onspeed

I started my post with "If I'm not mistaken" I've never tried hooking a WG to the manifold, so I'm not certain if it would open under vacuum, but it makes sense in my head that it would. I am fairly certain though that the IC pipe is never under vaccum. The stock hard pipe, as well as any aftermarket hard pipes I've seen, is drilled on the Turbo > IC pipe to allow for a vacuum fitting.

Black51

Well I'd be interested in trying that to see how that compares to my current routing. Where did you get your fitting for the pipe?

onspeed

Stock CV uses a banjo bolt. Some people get a fitting for the banjo bolt, personally, I took the vacuum nipple from the stock wastegate. Threads in perfectly in place of the banjo bolt.

Paulyy

correct me if im wrong but don't you see 2 different pressures from the boost pipe and manifold?

Black51

Ahh, well unfortunately the PO installed the WG so I don't have the original. I'm still confused on how 'only' the boost pipe could never be under vacuum. If the engine is pumping air out the back of itself, it shouldn't matter anywhere along the path leading up to it, it should all be under vacuum. Can you explain your theory?

Black51

I would assume so. The boost pipe has a different volume than the manifold. Which is why I would think to take a measurement from the manifold as opposed to the pipe, to get a more accurate reading of what's going into the engine.

So if you're reading boost from the manifold but the WG is hooked up to the pipe (which I would assume is a higher pressure environment), then the WG would be opening early under boost.

onspeed

Need to think about that and get back to you. I remember reading that when I was having issues with my MBC/WG though.

And I don't think pressure differences should matter for a wastegate since you're using a boost controller and not running off wastegate spring only. Pressure in manifold is what the cylinders are seeing, so that's important to monitor with a boost gauge.

Black51

So pressure differences don't matter. Ok. I understand all that you would need to do is set the boost controller to a higher setting so that the manifold pressure sees 16psi, or whatever the desired boost is, when the wastegate is hooked to the pipe.

But at all times off boost, air is being sucked into the engine starting from the intake, then turbo, then IC, then throttle body, then manifold, and finally the engine itself. So anything in front of the engine should be under vacuum when not under boost. That's why I don't understand why you couldn't just hook the wastegate up to the manifold. Seems to me like it wouldn't make a difference.

TurboTommy

The reason there is different pressures between the pipes and the intake manifold is because of the throttle. Only after the throttle plate will the sucking action of the engine cause a vacuum, so yes, the IC pipes will never be under vacuum. Even under partially open throttle (which is most of the time, unless track car) the IC and pipes will have absolute pressures significantly higher than the intake manifold, all the time. Only at full throttle will the pressures be almost even. One should always take the boost signal from just after the turbo discharge (IC pipe), otherwise there could be significant boost spikes and a loss of part throttle power (and loss of gas mileage, if you care about that)

Paulyy

I was going to say the same thing but wasn't to sure if i was right. i knew i've read it somewhere and heard it before also. i didn't want to give out wrong info

Black51

Ahh, thanks TurboTommy. Didn't know about that. I'll have to rig something up for the bolt hole in my pipe.

Black51

It still doesn't make sense to me why there wouldn't be vacuum before the throttle. Air is still displaced before it, by the engine when not under boost.

Are you saying the pipes are basically under boost at all times? That the turbo is spinning fast enough to pump enough air into the pipes to say they are under boost?

Even then, I thought the wastegate opens when the system is under boost, not vacuum. I would think it gets sucked shut when under vacuum, since the engine is pulling air out of the hose to the wastegate.

Educate me.

onspeed

But you would also have a vacuum on top of the diaphragm, working with the spring. I don't know if it would be enough to open the wastegate or not...

Wouldn't hurt to just switch to what everybody else is doing. And make sure your hoses are routed right. Then report back if you still are seeing weird spooling.

Black51

True dat.

Oddjob

Quick thought:

At full throttle full boost, then complete lift off of the throttle - throttle plate slams closed. Turbo (and intercooler pipes) still at full boost momentarily, but intake manifold goes to vacuum behind the closed throttle plate. Bypass valve opens to recirc boost/flow back to the inlet of the turbo to keep the impeller from stalling. What do you want the waste gate to do in that split second?

If the WG diaphragm line is coming from the intercooler pipe it will respond to what the turbo is doing, reacting to pressure at the outlet of the turbo, so the WG will be modulating open. Exhaust engergy thru the turbine will drop and the turbo will slow dropping boost pressure.

If the WG reacts to manifold pressure, it will see vacuum and the WG will close pushing all exhaust energy thru the turbine. Which might even increase the turbo output momemtarily, causing a pressure spike in the plumbing between the turbo and closed throttle plate. Maybe not significant. At most might only cause a little quirky behavior/response from the engine during full load throttle liftoff situations.