Two-Port Actuator Source - For Carb Drawthrough system...

Hello guys! I am planning to re-commission a past project with a 1978 Turbo Riviera drawthrough system. Trouble is, I can not locate a correct TWO-PORT actuator.

I ordered one off eBay, and it is a fake. It's a non-sealed actuator with 2 fittings. Unfortunately, there is no seal in the can around the rod, so the lower port (the vacuum port) is useless.

It's important on the drawthrough system that the actuator be referenced to vacuum on the rod-end. Else the turbo is going to be overspeeding at part throttle.



Well-Known Member
How will the turbo be overspeeding at part thottle? I have a two port actuator and disconnected the vacuum side long ago. The vacuum side was leaking (as most do) and the vacuum leak was causing issues. The spring inside the actuator is very tight, so I believe it is holding the wastegate closed at part throttle.
Well.... The wastegate actuator is trying to regulate the amount of pressure increase the turbo compressor is providing. It does this by moving the exhaust gas bypass valve, aka wastegate puck. Spring holds it closed, pressure opens it.

This is a simplified explanation. Not going to go into the effect of exhaust forces on the wastegate valve etc. The theory can get pretty deep.

The movement of the actuator is the result of balancing the force of air pressures versus spring force. When the air pressure difference on the diaphragm gets stronger than the spring force, the spring compresses. The actuator moves, opens the bypass, and the turbine slows down. The boost is lower, and therefore the air pressure holds at this level, with the actuator partially extended.

A normal 'blow through' system (majority of turbos in the world today) has an actuator which is sensing turbo compressor OUTLET pressure by a hose connection to the turbo, and balancing it against the spring. The other side of the diaphragm is open to atmosphere. The turbo inlet is also open to atmosphere without restriction, so the pressure is the same on the turbo compressor inlet and the wastegate actuator 'rod end.' Therefore when the turbo is raising the air pressure at atmospheric pressure, to the boost setting, the actuator will feel the same pressure as the turbo compressor.

With a drawthrough system, you introduce a new effect. You have a throttle body on the turbo compressor inlet. When the throttle is at less than WOT, the turbo compressor is receiving less than atmospheric pressure. If the turbo wastegate actuator does not have the second port connected to the turbo inlet, the wastegate will try to maintain the same turbo outlet pressure (relative to atmospheric pressure) as the throttle is closing. This causes the turbo compressor to speed up, making a high vacuum below the throttle plate. As the boost is reduced by closing the throttle, the wastegate is going to try to regulate the boost UP, by speeding up the turbocharger. Because the compressor is becoming 'unloaded' by losing air density on the inlet side, it will pose less of a load on the turbine, therefore allowing it to speed up. This will put more restriction on the exhaust, more heat on the turbo, and more centrifugal forces on the turbo components.

With the drawthrough system connected as designed, the wastegate actuator is working just like the blowthrough system. The compressor OUTLET pressure is acting on one side of the diaphragm, and compressor INLET pressure is acting on the other. The only difference is, here a hose connection is required at each end. One hose from the compressor outlet to the actuator base end; another hose from the compressor inlet to the actuator rod end. This configuration ensures pressure is the same on the turbo compressor inlet and the wastegate actuator rod end, as well as the compressor outlet and the actuator base end. When the throttle is closing, and the pressure at the turbo inlet is falling, the actuator will have less pressure on the 'rod end' and therefore it will take less pressure on the base end, to extend the rod. The pressure across the actuator diaphragm is the same, so the turbo compressor pressure rise is the same.


Mad Scientist
Its inconsequential.

Thus Buick elminated it on the later carb turbos.

it will have no effect on performance nor harm the turbo. we have the years and data to show it wont hurt it.

my PT6152 carb turbo has never had any problems either.

why re-invent the wheel.


Well-Known Member
OK, I understand what you saying and it makes sense in theory. Before I removed my vacuum line, I thought it though and came to pretty much the same conclusion, that the vacuum line was necessary for part throttle.

But the practical application for me, was the vacuum leak was much more serious than the part throttle operation. And I rarely ever find these engines operating long at mid boost levels. In normal driving, if I get to say, 5 psi, I will reach the speed limit in a few seconds and back off. I never find myself cruising along for extended periods of time while boosting. The quadrajet's secondaries won't open until about 90% throttle - at that point, it WOT/max boost all the way. So for me, it either full boost or no boost for the most part. I rarely find myself in between, and if I do, it's very brief. I will admit that my actuator was likely leaking from they day I got the car and I don't know what it's like to have a good actuator to compare to. I wouldn't know if the difference is noticeable or not.

You may wan't to confirm the year of your set up. The Turbo Riv didn't start until 1979. From what I have seen on the Regals, is that the early setups used a single port actuator and then switched to dual ports in either 1980 or 1982. My 82/83's are all dual ports. If you happen to have a 1981 Turbo, then it would use a normally open wastegate and that's a much different story.


Active Member
I'm in the same boat. I have been emailing Gpop shop so hopefully I will find something. Has anyone one found an aftermarket that will work? I'm not worried about original or the vacuum port. I just want the motor to operate properly. Originality means nothing to me if it means leaving the broke stocker on ams blowing my head gaskets haha




RIP Charlie!
Like Rich said, you can get a single port actuator and adapt it. That seems to be the most common repair. I decided to go external for better control of the boost and easier parts replacement. It requires a bit of fabrication but it gets rid of the original actuator.


Active Member
I was looking at one from kirban
ImageUploadedByTurboBuick Mobile1394417729.490408.jpg

Does it matter what it's for really? I am not scared of a little fab (if you want to call drilling out spot welds fab). Does the Diaphragm stroke make any difference( it can be adjusted with a threaded shaft).


Active Member
Did you opt for the $40 ebay one or the $75 turbonetics one from summit. I'm wondering if the ebay units are the kind you buy three of and hope one of em works. Ebay for finding stuff but the shipping takes eons up in canada. I had a parcel from a manitoba go to Tampa before coming back up to alberta. It's painful.