Nobody knows what a turbocharger does

Illustration for article titled Nobody knows what a turbocharger does

This is something even Car & Driver gets wrong, and they get it wrong all the time. You’ll be reading an article about a turbo dreamcar when you read a line about engine roaring, turbo whistling, “wastegates snorting” and you may keep reading without so much as a kindly “WTF?”

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It seems a lot of people have no idea what a wastegate does. It certainly doesn’t snort. It doesn’t sneeze. It doesn’t make any noise at all.

There are plenty of euphemisms that the term “Wastegate” could qualify for, and none of them falls below rated R unless I badly drew a cartoon butt, which I will do thusly:

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Illustration for article titled Nobody knows what a turbocharger does

That is unless you’re talking about an actual wastegate, which goes on a car

Illustration for article titled Nobody knows what a turbocharger does
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(or a dam, but whatever).

Let’s talk turbochargers: they’re spinny, whistly, POWERy things that make your car go from ho hum to explosion(!) two seconds after you hit the gas pedal.

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Normally exhaust gas is just routed from your engine out through tubes to the back of your car. Put a turbocharger in the way of that exhaust gas, and you can use it to make a LOT of power.

To use a line from a post I made before:

Turbos hear explosions and they don’t get scared, they get excited. They take those explosions and make them exploder.

Turbos take a boring old Volvo and turn it into a Mustang’s nightmare, by sheer force of forced air.

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Let’s get to how it works: The first thing that exhaust gas hits in the turbocharger is the turbine wheel (think very sharp fan blade). That exhaust flow past the turbine wheel starts the turbine spinning at extremely high speeds (more than 100,000rpm), which is then connected to a shaft, which is connected to a compressor wheel on the other end, which spins at the same speed and inhales ambient air and force-feeds it into your engine’s air intake.

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The result of this process is your engine has more horsepower than you had before because more air is being crammed in, and more fuel is going in, and the spark is happening at just the right time not to blow things to smithereens by accident.

But what if your turbo put out too much air, and you didn’t have enough fuel, or you didn’t have enough spark control, or, worst case scenario, your engine parts weren’t thick enough to withstand the power the turbo added?

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You need something to stop that from happening.

Before we get there, here’s a primer on turbo sizing: Turbos are sized to be a nice fit for your engine’s size, speed of turbo response, and top end power (though not always, as one reader pointed out). It’s a compromise all around that generally results in the turbo being capable of delivering so much power if you just let it spin free that it could break your engine.

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You need a way to keep the turbo from cramming so much air into the engine that it makes it explode. You need to limit the turbo’s voraciousness, because if you leave a turbo to its own devices, it will inhale and burn you and everyone you care about.

That’s where wastegates come into play.

They prevent too much air from going into your engine by preventing the turbine from spinning faster, so it won’t spin the compressor faster, and so the turbo won’t shove more air into the engine than the engine can handle.

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They do this by diverting away some of the exhaust gas that’s trying to pass through the turbine. A flapper or poppet valve opens and routes exhaust away from the turbine through a hole next to it (yes variable geometry turbos exist, but we won’t get into that here). The exhaust wants to go through the wastegate, because it’s easier than going through the turbine. In the example below, the exhaust enters through the flange at the top, and if the wastegate is closed (as it is here), exhaust would go through the turbine, causing it to spin fast. Open the wastegate, and exhaust gets diverted away from the turbine, keeping the turbine from spinning faster.

Illustration for article titled Nobody knows what a turbocharger does
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On anything short of a full blown racecar, that diverted exhaust is just routed into the exhaust pipe after the turbo, and continues down through the rest of the exhaust until it leaves the tailpipe.

The wastegate is controlled by a little actuator that senses how much boost the turbo is putting out. At a certain level you’ve set (let’s say 10psi) the boost pressure causes the actuator to open the wastegate valve gradually to bypass just enough exhaust gas to keep the turbine spinning the same speed and the boost pressure stable at the level you set it at. That gives you more controlled airflow into the engine, and keeps things safe instead of explosive.

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But people frequently mistake wastegates (which operate on the exhaust side of the turbo)for the other type of valve in a turbo system: the blowoff valve (BOV), or compressor bypass valve (CBV), both of which do the same thing: Ventilate boost pressure on the intake side of the engine. If you suddenly lift off the throttle while the turbo is making a lot of boost and spinning very fast, suddenly that pressurized air has nowhere to go, so it backs up instantly toward the turbo and violently slows down the compressor. That can cause damage over time. So the BOV or CBV sense that pressure change and instantly ventilate that excess pressure so that it doesn’t hurt the turbo.

You know this is happening, because it makes awesome noises.

Before you ask, no, I don’t know how that L’oreal cosmetics commercial ended up on a turbo noise compilation. Or at least I hope I wasn’t the only one who got it.

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Suffice it to say:

Those chirpy, pshhhhy, whistly sounds between shifts? Those are BOVs or CBVs.

Wastegates don’t make any noise. They don’t chirp. They don’t whistle. They don’t snort. They don’t Darth Vader.

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In fact, the only time you’ll ever hear a wastegate is when it’s vented to the atmosphere in a racecar, and you won’t even know you’re hearing it, because it sounds exactly like an open exhaust pipe, aka the sound of an engine with no muffler. It only opens at full boost (and at or near full throttle) so it’s only when the engine is already being its loudest and most violent.

When that wastegate opens to the atmosphere, it only sounds like the engine’s normal sound getting slightly louder.

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To sum up: If someone ever uses the word “Wastegate” and tries to tell you how it sounds, they don’t know what the hell they’re talking about. They just forgot what a BOV or CBV is. Educate them, lest they cripple us all with ignorance. Tell C&D while you’re at it.

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