What is a blow off valve and what does it do.

 

I'm not familiar with that term on a truck, can you explain please?

 

It a valve for dumping boost pressure.

It used to keep the turbo from barking... IMHO

 

I have heard of a blow-off valve before but only on turbo imports. You can hear it especially on cars (trying to race) every time they shift. Does it make the same loud noise on your truck? I know most 4 cly. are not making a lot more boost than we are.

 

I'm pretty sure the big rigs have them too. What would be the purpose of having one on the CTD? Anyone got one on their rig?

 

I have heard of a few twins setups with them. The only purpose on a CTD would be to (like Mopar said) keep the turbo from barking. It also keeps the turbo spinning when you let off to shift, which is why a lot of turbo cars have them.

 

Just curious what would they do for an auto? Still keep the turbo spinning when the truck shifts or would you not put one on an auto?

 

I wouldn't see it beneifting an auto..you want to keep the boost up between shifts, not let it drop. With a manual, it takes time for the shift to happen , automatic's are almost instant.

 

Knowledge is a wonderful thing. My dad has always told me to be smart you dont have to know everything just know where to look to find the answer! I am glad I found this site. I know it will save me wasted time and $$$!

 

I have a BOV as they are called on my 300zx turbo. It is boosting 12 psi and originally only came with a POV (pop off valve) which was mounted to the intake side and set at a psi so the thing wouldn't blow. if the wastegate didn't open, then pov would open and relieve pressure at a set point (wastegate set at 7pse, pov set at 8psi). The BOV is different, still relieves the pressure, but you will mostly find them on "intercooled" cars, which my 300z is now. By adding in the extra length of intake tubing you have to up the boost to achieve what was stock, in doing so, you create more boost which creates more lag if the pressure is not relieved in the intake line which slows turbo spool up which depletes horsepower which sucks. Just as was stated earlier, when you shift gears the boost is not created therefore the extra pressure has to go somewhere, hence the bov. If the pressure was not relieved then you would have backpressure pushing on the turbo which would spool it down instead of up.

 

More info here...for those inquiring minds that want to know!
http://www.tialsport.com/
http://www.answers.com/topic/blowoff-valve

RJ

 

The weird thing about putting one on a diesel is the hose configuration. On a gasser a hose is lead from the intake manifold back so it can compare pressures on each side of the valve. If the intake pressure is lower by a bit it opens the valve up, otherwise it stays closed. Of course this works because gassers work under vacuum, but diesels do not.

 

Basically the only benefit too a diesel is too stop the turbo from barking...on mauals it will let the turbo spin freely whil;e shifting...allowing for boost too build quicker....but on an automatic, it will only stop barking....Not worth the $$$ JMHO

 

Stock turbo is HX35 or HY35, HX40 is an upgraded turbo. A good BOV comes at a price, most BOVs are for gassers and aren't designed to hold the pressure a diesel like ours can make.

 

Bov's are made to instantly relieve the pressure inside the intake tract when the throttle is let up quickly. Turbos spin at very high Rpms at high boost levels. I think they spin around 20K and up. When you let up on the pedal quickly and that turbo is spooling good the air in the intake tract stops moving which will stop the turbo shaft instantly, thus the BARK. Bark is bad and on a spendy turbo like a 40 with a weak shaft to begin with, so the bark can be costly which is why a BOV is used. Most gasser bovs are boost activated. On diesels they dont operate that way so they usually have a electric micro switch hooked to the throttle. so when the throttle is released it will open the BOV. I have seen one one on here that doesnt use electronics and I havent heard how it works. It is on tasha's evil twin 3rd gen.