mk3cn4

iTrader: (3)

Ok, I know wastegate needs pre-throttlebody reference, but I'm the type of person who needs to understand WHY. I promise I've googled for an hour and can't find the answer WHY:

The Wastegate has a spring that will open at, say, 7psi in my case, when the boost pressure behind it gets to that level pushing it open. Why do I need a vacuum (actually pressure) reference AT ALL when the spring will open at 7psi and keep it from overboosting? What does that vacuum line really "do" (I know, there's no vacuum there, it's outside the manifold, just not sure what else to call it). I'm attaching to the bottom port of the wastegate so it makes it "easier" to open...? Why not just use a properly-sized spring?

I am finally done with the building part of my turbo project and will be TUNING now so I need to understand this.

Is my assumption wrong that the pressure in the hot piping is what opens the valve? I can push the valve open with my thumbs, seems like 7psi is about what it would take, so I don't think that's the case. So I don't see the need or use for the reference and the diaphragm.

Thank you to anyone who can explain.

NOTE: I am only referring to designs without boost controllers

GMCGreg

If you want any chance of hitting and holding a certain amount of boost you need to apply boost pressure to the underside of the gate diaphragm, that way boost pressure is what opens the gate . It may open just from exhaust backpressure but what boost pressure you hit will be unknown until you actually try it - risky . If you're tuning it yourself I would stick a light spring in it , hook up a boost reference below the diaphragm and start tuning. When you have everything perfect at a low boost setting then you can re spring it and tune for higher boost or add any type of controller and start turning it up

mk3cn4

iTrader: (3)

But in a non-boost-controlled environment, which lots and lots of people do, you rely on the spring to determine PSI. Why have the reference line at all? That's what I don't understand. There's no more or less risk with that that I can see. Also, the reference (being only positive pressure, and applied under the diaphragm via the bottom port) would reduce the spring's pressure, right? Why not just use a weaker spring to begin with? And... having the reference doesn't allow any better fine tuning of the PSI since it's just plugged straight into the cold piping. Still not seeing the reason to have it. Maybe the whole point is that if you're not doing a boost controller it really doesn't do anything but reduce spring pressure a bit?

mk3cn4

iTrader: (3)

Ok, this just clicked, I had to read it a few times. So I bet the reason is that the Wastegate is ONLY "calibrated" for certain springs at certain PSI using the diaphragm mechanism. I bet the wastegate manufactures don't calibrate the spring open pressure AT THE FACE of the valve, as it may be that it takes 7 or 10 or 20PSI at the face of the valve, it's just not calibrated that way. It's calibrated at 7psi AT THE DIAPHRAGM. So that's why it needs the pressure reference there, to guarantee a 7psi spring opens at 7psi for those who NEED it to be 7PSI.

So if that's the case, someone who is willing to just live with "whatever" the wastegate happens to allow at its face unreferenced would be fine just leaving the hose off...?

Me for example, I threw in a 7PSI spring, I really don't care what the PSI is as long as it's in that ballpark as I am going to tune around whatever PSI I get. Maybe I don't need the reference and I'll just get what I get. This theory kinda makes sense if that's the case.

EDIT: anyone reading this far, I am dead wrong on this point, please read down a little further in this thread.

GMCGreg

That's the balancing act , boost pressure vs waste gate spring . Without the reference line it would be an exhaust relief valve which can control boost but a lot of variables and likely boost would be higher than you are targeting. If you want to safely and accurately control boost you'll need a reference line , could be compressor housing, cold side piping or manifold-all will work. A lot of compressor housings are already drilled and tapped for a fitting, this location allows for the shortest hose which is a plus to me . Without the reference line you will have no idea what spring pressure provides what boost pressure - that's what would be risky. Doesn't it make sense that to control boost pressure you need to reference boost pressure?

mk3cn4

iTrader: (3)

BINGO. You nailed it. Using the reference gives CONSISTENCY. There are probably a lot of things that can cause that exhaust pressure to change from time to time, like a hot day might end up making way more boost. Using the reference ensures 7psi ALL THE TIME regardless of conditions in that hot side.

Awesome just having a forum like LS1tech to bounce things around like this. THANK YOU for the info, I think I get it now.

GMCGreg

Anytime, glad to help

ddnspider

iTrader: (26)

You can't not run a reference line to the wastegate or you'll make unlimited boost until your back pressure is high enough to push the valve open. The wastegate line pushes the valve open overcoming spring pressure. This is way more force than back pressure overcoming spring pressure unless your back pressure is through the roof. By the time you hit this value you're motor is toast in most instances unless the turbo is too small.

gametech

iTrader: (1)

I have never used a pre-throttle body reference. I only use actual intake pressure as a reference. However, I also always run a blow off valve. I can imagine that a pre-tb reference might have some small benefit if not using a BOV. As to the meat of your question, the wastegate is supposed to open from diaphragm pressure, not exhaust backpressure. If you only spring the gate to open with exhaust backpressure, the manufacturers could not possibly give you an accurate spring rating, as it would be entirely dependent on your entire engine combo.

