Published by Tim Scott on Jun 8, 2016

In part 1 of this series, we introduced turbocharger controls including wastegates, actuators, and electronic solenoid controls.

In part 2 of this series, we show different actuator designs and how they work with your turbo wastegate to control rotor speed and boost pressure.


Hey everybody. Thanks for coming back to Tim’s Turbos. This is part 2 of wastegates and actuators.

So, what I’m going to do is go over the basics of all of the stuff and try to keep it short.

This is a turbine housing.

This is a Turbine Housing

This is the actuator and flapper.

What happens is the exhaust goes in the inlet port and it comes out the turbine wheel and your wastegate is there to control the rotor speed and the turbine pressure. Now, rotor speed is very critical in keeping an efficient turbocharger, and just as critical as having the same drive pressure in relation to boost pressure.

Your Actuator is Going to Open Up the Wastegate

So when your turbocharger comes up to speed, your actuator is going to open up the wastegate, slow down the rotor speed, and try to balance it out so that you have the desired boost pressure.

This is a Boost Reference Actuator

Now, some vehicles are controlled differently. The most simple one is a boost reference actuator.

This one is off of a Dodge Cummins. It’s pretty simple – the older ones just ran a hose from here to the compressor cover, so whatever signal the compressor cover was giving out would determine the spring pressure on this one.

You can put different actuators on for heavier spring pressures and those are going to give you higher boost pressures. One thing you have to be very careful of is people like to tweak the turbochargers so the boost comes up, but if you don’t have full articulation of the wastegate, then you’re kind of defeating the whole purpose. You almost might as well weld the wastegates shut, which is a big no – don’t weld wastegates shut, it’s a waste of time.

The Way You Can Turn Up Boost Sometimes is by Screwing This In or Out.

The way you can turn up boost sometimes is by screwing this in or out. You’re going to be loading up the spring pressure here, so if it’s a 10-pound spring, you load it up to 12 pounds, and it’s going to take 12 pounds of boost to push this rod out and open the actuator.

BMW Boost Reference Actuator

Another way of controlling it is – this is a BMW, some of the old Duramaxes have this too, it’s a little different though, they didn’t use a spring. This one actually sucks in the shaft, then closes the wastegate, and so a lot of these cars you’ll hear a kind of wastegate rattle (what a lot of people complain about – it’s actually supposed to be there just because the tolerance is everything).

This one will be commanded by the computer and so it’ll suck it in, hold it, and then when the boost levels come out the computer just basically lets it go and that’s how it controls that.

This is a Dual-Port Actuator

Now, a very well-designed one – even though it’s been around forever – is called a dual port actuator. The topside is going to have a diaphragm in it that has boost pressure reference. The bottom will have the same thing, except it’s in a different direction.

So these are double sealed on the canister, but they still have spring pressure. The spring pressure can go different ways depending which one you have. For this one, the spring pressure is retaining upward.

To get the full articulation of a wastegate, if you put 10 pounds of spring on the top and then you add one pound of boost pressure, then technically you have an 11 pound spring. So it’s gonna take 11 pounds of boost pressure to lift this or push this actuator, depending on what type of spring you have in it.

A lot of external wastegates are set up specifically this way, because you have the full motion of the wastegate.

Spring Pressure on the Actuator

Now what I was saying is loading up the actuator – what can happen is if you have a whole bunch of spring pressure pulling on this actuary lever, then if you don’t have a full articulation of the actuator, it can be collapsing a spring and you only get maybe a quarter inch of travel compared to a full one inch or half inch, depending on which one you have, so that can cause a lot of problems. You’ll start seeing boost bleed off at real high RPMs when you’re racing and stuff.

The Critical Part of the Wastegate Control

The critical part of the wastegate control is when you’re under boost, your rotor speed comes up. If you add ten pounds of boost on the compressor side in a typical gasoline engine, they target for about one and a half to one. So, if this is one over here at 10 pounds, this is going to be about 15 pounds.

Once you get about that you start loading up the turbine pressure, then your efficiency kind of falls off the map. Then all you are doing is building heat on the compressor side and coming closer to a compressor surge. I’ll get more in detail on drive pressures and ratios on the next video. But, as a short description, I hope that helps.


-Tim’s Turbos