origionally posted by KirkMKIV:
The reason its not a good idea to vent to the atmosphere, is because the AFM reads how much air being ingested in to the motor threw the turbo. when the bov dumps back into the intake track the metered air remains consistent with what the meter has already read. the ECU determines how much fuel to add dependent on how much air the AFM recognizes. when you dump to the atmosphere the ECU thinks that air that is being ejected from the intake track is actually being ingested into the motor and supplies a given amount of fuel to match the metered air. but that figure is now incorrect because the air is not going into the motor its going into the air unmetered, so now you have a disproportionate air/fuel ratio between shifts. i hope that makes sense
This has been a topic talked about during some periods of time and I just wanted to clear up what happens and why it causes it to bog when you let go of the throttle when its not in gear. If you want a blow off valve then wait to get an aftermarket one, its not worth to vent the stock bypass valve to the air when it was meant to be vented back into the intake.
Here are the common problems you run into with a vented BOV, and how I fixed them:
Bad idle. The stock BOV is designed to fluctuate between open and closed at idle. Vented to atmosphere, you would constantly lose metered air. To get your car to idle properly, adjust the BOV so it is tight enough to stay closed at idle. Easiest problem to solve.
Bucking/stuttering under boost. Your BOV is leaking. Again, most likely it is set too softly. Tighten the valve.
Stumbling/stalling when coming to a stop. This one is a driving style issue. If you are on the gas, in the boost, and you notice the light ahead just went red you will probably put in the clutch and drift to a stop. When you hit the clutch, you go from having a lot of air coming through the MAS to a very small amount. Of course, the BOV vented to atmosphere dumping a lot of metered air, you run rich, the engine bogs. The solution to this problem is to back off of the gas first. When you back off of the gas, the amount of air coming in drops drasticly. In a split second, all of that extra air is out of the engine. Shifting to neutral now means much, much less metered air is lost and the engine will not stumble.
Running rich between shifts. This is the only real problem, in my opinion. The others are debatable at best, to some they aren't a big deal at all. Running rich, however is the real issue. However, no one has ever been able to tell me HOW rich and for HOW LONG. To date, the only way I have been able to come up with to test this (since I do not have a wideband in the car) is to use the stock o2 sensor readings. o2's will not be the same from car to car or even from day to day, but they should still be valid for this test.
At 4k RPMs in 3rd gear (a place where I would get instant boost), I floored it. At 4500 RPMs (20psi full boost) I let off of the gas and then instantly floored it again. This allowed the BOV to open and vent to atmosphere. My o2's increased by .1 after the venting, and lasted for less than a second.
Now, if my car was built with one purpose in mind - that is, going down the 1/4mi as fast as possible, this would not be acceptable at all. I lost control of fuel delivery for a second there, which is not good for making maximum power. However, seeing that my car has only been at the track for 2 of it's 170,000 miles, I think it's an acceptable loss.
So anyways, that is my take on venting to atmosphere. It's worked great for me for a while now, and it's worked for everyone that I have helped out. Installing and adjusting the valve properly is necessary, as is changing your driving style. If you don't do these things, it won't work. But then, what modification will work if you don't install it right, adjust it right, and utilize it properly?
The reason its not a good idea to vent to the atmosphere, is because the AFM reads how much air being ingested in to the motor threw the turbo. when the bov dumps back into the intake track the metered air remains consistent with what the meter has already read. the ECU determines how much fuel to add dependent on how much air the AFM recognizes. when you dump to the atmosphere the ECU thinks that air that is being ejected from the intake track is actually being ingested into the motor and supplies a given amount of fuel to match the metered air. but that figure is now incorrect because the air is not going into the motor its going into the air unmetered, so now you have a disproportionate air/fuel ratio between shifts. i hope that makes sense
This has been a topic talked about during some periods of time and I just wanted to clear up what happens and why it causes it to bog when you let go of the throttle when its not in gear. If you want a blow off valve then wait to get an aftermarket one, its not worth to vent the stock bypass valve to the air when it was meant to be vented back into the intake.
Here are the common problems you run into with a vented BOV, and how I fixed them:
Bad idle. The stock BOV is designed to fluctuate between open and closed at idle. Vented to atmosphere, you would constantly lose metered air. To get your car to idle properly, adjust the BOV so it is tight enough to stay closed at idle. Easiest problem to solve.
Bucking/stuttering under boost. Your BOV is leaking. Again, most likely it is set too softly. Tighten the valve.
Stumbling/stalling when coming to a stop. This one is a driving style issue. If you are on the gas, in the boost, and you notice the light ahead just went red you will probably put in the clutch and drift to a stop. When you hit the clutch, you go from having a lot of air coming through the MAS to a very small amount. Of course, the BOV vented to atmosphere dumping a lot of metered air, you run rich, the engine bogs. The solution to this problem is to back off of the gas first. When you back off of the gas, the amount of air coming in drops drasticly. In a split second, all of that extra air is out of the engine. Shifting to neutral now means much, much less metered air is lost and the engine will not stumble.
Running rich between shifts. This is the only real problem, in my opinion. The others are debatable at best, to some they aren't a big deal at all. Running rich, however is the real issue. However, no one has ever been able to tell me HOW rich and for HOW LONG. To date, the only way I have been able to come up with to test this (since I do not have a wideband in the car) is to use the stock o2 sensor readings. o2's will not be the same from car to car or even from day to day, but they should still be valid for this test.
At 4k RPMs in 3rd gear (a place where I would get instant boost), I floored it. At 4500 RPMs (20psi full boost) I let off of the gas and then instantly floored it again. This allowed the BOV to open and vent to atmosphere. My o2's increased by .1 after the venting, and lasted for less than a second.
Now, if my car was built with one purpose in mind - that is, going down the 1/4mi as fast as possible, this would not be acceptable at all. I lost control of fuel delivery for a second there, which is not good for making maximum power. However, seeing that my car has only been at the track for 2 of it's 170,000 miles, I think it's an acceptable loss.
So anyways, that is my take on venting to atmosphere. It's worked great for me for a while now, and it's worked for everyone that I have helped out. Installing and adjusting the valve properly is necessary, as is changing your driving style. If you don't do these things, it won't work. But then, what modification will work if you don't install it right, adjust it right, and utilize it properly?