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Blow off valve

5K views 29 replies 17 participants last post by  Ipkyss 
#1 ·
I for the life of me can not wrap my little brain around how a blow off valve does anything on a diesel motor. I fully understand how they work on a gas motor but the difference is they have a throttle plate that blocks airflow from the turbo to the cylinders when you let off the pedal. On a diesel there is not a throttle plate. When you let off the pedal the air keeps flowing into the cylinder, nothing with the airflow is changed. The only thing that happens when let of the pedal is the motor stops injecting fuel. The boost built up just gows through the engine when you let off the pedal.
 
#3 ·
Reverse air flow comes from when the boost has no place to go. I.e. in a gas motor it hits the throttle plate. In a diesel no plate, the air will just go through the motor. The engine does the same thing as a blow off valve. The only differnce is you now here the air come out under the hood instead of out the tail pipe.
 
#5 ·
Say you are in the throttle, 35psi of boost and the engine is taking it and you have back pressure to push it in. You suddenly let off the throttle and your back pressure is gone but you still have that 35 psi of boost that isn't getting forced into the cylinders by the back pressure via turbo, hence it goes backward (turbo bark)as there is more pressure in there (35psi) than there is back pressure since you let off the throttle. I hope this helps.
 
#6 ·
Me personally, I don't really get the point, nor would I spend 600 bucks to have it on there. But some guys say they will help save the turbos. What I don't quite understand is, if you are going to run one on compounds, and you run it after the second stage, what is stopping the air from the first stage or the big atmospheric age from running back through your high pressure or second stage turbo, and hurting it?
 
#8 ·
Have any of you tried spinning 60,000 rpm one way then reverse and do about 50,000 rpm the other in an extremely short amount of time? I haven't but I hope you understand what I'm trying to say.
 
#9 ·
:whs: for whatever reason it does it we all know when you let off hard the turbo chirps because it imiditly is forced to spin backward. why you ask??
Im not sure but we know it isnt good to spin backwards and we know wastegates prevent this so thats why we do it.
I wish i knew why they work... I also would think the air would contiue into the engine but i guess not:confused:
 
#13 ·
Cold side.
 
#16 ·
From my understanding. When you snap off the throttle. That cha cha cha noise you hear is boost pressure going back out the air intake. For that to happen, I believe the turbo would have to stop and turn backwards. The blow of valve is suppose to stop that. This does not happen under normally driving, or when you roll out of the throttle from high boost.

I believe the reason why this happens was said above. When you let off and remove the exhaust back pressure that is driving the turbo, it give the boost a quick chance to go backwards. Same basic thing that happens when the throttle plate is closed on a gas motor.

That being said. These motors do have throttle plates. Does anyone know how and when they operate? Is it just to aid the egr system? I removed my throttle plate from my stock intake elbow a while back. I am fairly sure I lost about 1mpg but I was too lazy to put it back in. Not sure of its actual purpose.

I personally think if you are beating on your truck quite offending, a bov would be well worth it. In extreme cases, it is possible to break the shaft in the turbo.
 
#17 ·
The turbo does not change direction. The noise is from the air slipping past the blades. the blades always spin the same direction.
 
#19 ·
I was just making a speculation. It's obvious that the turbo speed is drastically reduced very fast. I don't think there is any way to say if it stops or spins the opposite way for a fraction of a second without monitoring turbo rpm. Which you or someone else may have already done.
 
#22 ·
air going back into the blades can cause stall and damage turbos. BOV completely removes this possibility when enough throttle position is dropped off instantly. it keeps turbos alive. folks with very expensive turbos prefer to keep them safe.

thats why they, in the diesel world, are called turbo savers. folks with journal bearing turbo's are MUCH more likely to experience damage or failure from this condition. the bb units seem to handle the torsional load better. from experience with mine, i know when i got out of it at WOT, my turbo's never barked, but at 3/4 throttle they sure did when it didnt open. and if you think its still "going through the engine" you are dead wrong.
 
#25 ·
can someone with a bov explain how it is set up? what activates it? does it only work when you get off full throttle or all the time like normal? I am use to my 300zxTT with dual bov's. I am rarely full throttle in my truck, but 3/4 throttle is enough to make it bark. I am interested in one. I feel like I am running out of cheap things to add to the truck...
 
#27 ·
I understand bov's in general. I was just trying to figure out how they actuate it. I see there is a solenoid and a controller off some sort. I'm guessing it goes off throttle position? obviously there not run off vacuum...
 
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