It’s a commonly used term, but what does a wastegate actually do? Well, you’re about to find out with the Fast Car wastegate guide rammed full of the best tips, tricks and advice…
The wastegate’s sole job is to regulate boost pressure. It does this by letting exhaust gases out of the engine by bypassing the turbocharger rather than travelling through it. This prevents the turbo from spinning too fast, which in turn keeps the boost at the desired level. You generally need one wastegate per turbo, but running two is sometimes needed, and wastegates come in various sizes depending how much they are required to flow and how much room there is to fit them.
HOW DOES A WASTEGATE OPERATE?
It’s quite a simple device that mounts on the exhaust manifold before the turbocharger, and is basically a valve that is held shut by a spring. Your car’s boost pressure acts against a diaphragm to push against the spring and therefore open the wastegate. The process is slightly different on internal and external wastegates, so read on and we will explain…
CAN A WASTEGATE FIT TO MY CAR?
Well to start with, if you have a petrol turbo car, you will almost certainly already have a wastegate, and if you don’t have a turbo, then you don’t need one! Aside from a few very rare exceptions, production turbo cars come with an internal wastegate attached to the turbo, so to fit an external one you will need an aftermarket exhaust manifold with an external wastegate outlet on it. For many popular turbo cars, especially Japanese ones, these are widely available, but if not, you will need to get a custom exhaust manifold made up to suit your choice of wastegate.
DO ALL ENGINES HAVE A WASTEGATE?
Only turbocharged engines have a wastegate, but some of them don’t, and this is for two main reasons.
The most common reason is where the engine is running a variable geometry turbocharger (VGT), which instead of regulating boost with a wastegate, has sliding vanes in the turbine housing which effectively increase the turbine size to prevent the turbo overspeeding; although on some engines, a VGT turbo also has a wastegate as a secondary control measure.
The second reason is seldom seen these days, and that is because the turbo is either so big or is restricted by other reasons (inlet and exhaust restrictions mostly), that the turbo spools so slowly it cannot exceed maximum safe boost before the rev limiter. As you can imagine, this makes for a pretty horrible car to drive, with either huge lag, slow response, low rev limit, or a combination of all of these. Not good.
WHAT SIZE WASTEGATE DO I NEED?
Wastegate sizing is a common question with two answers, one of which is very complex and one that is far easier to understand. In layman’s terms, if your wastegate is too small it will not flow enough air, meaning your boost will be higher than you want it to be. In theory, if your wastegate is too big it may struggle to accurately keep the boost pressure stable. On the whole, the bigger your turbo is in comparison to your engine size, and the lower boost you want to run, the bigger the wastegate needs to be.
The more complex answer would involve some maths in order to calculate the minimum size wastegate that would still flow enough air for the given application, but in reality even large wastegates rarely suffer from boost instability. Our advice would be to run as big as you can fit as if you end up with a wastegate that’s too small, the consequences are far worse!
INTERNAL OR EXTERNAL WASTEGATE?
On some cars you have no option as the turbo will either come with an external or internal wastegate. They work in slightly different ways, and there are pros and cons to both.
Running an external wastegate will cost a little more, and take up more room (they can be as big as a can of lager), but have many advantages. The main advantage is flow, as they can be over three times the size of an internal wastegate. This is important when you need to run a big turbo at low boost, and also can help performance by improving exhaust flow; especially if it is vented into a separate exhaust pipe.
The function is similar to an internal wastegate, but rather than a hinged flap to vent the air, it uses a conventional valve that is held shut by a spring. A diaphragm is mounted under the spring in a sealed housing, and boost pressure underneath this diaphragm pushes the diaphragm up, along with the spring, and therefore lifts the wastegate valve off its seat thus opening the wastegate and venting the excess boost pressure away.
The advantage of internal wastegates is that they’re built into the turbo so are cheap and compact, but they generally aren’t as big as external units so can’t flow a huge amount of gas; sometimes simply not enough to keep the turbo at low enough boost pressures for the particular engine it is being used on.
