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View Full Version : FAQ: Blow-Off Valves and Bypass Valves/Diverter Valves



kristokes
03-27-2011, 11:14 AM
Blow-Off Valves available for the B7A4 2.0T as a direct bolt-on:

Forge Motorsport BOV (Diverter Spacer) (http://www.ecstuning.com/Audi-B7_A4-Quattro-2.0T/Search/Forge/Diverter_Valve/ES1832063/)
KMD Tuning's version of a Diverter Blow-Off Spacer (http://kmdtuning.com/index.php?_a=viewProd&productId=572)


Diverter Valves available for the B7A4 2.0T as a direct bolt-on:

Forge Motorsport Diverter Valve (http://www.ecstuning.com/Audi-B7_A4-Quattro-2.0T/Search/Forge_Diverter_Valve/ES1832057/)
Revised OEM Diverter Valve (http://kmdtuning.com/index.php?_a=viewProd&productId=578)


Blow-Off Valves (BOV) and Bypass Valves (BPV)/Diverter Valve (DV) are pretty much the same thing.

These devices are simply an actuator with either a built-in piston or diaphragm that allows air to pass through with the movement of the piston or diaphragm. The piston and/or diaphragm is controlled by the vacuum and pressure built up from your charge air stream (generally at or near the throttle body in the intake manifold), where it will open and close accordingly. When you let off your throttle going from a positive boost state into a vacuum state, your throttle plate will close - blocking air passage into throttle body. This excess pressure needs to be vented out ("blown off") to prevent it from going back into the turbo compressor and causing it to sometimes spin backwards. This effect is also known as "Compression Surge" and enough can severely damage a turbocharger. This is when a BOV/BPV comes into play. Most turbocharged cars come with a factory BPV, or Diverter Valve (DV) as we call it, that will "bypass" the excess boost out through the intake tract. A BOV simply vents the excess boost into the atmosphere.

The following animation demonstrates the operation of a built-in piston within a BOV (not 100% atmospheric ventilation)
http://x6d.xanga.com/289a0b174943254637297/b36614354.gif

Why Blow-Off Valves are not recommended for the B7A4?

I am not saying you "can't" use a BOV but it's not really worthwhile - will cause more harm than good. Venting into the atmosphere as opposed to back into the intake makes the sound a lot more aggressive - the #1 reason why people opt for a BOV over a BPV setup (with the exception of high boost applications). The B7A4 comes with a Mass Air Flow Sensor which meters all incoming and outgoing air. This allows the ECU to adjust your air-to-fuel (A/F) ratio accordingly. However, once the metered air is "blown off," the MAF will no longer see it and your A/F may be adversely affected as a result.

vhstejskal
03-27-2011, 11:18 AM
Great explanation and good visual on how it works... Goes well with all the other DV/BOV discussions/question.

Leo14
03-27-2011, 11:34 AM
I was just about to make a thread on this as I was having a conversation about BOV's/DV's and my friend said get a BOV and I was like no, my car doesn't like those for some reason. Now I can explain why, and thank you Stokes for (another) awesome explanation/write up [up]

rs182
03-27-2011, 12:32 PM
Riddle me this: The MAF in the B7 is upstream of the DV, so how does it meter the air the DV reintroduces?

Vorsprung
03-27-2011, 01:22 PM
Riddle me this: The MAF in the B7 is upstream of the DV, so how does it meter the air the DV reintroduces?

It already did. That's the point of using the DV, it keeps all the air that passed through the MAF IN the system

A 4 Awesome
03-27-2011, 03:32 PM
Wow! Great write-up [up] This clears up a lot of questions for me.

Tcass
03-27-2011, 06:09 PM
any one have KMD's diverter valve spacer?? i might pick one up i have the forge dv kmd's spacer is half the price

vbrad26
03-27-2011, 06:41 PM
did you not just read the thread?

Mister W
03-27-2011, 07:46 PM
What about the Forge for 1.8T engine. It's a DV and a BOV mix together? I read that it does the DV work as putting back the air in the intake, but use a small part of air to create the BOV sound.
What it good?
Why it doesn't exist on 2.0T?

tecnica
03-27-2011, 08:21 PM
It already did. That's the point of using the DV, it keeps all the air that passed through the MAF IN the system

Lets fix this.

If the MAF is post system, the reintroduced air will be recalculated as it passes the MAF for a second time. If the air is reintroduced pre-MAF, it really doesn't matter - regarding the rich-or-not situation.
I'm really not sure where the air is reintroduced in this car. I haven't had the weather to play around yet (damn Minnesota). Soon though. I'd be fairly surprised to find that Audi doesn't divert the air post-MAF though.

I'm not condoning the product. BPVs were made for a reason.

Since the system is at a state of vacuum, the pressurized air needs out. The BOV takes it and throws it away, making a whoosh sound. The BPV just capitalizes on this pressure by putting it pre-compressor again. The pressurized air expands towards the negative pressure of the still-spinning turbo and helps keep the turbo at spool, decreasing lag time for when the throttle is reintroduced.


