Turbo Bark Good or Bad?

[Alan Reagan]

My turbo makes a noise sometimes when the transmission shifts into OD while under a slight load, as in going up a hill. I was told this noise was from the wastegate relieving pressure do to the rpm drop. Is this "turbo bark"?

What you are probably hearing is the wastegate relieving excess pressure. That's not bark. That's the wastegate opening to avoid overspinning the turbine and overpressuring (based on what the engine is looking for pressure wise) the intake. If the wastegate is fooled or locked in a closed position, pressure continues to build and the turbine continues spooling to a higher speed creating more boost pressure.

 

[Alan Reagan]

Here's a pretty good discussion of bark.



https://www.turbodieselregister.com/forums/showthread.php?s=&threadid=65104&highlight=turbo+and+bark



Note, that Rusty (who I believe really understands this issue) indicates the turbo never actually spins backwards. I have to take his experience here and go with it. My understanding from when I was researching Blow Off Valve design a few years back is that the turbo can actually spin or attempt to spin backwards after it stalls. If it doesn't, I offer my apologies for saying it does. I'm not trying to mislead. The above discussions indicate that the damage comes from loading/unloading/loading the compressor during a sudden lift. There is reverse airflow and my earlier research indicated that a backspin was possible/likely in the right pressure conditions. Again, it was what I read and I cannot validate that opinion.

 

[Forrest Nearing]

it doesn't spin backwards... too much inertia at that speed... but it will lose axial and radial stability which is hard on bearings. it will also speed up and slow down RAPIDLY... the turbine wheel has a lot more mass than the compressor wheel, so when all that pressurized air pushes back on the compressor wheel, you get a LOT of twisting force on the shaft. the shafts are very strong, but there's only so much they can take.



in some turbos, the force on the compressor wheel will actually cause it to explode! our turbos don't have that problem, they snap shafts.



Forrest

 

[Alan Reagan]

I think this is where I got the backspin idea:



http://www.sydneyspeedsupplies.8m.com/GFB/GFB3.html



Also, do a search on Blitz Blow Off Valves and they indicate the turbines can backspin. I agree with you on the inertia, Forrest. I believe where a backspin may be possible is when the boost is in the 40-50 lb range in our trucks and the compressor is in it's unstable region (according to the compressor map for the turbine. ) If a turbo is operated in the region where it is unstable, as pressure fluctuates up and down, the compressor stalls, backward airflow occurs and if it occurs for a long enough period (because of extremely high boost pressure bleeding back through the intake) a back spin might be possible. What is saving our turbos from self destructing due to all of this jerking around is that now the pinwheels are forged aluminum. Now the bearings fail due to the rapid loading and unloading. The larger turbos (HX 40 and up) tend to ring off the shafts because of the inertia change.

 

[Matt400]

Mine has done it since day one for the past 26K. Only happens when OD locks under light pedal, about 50 mph. From my studies on it I learned that when the engine abruptly slows in RPM such as happens when the TC locks there is a sudden surge in air flow through the turbo causing an air reverberation sound.



I am hoping a better TC will help due to the shorter RPM drop.