On an internal wastegate the valve is actually a hinged flap built into the turbo exhaust housing, and is opened by a rod which is connected to a diaphragm inside a sealed container called the actuator. The rod is held in the closed position by a spring inside the actuator, and as the boost pressure rises it pushes on the diaphragm, which overcomes the spring pressure, and pushes the rod forwards, opening the wastegate.
As mentioned, the wastegate needs to be mounted on the exhaust manifold before the turbocharger, but the better the positioning of it, the better it will flow for any given wastegate size. The most important thing is the wastegate should be able to vent gas from all cylinders, which means it needs to be ideally mounted on the collector, just before the turbo. Not all wastegates are like this though, often out of necessity, but this significantly reduces flow and can lead to poor boost control and other issues.
Secondly, the flow to the wastegate should be smooth, less than 45deg to the direction of the exhaust flow is ideal, but if this is not possible certainly no more than 90deg if you wish to have any boost control at all. The same principles apply in twin wastegate setups, as shown below.
HOW DO YOU CONTROL WHEN THE WASTEGATE OPENS?
In its most basic form, the strength of the wastegate spring will dictate at what boost the wastegate opens, as the stronger the spring the more boost pressure it will take to open it. This can be adjusted simply by changing the wastegate spring to a stronger one.
The other way to control the boost level is with one of the many electronic or mechanical boost controllers on the market. These items don’t allow you to set the boost lower than the spring allows, but can fool the wastegate into opening later by stopping some, or all of the boost pushing against the wastegate spring.
As a general rule your wastegate spring pressure should be at least half what your maximum boost pressure is, and your maximum should be set with the controller, but it is possible to run wastegate spring pressure alone or run far more than double the wastegate spring pressure by using a controller.
You sometimes, even on standard vehicles in rare occasions, have two wastegates per turbo, and the reasons for this are twofold. The simplest explanation is if, for whatever reason, you cannot fit a single wastegate that’s big enough to bypass enough exhaust gas, this can either be for space considerations or simply because you are running an enormous turbo at relatively low boost.
The more complex but common reason is because you are running a divided turbo setup. Divided turbo setups theoretically give you better overall performance, but to be fully divided you need one wastegate for each side of the divide, and late-spec FC RX-7 Turbos even came with a turbo that had twin internal wastegates as standard, although we have never seen an aftermarket turbo with twin internal wastegates before.
SCREAMER PIPES AND WASTEGATE OUTLETS
Another advantage with an external wastegate is that you can run a separate exhaust pipe from the wastegate, usually called a screamer pipe due to the noise it makes at full boost if unsilenced. Running a separate pipe from the wastegate means less restriction in the main exhaust, potentially giving more power, but also making the car incredibly noisy at full boost, though it doesn’t emit noise when the wastegate is shut.
Because of this people usually just plumb the external wastegate outlet back in to the exhaust system, but that can be restrictive unless you have a very large exhaust, so the ideal solution is a separate pipe from the wastegate, but with a silencer on it too – effectively a screamer pipe that doesn’t scream. On an internal wastegate the outlet is just as important for good flow, and many standard cars have a very restrictive outlet which means a redesigned and larger turbo elbow or downpipe gives good gains and better boost control.
There are internal wastegate screamer pipes on the market, but these really aren’t recommended as they rarely fully seal off the wastegate which means your car sounds like it has an exhaust leak at all times, and I don’t think we need to tell you how rubbish that sounds.
There is a common myth in the performance tuning scene, often even talked about by other magazines and TV shows. It’s what most people call ‘wastegate chatter’ – the fluttering noise that happens when you lift off the throttle on some turbo cars. In reality that noise has nothing to do with the wastegate, and is in fact the air compressed by the turbo escaping back through it after having hit the closed throttle butterfly and having no other means of escape.
There is also a less common myth that certain types of dump valve make the chattering noise, but a dump valve will actually reduce this chattering as its job is to vent boost when the throttle is closed. However, different types of dump valve can give out a similar noise.
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