Wastegate Noise

Mod Description

For many years, the chatter/twitter noise heard when changing gear or lifting off the accelerator was thought to be the wastegate. I proved this to be wrong and now it's common info. But what is it? Is it good or bad?

Mod Details

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What’s The Flutter Noise When I Change Gear?

There is a wide common misconception within the turbo community that the noise is…

It’s Wastegate Chatter Right?

Wrong. Wastegates don’t chatter, twitter or make the fluctuating noise you hear when you change gear on a turbo smart (mainly 450 and Roadster). So many people believed that it was wastegate chatter that the misinformation spread as if it was fact. Don’t feel bad, more people think the noise is wastegate chatter than the people who know what it really is. Prepare to learn the truth.

Why Don’t Wastegates Chatter?

Wastegates don’t open and close rapidly so they cannot possibily make that noise. They open, they close, they do NOT open and close in quick succession. This would cause massive fluctuation in boost pressure and that’s not good for the turbo.

Dump valves chatter, external wastegates scream. Internal wastegates make no audible noise.

It’s Good Though Right?

It might sound good because you have been trained into thinking it’s wastegate chatter and therefore a good noise, however, it’s not. If anything, it’s a bad noise and it’s damaging your turbo.

What Is That Noise Then?

Its proper name is Axi-symmetric stall (AKA compressor surge, compressor stall or pressure surge).

To quote Wikipedia:
…a complete breakdown in compression resulting in a reversal of flow and the violent
expulsion of previously compressed air out through the engine intake, due to the
compressor’s inability to continue working against the already-compressed air behind it.

In other words, when the throttle body closes, the turbo continues to spin and create boost. This boost has nowhere to go to so the pressure between the turbo and the throttle body increases. The turbo (not being loaded because the throttle body is closed) slows down to the point where it cannot increase or maintain the pressure ahead of it. The boost that has built up between the turbo and throttle body pushes back passed the turbo and back out the air intake pipe.

The noise you hear is the air rushing back over the compressor as it fights against it. Imagine blowing into a fan.

The “flutter” reason is very similar to the “water hammer” effect that you can get in plumbing to float valves.

Once the pressure releases passed the turbo slightly, the pressure ahead of the turbo reduces, this allows the turbo to overcome the escaping boost and partially recompress it. Again, eventually the pipe pressure exceeds the turbo pressure and the boost gasses will pass the wrong way through the turbo once again.

Because the turbo isn’t under load, the build up is less and less each time which is why the noise starts loud and each recurring flutter is quieter than the previous one. This will happen very quickly over the space of 1 second and will stop as soon as the throttle body opens or the turbo stops spinning enough to create boost over that of the engine’s natural aspiration.

So, Why Is It Bad?

The turbo spins approximately 80,000 rpm (estimate) at full load. Despite the compressor and turbine wheel being balanced, the rotational movement at such high speeds creates enormous rotational inertia (angular mass).

As the air rushes back passed the turbo it rapidly stalls and releases the compressor. This energy creates an imbalance momentarily which can cause tiny vibrations. These, over time, can damage the oil seals, bearings, compressor, turbine and shaft.

What Can Be Done?

There are 2 main options.

Firstly, fit a dump valve or recirculating valve. Get the right type though.
Secondly, ignore it. Smart never saw the need to fit a valve so why bother?

OK, So Why Bother?

Exactly. Although a dump/recirculating valve does cost a lot less than a replacement turbo. On the flip side however, the turbos do tend to outlast the engines on the 450 and Roadster. Do read up about dump valves on here before committing to one as they aren’t all the same. There are currently no recirculating valves available but I have fitted one. I’ll add info one day.