Boost control....what a pain in the rear

[Legacy777]

I've spent several hours searching on nabisco, and come to the conclusion that our stock boost control is far better then the WRX. (maybe this was already known...)

I've also reconfirmed that the so-called "guru's" over there are pompus retards.

The WRX & other late model subarus use a bleed type setup on the line connected the compressor & wastegate actuator. While on our cars, the line coming from the compressor goes to a three-way solenoid, and then to the wastegate actuator. This prevents the actuator from seeing boost until the turbo is fully spooled, or until the ECU says, ok I need to start bleeding boost to the wastegate actuator.

Depending on how the ECU is programmed, my previous thread may not be so true. The reasoning is because if the vf11 spool characteristics are programmed into the tuning, instead of just a feedback loop, then the ECU will be allowing boost to bleed to the actuator prematurely, and slowing spool. I'm going to hook a vacuum/boost gauge up to the line going to the actuator and see what's going on.

Irregardless of this, it seems like any aftermarket boost controller you go with will have some form of PTFB issues. I've read some stuff on here regarding MBC's and if you hook them up to the compressor housing, you can minimize/not get PTFB issues. However I haven't really looked into that too much. With EBC's, it seems like some have gain settings and other things that may help, but don't know if any have logic directly built in to prevent it.

So.....just venting a little on what I found out.

Any comments are by all means appreciated.

[free5ty1e]

Hey Josh --

I'm not sure yet either if our stock ECUs use a feedback loop to control the wastegate solenoid's duty cycle and promote spoolup, that's one of the things I plan to look into once I've got a garage with a shop and can put my oscilloscope on the car's various signals... I would imagine that any of the EBCs that use feedback loops (not sure which do) would take care of the PTFB as well as promote the absolute fastest spoolup (not opening the wastegate at all until the desired boost level was reached). If they don't, they are missing the boat. My unit will definately do that... man I can't wait...

Every MBC I've ever tried (I've tried both regular ball-and-spring and hybrid reduced mass ball-and-spring types, as well as the turboXS single-stage bleed style) has had boost spikes as well as PTFB (no way to differentiate between partial and full throttle), although they do promote the absolute fastest spoolup until they begin to open.

This is probably just rambling, as I tend to do, but maybe it'll help someone somehow somewhere down the line. Quotable quotes by Chris.

[rallysam]

My understanding is PTFB should have nothing to do with the type of boost controller (electronic vs. manual). It just depends on where you sense pressure - before or after the throttle plate.

Our stock boost control is an exception because it has access to other sensors.

[rallysam]

Ok, now I've got more time to respond.

Josh - I'm not familiar with your story, but aftermarket boost control should not be giving you this much trouble. It was the easiest and most reliable mod I ever did. Hook up any variety of control system to the correct connections (the pressure sensing from the compressor not the manifold) and you should be ok.

Those connections aren't witchcraft. My Hallman came with very clear instructions for how to connect it. Also, once you really understand how the control system works and how the throttle plate interferes, it makes perfect sense why everybody has problems when they hook it up the wrong way.

A ball and spring is cheap and works great in all respects. Your spool up should be very close to optimal and you shouldn't get any PTFB. Anything else (like electronics) will be a very small improvement, if any, but maybe you like icing on the cake like on-the-fly adjustment.

[free5ty1e]

Sam - that kinda makes sense, you're saying to hook the input of the MBC up to the intake manifold instead of the nipple on the turbo compressor, and just plug that up instead? I'd never tried that setup, but it sounds like it would work better. Hmm.

Edit: Wait, I read that backwards. I've always used the compressor nipple for the pressure feed to the MBC. But the boost always spikes to ~2 psi over what I set it at, and then settles down after a second or so. Or maybe what I'm seeing is the inability to hold full boost to redline, although uncontrolled even the stock VF11 easliy rocketed up to 21+ psi, so I can't see why I can't achieve stable boost levels without electronic control.

Edit 2: Also... I can't see how you could avoid PTFB with a MBC --- if your turbo is capable of outputting up to the set boost level at the current throttle position (and resultant exhaust output), then it will. There is no partial throttle boost setting, nor is there a way to do that without at least a solenoid. (at least not one I'm aware of).

[rallysam]

Sam - that kinda makes sense, you're saying to hook the input of the MBC up to the intake manifold instead of the nipple on the turbo compressor, and just plug that up instead? I'd never tried that setup, but it sounds like it would work better. Hmm.

Edit: Wait, I read that backwards. I've always used the compressor nipple for the pressure feed to the MBC.

Yes, I'm saying hook it up to the compressor.

But the boost always spikes to ~2 psi over what I set it at, and then settles down after a second or so. Or maybe what I'm seeing is the inability to hold full boost to redline, although uncontrolled even the stock VF11 easliy rocketed up to 21+ psi, so I can't see why I can't achieve stable boost levels without electronic control.

Yeah, I don't know your setup but I'd guess that is inability of the turbo and intercooler to flow a large volume of air. On my car, the manifold reads 13 psi @ 2krpm, but only 10psi @ 5krpm. The turbo can make lots of boost - but it can't flow a lot of air. So, as the engine picks up speed and asks for more air the turbo (and intercooler!) can't flow it.