pknowles

iTrader: (1)

Using the reference will get you mostly 7 psi all the time. However depending on setup, you may see boost drop off with RPM and that is backpressure adding to open the valve earlier. Backpressure is often higher at higher RPM. The waste gate manufacturers have no idea what everyone's backpressure curves look like, so they make some assumptions. On my LQ4, I have a 7875 (a hair small for the 6L) and before adding the electronic boost controller, boost would tail off a little after 3600-4000 rpm. It's not a big deal, it's just backpressure from all that flow trying to push through the turbine of that little turbo. When hooked up properly, you have reference pressure and backpressure pushing the wastegate open. Without the reference, your low rpm boost pressure before the valve would open would be very high until the rpm kicked up enough for backpressure to open the valve. I imagine your boost curve vs rpm would not be flat at all.

Jwooky

First the reference line should be the intake, not pre throttle blade.

Second, it would be virtually impossible for a gate supplier to make a valve that would open a specific pressure. There is just too many variables.

Readings the actual pressure is the simplest, most accurate closed loop system. Don’t over think it.

mk3cn4

iTrader: (3)

For the sake of others searching for this answer, here is where I landed.

Lots of good info, I appreciate everyone's posts!!! Makes sense to me now.

I was wrongfully assuming a static and predictable ratio between exhaust pressure and boost pressure. Now that I think it through and after talking with others and reading the posts here, the exhaust side pressure versus corresponding boost pressure could be all over the map based on turbo selection, pipe design, environment, back pressure etc. The wastegate needs to have the ability to regulate the boost pressure with consistency in spite of all these variables, so the only way to ensure X PSI boost is to actually measure the boost PSI and regulate against that.

In my example that I reference above where I would not reference the wastegate, if I throw in a 7lb spring at the time I tune I will likely get a particular boost PSI that may be in an acceptable range for me ONLY AT THAT TIME and I may even be able to tune around it. But when the engine cools down/heats up, or the weather changes, or maybe I change the muffler, or even if the fuel and altitude changes then the exhaust pressure will change and now I have entirely different boost PSI. The reference fixes this and makes the boost a more consistent/safe/tunable PSI.

mk3cn4

iTrader: (3)

I don't think this is correct. BOV is vacuum, but Wastegate "technically" should not be able to see vacuum. Many do it and have no problems, but being outside the throttle blade is the "correct" reference for a wastegate is my understanding. Some wastegates can be damaged by vacuum even from what I understand.

But your other points make sense and are very helpful. Thank you.

vorteciroc

Wow!
This could have gone badly.

mk3cn4

iTrader: (3)

Not following you. You mean running with no boost reference?

My initial thought was to tune around it and accept whatever PSI the WG spring provided, it would be in a constantly changing but still rough neighborhood of whatever PSI that particular spring provided I'd expect. Would be a bit challenging to tune and certainly not optimal but not a catastrophe, right? The tune would compensate for the varying MAP levels it would create. Trying to understand your point of it would have gone badly.

Full Power

.
.
Zombie thread.
, Please elaborate a bit, if you will.
I am having similar questions, regarding BOOST pressure source point. Seems that wastegate would more accurately maintain stable boost pressure with PRE-THROTTLE BODY port pressure taken from cold side pipe.
Obviously Boost reference fuel pressure reg and MAP sensor need actual ( post throttle plate ) manifold pressure.
Am I on the right rack here ?
.
Wild card: Where's the smart place to pick up reference pressure for BOV ?

Full Power

...
.
.I am NOT picking on the OP, I promise. Have we thoroughly de-bunked the above ?
I am plumbing vacuum ( boost ) lines now, on my first Turbo installation. Experts please confirm: ALL BOOST activated pieces are properly sensing Intake manifold pressure, correct ??
I have:
Fuel pressure regulator
Blow off valve
Wastegate.
.
<May have a Possible, future addition of boost activated exhaust "Loud Valve", but realize that is a long way for boost reference hose, and a LEAK in such a hose could cause bad things to happen closer to the turbine. Maybe smart to pull boost for LOUD VALVE from cold side plumbing, ie before throttle body ?
as usual, tips, advice, opinions and expertise much appreciated..

gametech

iTrader: (1)

I use brake line for those odd situations where I need to run an overly long boost/vacuum line. Less chance of burning or rubbing through, less chance of dry rotting, and less chance of eventually getting pinched by anything.

jack10685

Fuel pressure and BOV should be on the manifold, wastegate reference should be intake as close to turbo as possible

Plumbing a line for a loud valve will make negligible difference where you put it, I don't see an issue with having the reference on the turbine housing, plenty of people put one there and pretty much all IWG turbos reference from turbine housing

LS1Formulation

iTrader: (13)

This video gives an excellent explanation of different methods of boost control and plumbing. I was significantly less confused after watching it: