turbo chatter when no dump valve is used

[Heeltoe]

I have always wondered what makes the chirping, chattering "pju-pju-pju"-sound often heard on turboed rallycars when they close the throttle.

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it is not the dump valve. a manifold vacuum controlled dump will fully open when the throttle plate is closed and make a "pschhhh"-sound. the sound I am talking about comes when no pre throttle plate pressure release valve is fitted.

what makes the sound then ?

some people are saying it is the turbo impeller wich cuts up the returning air. I totally disagree. at this level, the compressor wheel has a rotational speed of ca100krpm.
a little math says that if this theory is to be right, the sound would have a frequency of 20kHZ, wich is barely noticable by the human ear.

besides, a radialcompressor (turbocharger) is constructed in such a way that it cannot cut up high pressure returning air into pulses and make music. what happens to a radialcompressor when backpressure is exerted - plain simple surge..

anyone have an idea on this phenomena ?

[Notso Swift]

I have not vents on the inlet side and only an intergrated wastegate on the turbo, so no recirculating of air, and I get it, I have always thought it was "wastegate chatter" When the wastegate opens quickly, vents the residual pressure, closes, preassure builds ou again, wastegate opens,.. repeat cycle 3 times a second

[Heeltoe]

the wastegate have both exhaust pressure and turbo pressure working to get it open.
the turbo pressure won't pulse, because the air doesn't have anywhere to go, only through the idle-opening of the throttle plate, wich take some time.
the wastegate valve is opening in the same direction with the exhaust gases, so the exhaustflow will acually help holding the valve open until the high pressure plenum chamber has been emptied, and the engine is producing low-idling exhaust flow/pressure.
because it is only a spring counter acting the force of the turbo/exhaust pressure, the wastegate becomes a progressive opening valve opening slowly as the two pressures escalades. therefore, the valve is open before the max boost figures (gradually opening towards the end of the rpm band) and hence open before you close the throttle.

all this makes for a slow opening/closing valve, and therefore, the wastegate can't make those pulses.

[1200Datto27]

It's the presurised air in the inlet tract reverting at the throttle plate and bouncing between the turbo and throttle plate, some of it bleeds out through the turbo impeller blades, making the noise.

[zac510]

Although sometimes with a dumpvalve, the shape of the outlet can cause different sounds to be made.

[tzei]

Originally Posted by Heeltoe

the wastegate have both exhaust pressure and turbo pressure working to get it open.
the turbo pressure won't pulse, because the air doesn't have anywhere to go, only through the idle-opening of the throttle plate, wich take some time.
the wastegate valve is opening in the same direction with the exhaust gases, so the exhaustflow will acually help holding the valve open until the high pressure plenum chamber has been emptied, and the engine is producing low-idling exhaust flow/pressure.
because it is only a spring counter acting the force of the turbo/exhaust pressure, the wastegate becomes a progressive opening valve opening slowly as the two pressures escalades. therefore, the valve is open before the max boost figures (gradually opening towards the end of the rpm band) and hence open before you close the throttle.

all this makes for a slow opening/closing valve, and therefore, the wastegate can't make those pulses.

I have to disagree. I´d say it´s a wastegate. I think it would be ideal to keep it close until max boost is aquired, then it rapidly operates to maintain that boost level. Slowly opening/closing valve is poorly managed system and loses power imo.

[zac510]

Originally Posted by tzei

I have to disagree. I´d say it´s a wastegate. I think it would be ideal to keep it close until max boost is aquired, then it rapidly operates to maintain that boost level. Slowly opening/closing valve is poorly managed system and loses power imo.

What heeltoe said is true, the valve does open progressively and indeed improvements can be made.

Some people install a valve in the line to stop the wastegate opening prematurely.

[Notso Swift]

Mine is triggered by the ECU, so maybe that has something to do with it.

Besides doesn't the quck close of the throttle create a vacuum on one side of the waste gate?

[zac510]

Some actuation diaphragms of external wastegates do have vacuum and boost ports.

[tzei]

I use boost to open and to close so no vacuum anywhere.

[Heeltoe]

Originally Posted by 1200Datto27

It's the presurised air in the inlet tract reverting at the throttle plate and bouncing between the turbo and throttle plate, some of it bleeds out through the turbo impeller blades, making the noise.

are you sure ?
I understand you are saying that a high pressure front develops when the throttle is abruptly closed. the pressure front is reflected, and reverberates between the butterfly and the radialcompressor and the radialcrompressor surges every time it recieves the pressure front.

it would seem logical, but I have never heard of this happening in any other system.

I have also never heard of surge making an extra sound.
try blocking the exhaust jet of a leaf blower for example, the only thing happening is the radialcompressor surges, stops doing work, and the power needed decreases, hence the rpm increasment. no sound !
on vacuum cleaners also, only an rpm increasment..

we should have a fluid dynamicist here. or turbomachinery engineer.

I am very interested in the topic