Edit 2: Also... I can't see how you could avoid PTFB with a MBC --- if your turbo is capable of outputting up to the set boost level at the current throttle position (and resultant exhaust output), then it will. There is no partial throttle boost setting, nor is there a way to do that without at least a solenoid. (at least not one I'm aware of).

Here's the confusion - what do you mean by FB? Full boost at the compressor (ok) or full boost in the manifold? (bad).

The boost controller is going to create full boost in whatever pipe it is plumbed in to sense. So yes, if you hook the controller up to the compressor like you should, it will allow full boost at the compressor even when you are at part throttle. That's ok. I don't think that's what people mean when they say PTFB. That situation is ok because your throttle cuts that pressure down and you don't get full boost in the manifold. That's what my car does now and it's happy.

On the other hand, when you hook the pressure sensing up to the manifold, it will try to create full boost in the manifold at all times. If you pull back on the throttle to obstruct the airflow, the boost controller sees the pressure drop and tries to fight it by spooling the turbo up to even HIGHER pressures than you intended. The engine runs like it's at full throttle even when you've pulled back. Badness = "P.T.F.B - in the manifold".

[free5ty1e]

That's right, that was the confusion. PTFB in the manifold is the one you definately want to avoid, overworking the turbo to achieve the set pressure.

PTFB in the plenum is not nearly as bad, I still want a separate partial throttle boost setting with an adjustable throttle threshold though, because I don't necessarily want full boost when I'm cruising.

So yeah it does seem like most of us experience running out of steam in higher RPMs, even though the turbo can put out the pressure. Think that's the compressor's ability to flow enough air, or the restrictive stock heads?

[Legacy777]

See....that's sort of the issue too....I haven't tried a MBC, so I really don't know how it would react/work. It may work fine, minus having to tweak it for ambient conditions......but I don't know.

[rallysam]

Josh - have no fear

So yeah it does seem like most of us experience running out of steam in higher RPMs, even though the turbo can put out the pressure. Think that's the compressor's ability to flow enough air, or the restrictive stock heads?

It's not the heads. We see the pressure drop on the boost gauge, which tells you the bottleneck is upstream of the gauge. I'd guess stock turbo and possibly the intercooler also, depending on the setup. BAC5.2 said the drooping went away when he replaced the stock turbo with an upgraded one, so I'd guess that's a bottleneck for us too.

[Imprezive]

rallysam, where would you hook up the boost gauge then? I would assume the manifold, that why you can monitor the actual pressure inside the manifold.

[dzx]

Sam - that kinda makes sense, you're saying to hook the input of the MBC up to the intake manifold instead of the nipple on the turbo compressor, and just plug that up instead? I'd never tried that setup, but it sounds like it would work better. Hmm.

Edit: Wait, I read that backwards. I've always used the compressor nipple for the pressure feed to the MBC.

Yes, I'm saying hook it up to the compressor.

But the boost always spikes to ~2 psi over what I set it at, and then settles down after a second or so. Or maybe what I'm seeing is the inability to hold full boost to redline, although uncontrolled even the stock VF11 easliy rocketed up to 21+ psi, so I can't see why I can't achieve stable boost levels without electronic control.

Yeah, I don't know your setup but I'd guess that is inability of the turbo and intercooler to flow a large volume of air. On my car, the manifold reads 13 psi @ 2krpm, but only 10psi @ 5krpm. The turbo can make lots of boost - but it can't flow a lot of air. So, as the engine picks up speed and asks for more air the turbo (and intercooler!) can't flow it.

Edit 2: Also... I can't see how you could avoid PTFB with a MBC --- if your turbo is capable of outputting up to the set boost level at the current throttle position (and resultant exhaust output), then it will. There is no partial throttle boost setting, nor is there a way to do that without at least a solenoid. (at least not one I'm aware of).

Here's the confusion - what do you mean by FB? Full boost at the compressor (ok) or full boost in the manifold? (bad).

The boost controller is going to create full boost in whatever pipe it is plumbed in to sense. So yes, if you hook the controller up to the compressor like you should, it will allow full boost at the compressor even when you are at part throttle. That's ok. I don't think that's what people mean when they say PTFB. That situation is ok because your throttle cuts that pressure down and you don't get full boost in the manifold. That's what my car does now and it's happy.

On the other hand, when you hook the pressure sensing up to the manifold, it will try to create full boost in the manifold at all times. If you pull back on the throttle to obstruct the airflow, the boost controller sees the pressure drop and tries to fight it by spooling the turbo up to even HIGHER pressures than you intended. The engine runs like it's at full throttle even when you've pulled back. Badness = "P.T.F.B - in the manifold".

It's my understanding that the pressure from the compressor varies a lot. If you hook up a boost gauge there, it doesn't work too well. I have my hallman hooked into the intake manifold and the car hits 18psi consistently

My boost spikes also just before the wastegate opens and my turbo is large enough to hold well over 20psi through the redline. I would imagine that it's similar to static friction. Where once you break the static friction, the needed pressure to open the wastegate more drops, so you see the boost settle after a moment.