When watching Truck pull competition I hear some of the trucks at the end of their run let off a squeaky chuff, chuff, chuff sound. I assume this is the excess boost being let out. Where does it come from? a waste gate (not sure on how a waste gate works either)? or there some sort of blow off valve?

 

That noise is made when the turbo charger changes directions rapidly , as boost tries to escape the intake because there is nowhere else for it to go, at the same time there is still exhaust trying to drive the turbine side.

commonly known as a "turbo fart"

 

Thanks! Is it dangerous? If I am not mistaken turbo ricers have a blow off valve, is there something like that for diesels?

 

turbo bark, can be pretty hard on the shafts of a turbo as the presures xaused when this happens is very excessive. That being said I have barked my stock turbo very hard a few times with no ill effects yet.

 

On a stock HX35 , you probably won't hurt it too much if by chance you fall into a situation where this occurs.

On aftermarket turbos, with bigger , heavier wheels , the odds of damage increase.

There are aftermarket BOV's available for us , but they are VERY expensive. The BOV's that ricers use are vaccum operated , when the intake closes, the engine draws a vaccum in the intake manifold, and that vaccum opens the BOV and releases pressure before the throttle body.

Since diesels to not have a throttle body , and do not produce vaccum in the intake, the BOV for a diesel is electrical.

BD makes one :

http://www.bd-power.com/ram/product....boGuard&tt=ram

 

What you are hearing is simply Turbo Bark (aka Compressor stall).

Let's say that you are accelerating hard with 30 lbs of boost pressure (for example), and you have to let off suddenly. Here's what happens:

The amount of fuel injected is cut off. With little to no fuel being injected, the exhaust output of the engine is likewise reduced as well as the EGT's. The turbo's exhaust turbine recovers some the energy contained in the heat of the exhaust gas, and some from the flow of the gas (more heat energy than flow). Since the exhaust turbine no longer has the energy it needs to spin the shaft to the compressor, it starts to slow down.

Now, the same time that is happening, the cylinders demand for intake air is dramatically reduced since there is little to no fuel being injected. Since the turbocharger compressor was producing 30 lbs of boost for the engine, and the cylinders no longer need 30 lbs of boost, now there is 30 lbs of pressure in the intake tract and intercooler with nowhere to go.

That 30 lbs of pressure has to go somewhere, because the compressor can no longer maintain that pressure, since the exhaust turbine has nothing to drive it. Since the 30 lbs of pressure can't go into the cylinders, the only place for it to escape and equalize is back out through the intake. To go that way it has to go back through the compressor. In doing so, it causes the compressor/exhaust turbines to rapidly decelerate. When is does this it also creates pressure waves, which makes that fluttering noise you hear. The turbine will never stop rotating or ever rotate backwards when this happens.

All of this happens in the span of a few seconds, and is normal, even for a truck that is stock (mine has done it at the drag track before).

The bad thing about turbo bark/compressor stall is that you can weaken the shaft or even break it, due to the sudden slow down, and uneven forces from the stall/bark. This has to be done frequently before that become an issue.

Hope this helps explain it.

 

Thank you very much that clears it all up!

At what point would would a blow off valve be warranted? ie high boost level or frequency of Compressor bark/stall?

 

I think most guys put them on after the 3rd or 4th grenaded turbo

 

i've barked mine a few times. toronto drivers are great. you try to accelerate and the guy in front of yo decides to change lanes last minute.

 

I've barked mine a few times when some retard pulls out in front of me when I'm loaded (or during bat turns with Dennis following me) and had to get out of it real quick. I personally try to avoid it if at all possible. I've already got enough dollars in my engine.

 

BOV's on diesels aren't really used to control bark, as far as I know, but rather to control boost when a standard wastegate won't cut it. The best (only?) way to control bark is with your right foot. Watch some of the pull videos and, at the end, you'll notice a lot of them bring the revs down slowly by burping the throttle. This technique keeps the pressure moving in the right direction as the boost level comes down.

 

If I get on my 66 and let off at about mid acceleration while it's still building boost man that thing will bark. I thought it was "Turbo Bark" not "Turbo Fart".

 

I just looked it up...Over $600.00!!!

 

we can all admit. it does sound pretty sweet.

 

Wrong , that's called a Pop off valve , it opens at a pre set pressure, and closes again under that pressure.

A BOV releases ALL pressure from the intake.