spring tension and pre load, mainly determines at what points the DV responds and determines DV behavior when the vacuum/pressure applied at the contol port is low or zero.

The internal balance of forces can be evaluated by summing the forces applied on the piston or diaphragm at any given instant and a net upward force (+)vector will indicate an opening or open DV, and a downward net force (-)vector indicates a closing or closed DV.

(Frame of reference depends on if using gauge pressure or absolute pressure for the balance of forces comparison.)

[Casius]

I knew this question would bring you out of the woods. That's how I figured it worked thanks man :)
Now, is the vacuum related to how much boost you run? Cause the vacuum opens the DV, but first it must overcome the spring force so if you have a stiffer spring it would stand to reason that you'd need more vacuum pressure. Unless that vacuum is at a steady number and all stiffening the spring does is delay the open time of the DV?
So a stiffer spring would keep the DV closed longer at low pressures right? Basically if you put a stiffer spring in the DV wouldn't open at low boost if youre driving around applying low throttle like if you're just driving around normally... but that also wouldn't release that pressure so could stall your turbo. Keep some pressure in the manifold so that when you get back on the gas you already have pressure in the mani.

Question central

EDIT: I know what I was trying to say, you could adjust the amount of pressure left over in the manifold by stiffening the spring? I'm just missing the piece of info regarding where and how large that vacuum pressure is

[diagnosticator]

The amount of vacuum depends on RPM and how far open the throttle is, before boost is developed, and if the engine is in overrun or not. Your other conclusions are essentially correct regarding spring tension delaying the response of the DV as spring tension increases. Higher spring forces also advance DV closing compared to manifold pressure/vacuum. When the DV is installed correctly, allows for using the lightest spring available for best DV response. The spring tension is not the force that holds the DV closed against boost pressure, the boost pressure itself does that. Using different spring pre load and or spring stiffness allows for fine tuning the DV response. But in general, using the lightest spring is preferred.

[Casius]

Lightest spring to release pressure as fast as possible and prevent surge?

What would be the limiting factor for how light you go with the spring then? The boost from the manifold closes the DV anyway right. Let's, for the sake of argument, say that engine rpm and manifold vacuum pressure have a linear relationship. By increasing spring tension you could adjust at what rpm the dv closes after being open. But then you need it light enough that it can be open at idle. Hmm... Sorry once i get a hold of something like this I need to figure it out. Takes some time sometimes but once I get it I get it

[walky_talky20]

There are some things to discuss here specific to the splitter valve. The spring must be kept stiff enough that a leak to atmosphere is not created at idle. If you know the operation of the splitter valve, you will know that only at the full open position will it open the charge pipe to atmosphere for the "pshh" sound. You want to keep the spring tight enough that it isn't leaking to atmosphere during idle or part throttle, only when closing the throttle quickly under boost.

This is part of the reason why they use a progressive rate spring. They want the last little bit of piston travel to require more than just vacuum from the control port. It also requires some boost pressure pushing against the piston from the other side. This is also why it can only be installed the "correct" way, with the piston opposing the charge pipe. It needs the boost pressure from the charge pipe to push back on the piston to get it to full deflection. Only at full deflection will you get the "pshh" noise.

There is one other factor that must be discussed here. That is the N249 valve. This valve, controlled by the ECU, determines what the DV control port sees. It can change it between intake manifold pressure or full vacuum from the vacuum reservoir in the fender. To understand what it is doing, and why this *specifically affects the splitter valve*, you must first understand the purpose and function of the N249 valve and it's ECU control.

The N249 main job is to pre-spool the turbocharger. It opens the DV at part throttle to keep the turbo RPMS high. Basically it does this by creating an internal boost leak - the open DV. With the turbo rpms high, when a boost request is made, the N249 closes and DV and the boost is [supposedly] available slightly sooner. It is popular to bypass this valve so as to remove the slight delay it can cause when closing the DV.

There is another problem that is specific to the splitter valve. Because the N249 is keeping full vacuum to the control port during the pre-spool period, there is a propensity for it to leak to atmosphere at part throttle acceleration. During pre-spool there is very slight boost pressure in the charge pipe. This isn't shown on your boost guage as the throttle plate isn't fully open. This slight pressure applied on the piston, in conjunction with FULL vacuum to the control port provided by the N249 is enough to move the piston far enough to leak to atmosphere. This is audible and repeatable.

This leak problem is enough to affect fuel trims by as much as 5% in my experience and in general make the car run crappy at part throttle. By deleting (looping) the N249 and connecting the control port of the splitter valve directly to the manifold, this leaking was completely gone. The other option is simply to tighten the valve a lot more. This would work, but would make the spring much tighter than it otherwise would need to be. It would also make the valve only "dump" on very aggressive shifts and possibly create valve flutter.

Sorry for the long post, but this is difficult to explain without being detailed.

[diagnosticator]

You need the spring tension high enough to close the DV or hold it closed with no pressure applied to the control port. Increasing spring rate changes the % opening of the DV as vacuum varies. Changing the spring pre load effects the point the DV begins to open in relation to the amount of manifold vacuum.

A graph of DV position vs. manifold vacuum/boost pressure can be prepared using the summation of forces occurring in the DV at defined intervals covering the compete operating range of the DV vs. the range of manifold vacuum and boost pressure. A graph will provide clarity regarding the interrelationships between the forces controlling the DV and the resulting DV positions.

My posts are limited to standard DV theory, I an not familiar with dual mode or split mode DV/Blow off valves.

[diagnosticator]

The N249 solenoid valve controlled by the ECU can also apply vacuum to the DV even if boost is present, to open the DV in certain situations to reduce pumping losses and increase fuel economy.

[Casius]

Haha I gotta find a few mins to read all the posts and think about it some but thanks for the replies, awesome info :)