By venting the blow off , what does this do to you engine. I have heard some stuff such as just burning rich, but what else would happen?


And on the highway when you have it on cruise control etc. Is this bad on the engine with the blow off not (blowing off) from what i heard from some people.

I can't really answer the first one, however the second one, I know that my BOV goes off when I go up a hill or something with cruise control on.

by venting the BOV you are protecting the turbo from impoding or exploding (says my friend) cause the pressure you destroy the spool in the turbo and make it not work

^ haha...

The main function of a blow-off valve is to vent excess boost pressure when the throttle butterfly is closed. The lower chamber of the blow-off valve is plumbed into the intercooler front pipe while a small vacuum hose leads from the top of the valve to the inlet plenim on the engine. Inside the blow-off valve is a diaphragm (usually made of rubber) and a spring with a small insert to support the diaphragm where it seats against the valve body.

When the throttle butterfly is open, the boost pressure that enters the upper chamber above the diaphragm, via the small hose from the plenim chamber in the top of the blow-off valve, is equal to the manifold absolute pressure (MAP). Because the pressures are equal and positive, the spring holds the diaphragm on the valve seat and stops the pressurised air from being vented away. When the throttle butterfly is closed, the MAP is in vacuum and the diaphram lifts away from its seat, allowing the boost pressure to vent to atmosphere.


When the throttle butterfly is reopened, the pressure in the blow-off valve becomes positive once more, the spring pushes the diaphragm onto its seat reventing boost pressure from being vented away. There are a number of reasons why a blow-off valve is a good idea. When the throttle butterfly is closed, the spooled-up boost pressure cannot escape and subsequently puts tremendous stress on the turbo by stalling the compressor wheel, compressor shaft, and turbine wheel. The blow-off valve vents his boost pressure allowing the turbo components to spin freely in a vacuum. This prevents significant turbo lag and allows the turbo to spool-up more quickly. Over time, the blow-off valve can help to minimise stress on the turbo components and prolong the turbo's life.

While in boost, the valve remains closed and the turbo pumps air into the engine normally. Without the BOV, when the throttle is closed the turbo is suddenly trying to pump air against a closed throttle plate. This creates a pressure spike in the turbo output hose and sends a pressure "wave" crashing back and forth between the throttle plate and the turbo compresser blades. The pressure spike quickly slows down the turbo and the pressure wave can actually damage the turbo. On intercooled engines, this pressure wave effect is suppressed but the pressure spike still occurs. When the throttle is opened again, the turbo has to spin up again, creating turbo lag. If a BOV is present, the BOV will open as soon as the throttle is closed, releasing the pressure spike into the airbox and avoiding the pressure wave phenomena

and here I always thought the BOV was just to make cool sounds to impress chicks.

By venting the blow off , what does this do to you engine. I have heard some stuff such as just burning rich, but what else would happen?

The air enters the motor and passes by the MAF (Mass Airflow Sensor). This sensor measures the amount of air entering the motor.

The MAF sends that data to the ECU (the brain of the car). The ECU takes that information and decides how much fuel it needs to fire via the injectors to match that amount of air. If all goes right, the car runs beautifully.

However, Joey Tuner decides to install a BOV that vents to atmosphere (VTA). Thus, when the BOV is open, the air that was already measured by the MAF is being forced out of the BOV before the motor ever sees it.

The issue here is that the ECU has NO idea that the air was purged from the system, so continues to fire the injectors according to the amount of air it THINKS is in the motor. So in the end, you end up dumping in a fair amount of fuel and have almost no air to burn said fuel.

In some cars, this will cause the motor to bog down and/or stall. In other cars, it's not that big of a deal, and the car can continue on.

There are a few ways to solve this issue, but the most "theoretically proper" way of doing so is to blow off the air before the MAF ever sees it. This would involve moving the MAF the the pressurized side (post turbo side) of the engine piping and to mount it AFTER the BOV. That way you could VTA all day long, and all would be well...the MAF (and thereby, the ecu) would only see the air that ran past it...not the air that was purged.

Ya dig?

^^^^ great explanation!

niice kooldino

so dino does that mean moving the bov to the pressurized side of the turbo as well?? or retain stock location and just moving the MAF??

There are a few ways to solve this issue, but the most "theoretically proper" way of doing so is to blow off the air before the MAF ever sees it. This would involve moving the MAF the the pressurized side (post turbo side) of the engine piping and to mount it AFTER the BOV. That way you could VTA all day long, and all would be well...the MAF (and thereby, the ecu) would only see the air that ran past it...not the air that was purged.



noob question:
so in some pics the BOVs are shown up front closer to the front bumper and further back, where the stock one would be, which of these is correct to "VTA all day"?

and here I always thought the BOV was just to make cool sounds to impress chicks.

Ya thats their main function. Do aftermarket BOV do the job better thn oem>

from what I have read and heard the stock X BOV is actually really good and can handle up to 400 hp. I think. There was guy on the other forum who put on a new turbo and after a few tuns had to change it becuase it could handle the power and would bleed or something like that.

I know alot of people get aftermarket BOV for the "pissing" sound.

I am not sure if it will attract chicks, really, but I do know alot of folks (I am talking about pedestrians, passerby) dislike the sound.

Also, correct me if I am wrong, most stock car comes with bypass valve & not Blowoff valve. That is why stock STI or Evo do not produce the "pissing" sound. While the BOV release air to the atmosphere, the bypass value have a pipe that connect the air back to the inlet.

Blow Off Valve Frequently asked questions: http://www.alltrac.net/tuning/bovfaq.html (http://www.alltrac.net/tuning/bovfaq.html)

I know alot of people get aftermarket BOV for the "pissing" sound.

I am not sure if it will attract chicks, really, but I do know alot of folks (I am talking about pedestrians, passerby) dislike the sound.

Also, correct me if I am wrong, most stock car comes with bypass valve & not Blowoff valve. That is why stock STI or Evo do not produce the "pissing" sound. While the BOV release air to the atmosphere, the bypass value have a pipe that connect the air back to the inlet.

Blow Off Valve Frequently asked questions: http://www.alltrac.net/tuning/bovfaq.html
It's bypass. But get an intake and you'll hear it like a vta blowoff. I think the air sound resonates better in the metal tubing of the intake and the sound travels through the filter. And yes, it only scares chicks, unless they're in the know...it's my experience that very few are...

...so then BOVs are bad? Because I love my MAF and don't wana confuse my ECU....

OR

...so then BOVs are good? Because I certainly don't want to damage my Turbo....


.Rant

Good thread i'm learning a lot... So all these BOV people are installing are they Recirculating or Atmosphere?

...so then BOVs are bad? Because I love my MAF and don't wana confuse my ECU....

OR

...so then BOVs are good? Because I certainly don't want to damage my Turbo....


.Rant

Keep your stock recirculation valve. It's a BOV that dumps the excess air BACK into the inlet stream. It actually helps keep the turbo spooled and keeps excess fuel from getting dumped into the cylinder walls washing them out.

from what I have read and heard the stock X BOV is actually really good and can handle up to 400 hp. I think. There was guy on the other forum who put on a new turbo and after a few tuns had to change it becuase it could handle the power and would bleed or something like that.
What the BOV is dependant on is not HP, from what I understand, but psi of boost.

What the BOV is dependant on is not HP, from what I understand, but psi of boost.

Yes\No

When you start moving a high volume of air, much will depend on the size of the valve and it's ability to hold (spring) and release the amount of volume and pressure.

and here I always thought the BOV was just to make cool sounds to impress chicks.
That's just a cool bonus to the mod.:ghey: