Wastegate.

Chris1

Member
One thing that I've never really 'got' is external wastegates.
I've heard that they 'chatter' which I assume is from releasing pressure caused by the turbo spooling up. Is that true?
Can someone shed some light for me?
 

RU55 GTR

Member
From my understanding, all wastegates 'chatter'?! The sound is caused by the stalling blades of the turbo / air over them.

Hope this helps! :)
 

Chris1

Member
I thought that having a BOV venting to atmosphere cured the stalling turbo problem?
So what are the benefits of having an external wastgate? What does it actually do?
 

RU55 GTR

Member
I thought that having a BOV venting to atmosphere cured the stalling turbo problem?
So what are the benefits of having an external wastgate? What does it actually do?
The BOV does stop the stalling blades as the venting enables the turbo to continue spinning while slowing and depressurising the system. In my view, the stalling of the blades is not a problem until you start getting the turbo to run higher boost i.e. 1.2bar and above.

The whole purpose of an actuated-wastegate is to control the pressure at which the turbo runs. It controls the level of boost the system maximises to (depend on how it's set-up) and 'dumps' the pressure out of the system when the engine rev's drop.

To my understanding, an external wastegate enables more and easier adjustment of the boost pressure. I believe that they are usually louder, but to be honest, my standard wastegate yaps quite nicely. I think you can hear it in this video on youtube?! - http://www.youtube.com/watch?v=iWY2WQ-j3aI

Hope this helps!!!! :-D
 

ChrisS

New Member
An external gate allows better control of boost especially when running a big turbo or boost level. Instead of there being a small internal gate allowing the exhaust gases past when boost reaches the required level which can be restrictive because of the available size, you have a standalone unit allowing the exhaust gases out which can be much larger.
 
External wastegates have a few advantages over internal ones but the main one is they allow for better control of boost pressure on high boost applications. They do this in two ways; first they prevent boost creep, basically the internal wastegate port is too small for lots of flow so after a while it will choke (can't get any more flow through it) this means more flow goes through the turbine and pushes up the boost produced by the compressor. Secondly they help prevent boost fluctuations and boost spikes, again the small diameter of the port/penny valve means that they are slow to respond to increases in boost pressure (e.g. as you come on boost) causing an over shoot in the boost level. In worst case this can lead to oscillations as the wastegate struggles to achieve the desire boost pressure.

For most GTiR’s there’s very little point in an external wg and most of the above can be achieved by porting the wastegate port.

As for chatter its best thought of as "Transient surge" and is a compressor phenomenon that has next to nothing to do with wastegates. I f you look at a compressor map, under normal operation (on a well matched turbo) you will be somewhere around the middle of the map. When the throttle body (/ies) close the boost pressure is still maintained as the turbo is still spinning very fast due to the inertial of the rotor system, but the flow decreases; this moves you to the left of the comp’ map and into the surge region.

The flow separates from the compressor blades (stall in the aerodynamic sense nothing to do with the turbo not spinning) and the compressor operation becomes unstable, leading to audible pulses or “chatter”.
 

RU55 GTR

Member
External wastegates have a few advantages over internal ones but the main one is they allow for better control of boost pressure on high boost applications. They do this in two ways; first they prevent boost creep, basically the internal wastegate port is too small for lots of flow so after a while it will choke (can't get any more flow through it) this means more flow goes through the turbine and pushes up the boost produced by the compressor. Secondly they help prevent boost fluctuations and boost spikes, again the small diameter of the port/penny valve means that they are slow to respond to increases in boost pressure (e.g. as you come on boost) causing an over shoot in the boost level. In worst case this can lead to oscillations as the wastegate struggles to achieve the desire boost pressure.

For most GTiR’s there’s very little point in an external wg and most of the above can be achieved by porting the wastegate port.

As for chatter its best thought of as "Transient surge" and is a compressor phenomenon that has next to nothing to do with wastegates. I f you look at a compressor map, under normal operation (on a well matched turbo) you will be somewhere around the middle of the map. When the throttle body (/ies) close the boost pressure is still maintained as the turbo is still spinning very fast due to the inertial of the rotor system, but the flow decreases; this moves you to the left of the comp’ map and into the surge region.

The flow separates from the compressor blades (stall in the aerodynamic sense nothing to do with the turbo not spinning) and the compressor operation becomes unstable, leading to audible pulses or “chatter”.
Well done poppet! Thank you for the clarification. :thumbsup:
 
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