ScepTTor

Both devices serve the same purpose. When you let go of the gas, the throttle plate closes and a vacuum is created. In order to keep the pressure that is built up by the turbo from warping the throttle plate and its self, it will need to be vented. The vacuum that is created pulls the vacuum line on the BOV/DV and releases through a piston type device.
The way that a BOV Valve functions is to release it back into the atmosphere while a DV releases it back into the intake track. On most stock turbo systems a DV is used to vent back into the turbine to help keep the turbo spinning to reduce lag and keep the car from running rich.
A rich condition is when there is more fuel in the cylinders then is the ideal fuel/air ratio. I think it was like 14:1. Rich conditions can be used to cool the engine while lean conditions will generate more heat. You can lean a car out to make more power but should be very very careful becuase more power means more heat which will eventually melt parts if it gets too hot. Lean conditions are how NOS destroys engines. (A/F ratio and Exhaust Temp gauge is used to monitor this.)
When a BOV vents to the atmosphere, it will cause a rich condition that on our model can throw codes and cause a rich condition that the multitronic may bark on and choke. I would not use this for our cars because I think that is annoying. Drag Cars often use BOV because they often run Lean to generate more power and will burn the excess fuel that the sudden rich condition creates.
Most DV and BOV on the market can be interchangeable. They can be vented into the atmosphere or vented back into the intake track. HKS even use to have an adaptor to vent their BOV back into the intake track making it a DV.
On racecars, you will sometimes find no BOV or DV. This is because the engine and turbo are built to handle the stress. Sometimes they use something called a pop off valve that is a spring-loaded device set to vent if a pressure spike goes beyond the safe limit.

orangeTT

Let the discussion begin =).

hoTTub

You are correct, a DV and a BOV do the serve the same purpose which is to release excess boost pressure, but not for fear of bending the Throttle Plate...A turbo and its bearings will let go long before the Throttle plate will. 25-40psi of pressure is not going to bend a throttle plate. A turbo spinning at 50k RPM's producing 40psi that hits a wall with no where for the air to go, will cause HUGE back pressure and cause the turbo to twist/tweak and ruin the bearings.


Anyway, there is also a misconception that a DV is used in order to keep the turbo spinning. This is also incorrect. A DV/BOV both do the same exact thing in terms of keeping the turbo spinning, neither in proper working order do <b>ANYTHING</b> differently. The sole purpose of either the DV or BOV is to release the excess boost pressure that the turbo is producing once the throttle plate closes. A DV does not recirculate the *pressurized* air back to the turbo in order to somehow keep it spinning.

<b>The only reason a DV is used in the Audi/VW application is because of the type of air metering system that Bosch uses.</b>

Bosch uses a Mass Air Flow sensor in the very beginning of the intake tract that measures incoming air...The ECU then uses the reading from the MAF along with an IAT (Intake Air Temp) and a MAP (Manifold Absolute Pressure) to determine how much fuel is needed. If you realease the excess boost into the atmosphere, you have now let air escape without accounting for the extra fueling. This is what causes the rich condition. If the ECU was able to somehow *recognize* that the air was released than it would be ok, but Bosch has not programmed the ECU for this type of operation.

A DV releases the excess boost pressure, and recirculates it to the <b>INTAKE</b> side of the turbo, <b>not</b> the <b>exhaust</b> side. There is no way that the recirculated pressurized air is helping to keep the turbo spinning.

Drag cars do not use a BOV in order to make the car run leaner, hence making more power. Drag cars typically are able to use BOV's without consequence is because most/if not all of them are using some sort of Speed Density sensor in order to determine the amount of air entering the engine. These are typically located in the manifold or downstream of the BOV so it is measuring ACTUAL air/boost that is about to enter the cylinders as opposed to the Audi setup that measures air at the very beginning of the intake tract...

And as far as the *pop off valves* are concerned, I think that they are the same thing as a BOV, it is just what the Brits call 'em


&lt;whew&gt; that was alot =-D

Lightweight-USPowah

I don't have access to my email account from work.. I also have a class tonight so I won't be home untill 9:40ish..

On a seperate note, I like your post about the DV/BOV, now maybe you know something about the ECS BOV.. I saw on their website that there is an option of redirecting air back into the intake system.. any idea how that works and if it does work? So the DV takes the excess boost which is extremely hot air and puts it back into the intake system.. Now, it seems as though this is kinda defeating the purpose of obtaining the coolest air in the system.. I mean, the hotter your turbo gets the hotter the intake temps will get no matter where your source of air is from which is why blowing the hot air into the atmosphere would be benificial (of course forget the running rich part).. Well, maybe thats wrong, maybe the boost being released isn't enough air to really have a large impact on the intake temps.. Any idea how much air is redirected back into the intake system?

ScepTTor

Quoting myself "warping the throttle plate and its self" and quoting you "A turbo spinning at 50k RPM's producing 40psi that hits a wall with no where for the air to go, will cause HUGE back pressure and cause the turbo to twist/tweak and ruin the bearings."
I did mention warping itself and I have seen throttle plates warped on cars with improperly set up turbo systems caused by the lack of venting. And guess what? the turbo and the bearings where okay!
"A DV releases the excess boost pressure, and recirculates it to the INTAKE side of the turbo, not the exhaust side. There is no way that the recirculated pressurized air is helping to keep the turbo spinning."
Hello! the turbine is on a shaft! Did you get a chance to blow on your ball bearing turbo when it was out of the car? (I am serious about this not joking or making fun of.) Did you see that when one side turns so does the other?
"Drag cars do not use a BOV in order to make the car run leaner, hence making more power"
And I never said they used the BOV to make the car run lean.
Please explain how the Aftermarket for Eclipse Turbo cars and WRX favor the BOV to the DV since they also have their MAF Sensor mounted on the "very beginning of the intake tract." Are you saying that they have superior electronics sensors to our TT?

hoTTub

when you think about it, a turbo that has been run hard can be <b>GLOWING</b> red hot, I dont think that some redirected air is going to hurt much

hoTTub

Please dont try and convince me that because the turbo (exhaust and intake wheel) are connected by a shaft that this is why the pressurized air is keeping it spooled. The pressurized air is more likely to escape out back through the MAF housing and air filter than it would to *pressurize* the turbo, only because the turbo is already spinning is the air drawn back through the inlet side of the turbo

As far as your other statement, I did misread your drag car statemnent.

<b>"Drag Cars often use BOV because they often run Lean to generate more power and will burn the excess fuel that the sudden rich condition creates."</b>

I still dont think that statement is true but now
that I read it, for a different reason =0)

These cars are not running MAF's, they are running MAP sensors etc..

That is also the same answer for the Mitsu's, I am not an expert but my recollection is the these cars do not use MAF's, they use MAP's or speed density setups, and in other cases, the ECU compensate/detect for lost air whereas Bosch does not.

ScepTTor

then the intake track. Sean is right Turbo's do Glow red on the exhaust side when run hard

Lightweight-USPowah

air redirected? I guess if one could get some numbers as to how much air it was you can figure out how badly it was affected..

Lightweight-USPowah

odds are it doesn't have any affect since all that air will be going through the compressor outlet of the turbo anyway..