Turbo blowback or wastegate actuation??

[CGA97]

Can anyone describe what each sounds like? Under load (10k) when the transmission shifts into OD, the rpms and boost both drop with a fairly loud and quick "clunk clunk" coming from the turbo area. Doesn't happen all the time. Boost psi was around 20 and dropped about 10 psi then started to build again. How do I figure out what exactly is happening?

 

[Alan Reagan]

I don't know what the wastegate is set at on the 03's but on the 05's it is higher than 20 psi. When mine shifts into OD under load, it is a doulble clunk but that's the transmission.



It wouldn't be blowback or "bark" because you still have sufficient boost (10 psi) and the engine is under load when it happens. Blowback or bark is when there is a sudden lift off the throttle during hard acceleration.



Now, if the wastegate is opening at 20 psi, then you will usually just hear a hard swish sound (pressure release). In your case, that may be what is happening. It's opening and then immediately closing back. Do you ever get more than 20 psi boost when the engine is under load (accelerating, pulling hard)? If not, then it's probably the wastegate.

 

[ASHughes]

Is Blowback a bad thing? Dose it cause any damage?

 

[Alan Reagan]

Is Blowback a bad thing? Dose it cause any damage?

You'll get mixed answers to these questions. In these trucks, in the stock configuration, it shouldn't be a problem. With higher HP, higher boost, the theory is that the blowback or bark is attempting or actually backspinning the turbo. Some say that is impossible. Others have had the turbo shaft twist and break in high boost applications. What you are hearing on a stock truck is built up boost being bled off back through the turbo and out of the air filter.



That will cause a quick slow down of the turbo but should not be too harmful. I personally wouldn't make mine do it for fun. But some do and have no problems with their turbos. To me, if I can avoid it, I will.

 

[CGA97]

Alan,

the most boost I have seen is right around 20 psi, 23 psi was the highest. I think that is one of the features of the Quad box that they don't use such high boost pressures to increase hp. My issue only happened when I was under load and the transmission shifted to OD, the rpms dropped and so did the boost. You make sense with the boost still around 10 psi, the pressure should not go back through the turbo. This only happened a few times and was very inconsistent (it didn't happen at every shift to OD). I tried to duplicate it a few times, by backing off the accelerator and got the noise 1 time out of about 10 tries. I guess I should take it to the dealer to see what one of their techs say. I really can't see how the wastgate would open at such low pressures anyways...

 

[rbattelle]

It wouldn't be blowback or "bark" because you still have sufficient boost (10 psi) and the engine is under load when it happens. Blowback or bark is when there is a sudden lift off the throttle during hard acceleration.

I don't think this is necessarily true. I believe bark can happen under high-throttle (high boost) when automatic transmissions switch gears and the RPM suddenly drops off causing a sudden loss in turbine power output. That loss in energy manifests as an inability to continue to power the compressor at the current pressure ratio, which leads to stall (which is what bark is).



The problem can be alleviated by lifting off the throttle before the transmission shifts to reduce manifold pressure. Stalling a compressor is generally not good, but Alan is probably right that if it's happening on a stock truck the turbo is probably built to handle it.



I could be wrong, but that's the way I think it works.



-Ryan

 

[Alan Reagan]

Ryan has a good point on lifting at the shift. My truck will do the "double shift" under hard acceleration as it is going into OD. If I lift a little at about 50 it is smooth.