North American Turbocoupe Organization



Can someone explain to me how a wastegate works?
cbpldc Offline
Member
#1
Basically, I have a friend who's swapping a good engine into an SVO and the old engine would run right up to 30 psi and revved there in a hurry. He wants to put a gillis valve in to regulate this.

First, why did that old engine do that? He mentioned something about using vacuum line that was too large.

Second, explain the relation of vacuum and boost in regards to how a wastegate opens and when it opens and works.

Thanks,
Chris
Blue 87 T/C, basically stock and slow.
Black 85 T/C. The race car.
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martin0660 Offline
Posting Freak
#2
The wastegate is a very simple device.

For our application the wastegate actuator canister has a diaphram inside it. On one side is a spring, on the other is a supposed to be a signal. In our case the spring is rated to approximately 10 psi.

If you were to run a boost signal line from the turbo outlet, to the wastegate actuator, the wastegate would open at 10 PSI. This is because the signal pressure overcomes the spring, opens the wastegate, and bleeds exhaust energy off the turbine.

If you do not connect a signal to the actuator, it will basically never stop building boost. This is because the 10 # spring is holding the WG closed, and nothing is telling it to open. This is not entirely true, as you will sooner or later build enough pressure in the turbine to force open the wastegate, but in our application, bad things will happen first.

All manual boost controllers work on the same principal. The line from the boost signal (turbo outlet) to the actuator basically has a tee with an adjustment. In reality, this is a leak. This fools the actuator into not seeing 10# unit the leak cant bleed any more pressure. If you make the leak bigger, you get more boost (like no signal).

The Hallman valve, Gillis, etc, are ball and spring valves. All they do is stop the signal completely, until the spring in the valve is overcome, then they work just like a bleeder until the pressure dropes below the spring pressure.

The factory BCS works the same way, except the BCS is a solenoid valve that the EEC tells to open. The routing looks more complicated because of the tee on the compressor outlet, and the return into the inlet, but its still a leak. The orifice in the BCS is sized so that it can only leak enough to create 15 #.

In the description you gave, your buddy either didnt have the boost signal connected to the actuator, or he had a HUGE leak in the signal line.

HTH
Bob Myers
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martin0660 Offline
Posting Freak
#3
Also, Vacuum is not in any way involved in the workings of boost control on our application.

Technically, the throttle body created the vacuum, and the boost control stuff is in front of that.

Bob Myers
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Pete D2 Offline
Member
#4
First, the waste gates (WG) are actuated by a waste gate actuator (WGA) which is a diaphram that is acted on by boost pressure. When boost reaches a certain point (9-10 psi for our cars), it overcomes the return spring and causes the diaphram to move. There is a rod attached to the diaphram and when it moves due to boost, the rod opens the WG which allows some exhaust gas to bypass the turbine wheel so that it slows down which decreases the boost being generated by the compressor wheel. When boost decreases below the threshold level the return spring moves the diaphram and rod in the direction that closes the WG and allows more exhaust to pass over the turbine wheel, speeding it up and allowing the compressor to build boost.

The reason your friends engine went to 30 psi is related to a WG deficiency. It may be that the vacuum line that carries the boost signal to the WGA was missing or cracked (leaked) so that the boost signal never got to the WGA to cause the WG to open and limit boost. The diaphram in the WGA may have been ruptured. The rod from the WGA to the WG may have become detached from the WG pivot arm. No matter what, for some reason the WG was not opening to limit boost.

Vacuum does not play a part in WG operation. The WGA only responds to boost.

Now let me really confuse the issue. There are two places that a boost signal can come from, for discussion purposes. One is the compressor outlet. The other is the vacuum tree which is fed by a line directly to the upper intake manifold. Every system connected to the vacuum tree sees what ever the upper intake sees, be it vacuum or boost. EXCEPT those systems that have a one way check valves between the vacuum tree and the control mechanisms for those systems. For example, only vacuum is allowed to flow to the cruise control amplifier. Boost is kept out by one way check valves because it would harm cruise control components.

The boost signal, commonly measured in PSI, as measure at the intake manifold/vacuum tree, may be somewhat lower than the signal measured at the compressor outlet, due to pressure drop across the inter cooler.
Turbo
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Pete D2 Offline
Member
#5
Bob, you win this weeks typing contest [Image: biggrin.gif]
Turbo
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martin0660 Offline
Posting Freak
#6
At least we gave the same answer [Image: smile.gif]

Bob Myers
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cbpldc Offline
Member
#7
So, let's assume I run my line to my gillis from the compressor outlet instead of the vacuum T. Would that make it spool up quicker?

I just have to wonder what caused it to spool so fast as compared to what the cars normally do.

Chris
Blue 87 T/C, basically stock and slow.
Black 85 T/C. The race car.
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Pete D2 Offline
Member
#8
It going to spool up at the same rate no matter where you put the source line for the gillis. What will be different is when the signal, through the gillis valve, reaches the trigger point in the WGA.

Put another way: Let say there is a 2 psi pressure drop across the inter cooler.
So when there is 20 psi of boost in the compressor outlet, there is only 18 psi of boost in the upper intake/vacuum tree.

If the gillis valve gets it's signal from the compressor outlet, it will, in thory, see the "set" boost slightly quicker and send the signal on to the WGA quicker

If you want 20 psi of actual boost at the intake valve then you should source the gillis valve boost signal at teh upper intake/vacuum tree. If you want the turbo to not produce more than a certain psi, irregardless of pressure drops, then source the gillis at the compressor outlet,

[This message has been edited by Pete D2 (edited 09-02-2004).]
Turbo
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