As for the sound. You can accomplish quite a bit with a new intake. You get the whoosh when the air hits the intake piping from the DV. And with none of the drawbacks!

mauismile
03-27-2011, 09:58 PM
So which is better?? The OEM DV or the forge?? Will you notice a difference between the two??

jimrobbington
03-28-2011, 08:36 AM
So which is better?? The OEM DV or the forge?? Will you notice a difference between the two??

The OEM piston is electronically controlled by the computer, whereas the Forge is actually controlled by vacuum alone. Correct me if I'm wrong, but it's all in how you look at it. Some say the OEM is better because it reacts faster to the electronic signal. Some say the Forge is better because the vacuum control is more accurate. Discuss.

ny2flb7a4
03-28-2011, 08:42 AM
Good info. My DV is shot but I have not had the time to get it replaced with the updated OEM part.

VZWalex
03-28-2011, 10:16 AM
lets end the discussion right here .... dont waste your money on the Forge ... it gets stuck and you have to maintenance it every month so that it operates properly ..... just get the latest OEM revision and DONE!.

If you really want added peace of mind go with AWE's DV for your big turbo applications.

That was simple ... there should be no more DV discussion threads ever made ... have a great day!

mr shickadance
03-28-2011, 10:31 AM
lets end the discussion right here .... dont waste your money on the Forge ... it gets stuck and you have to maintenance it every month so that it operates properly ..... just get the latest OEM revision and DONE!.

If you really want added peace of mind go with AWE's DV for your big turbo applications.

That was simple ... there should be no more DV discussion threads ever made ... have a great day!

...what.....try every 10,000-15,000 miles.....so i guess if you drive that much every month then yeah....every month you would have to do maintenance on it....i do my maintenance every 15,000 miles....which is a good bc its also when i check my dv, and check my fluids and pretty much complete a tune anyway so its really no inconvienance for me at least to check the dv for sitckiness

also, the forge dv has never given me problems and when i do check it at 15,000 miles, it is always running as good as it was when i re-apply grease, i just go ahead and do it anyway cause why not?





bottom line, there are two options for dv valves, both are adequate, one is cheaper, you can't really go wrong with either one



also i don't think the bov spacer actually lets anything out into the atmosphere i always thought it just created a big enough gap that it just made the bov noise....but i could be wrong

b7kevin
03-28-2011, 11:35 AM
There is a lot of pertinent information missing from this thread regarding the advantages of either system

A BOV has the fastest cycle rate. That means that anyone running a BT should be using one to help stave off compressor surge. Many people in this category are also running MAF-LESS. Now for a 2.0t BOV are not ideal. They cause the motor to run rich and can lead to stalling, by default these cars run rich. The metered air is meant to be recirculated at idle; both a DV and BOV are opening and venting. A BOV will cause idle fluctuations as its essentially a vacuum leak at this point. To combat this issue companies like forge have developed BOV that stay closed at idle, the 004 for instance leaves the piston closed and opens under normal circumstances.

Also another option for BT guys is to run twin DV's, this setup is becoming increasingly popular as it allows for different tuning, IE. BT files with a MAF.

The stock DV is electronically controlled and rev. D is maintenance free. This is the slowest possible way for boost charge to be vented preventing compressor stall. A vacuum set-up like the forge is the only tried and true system that quickly vents air (this goes as well for the BOV).

I've ran all 3 styles listed above on my 1.8t GT3071r. Dual DV set-up was my overall favorite.

I personally dislike BOV's, and for the purposes of BT do not plan on running one either as I plan to maintain a MAF based file.

jimrobbington
03-28-2011, 12:21 PM
There is a lot of pertinent information missing from this thread regarding the advantages of either system

A BOV has the fastest cycle rate. That means that anyone running a BT should be using one to help stave off compressor surge. Many people in this category are also running MAF-LESS. Now for a 2.0t BOV are not ideal. They cause the motor to run rich and can lead to stalling, by default these cars run rich. The metered air is meant to be recirculated at idle; both a DV and BOV are opening and venting. A BOV will cause idle fluctuations as its essentially a vacuum leak at this point. To combat this issue companies like forge have developed BOV that stay closed at idle, the 004 for instance leaves the piston closed and opens under normal circumstances.

Also another option for BT guys is to run twin DV's, this setup is becoming increasingly popular as it allows for different tuning, IE. BT files with a MAF.

The stock DV is electronically controlled and rev. D is maintenance free. This is the slowest possible way for boost charge to be vented preventing compressor stall. A vacuum set-up like the forge is the only tried and true system that quickly vents air (this goes as well for the BOV).

I've ran all 3 styles listed above on my 1.8t GT3071r. Dual DV set-up was my overall favorite.

I personally dislike BOV's, and for the purposes of BT do not plan on running one either as I plan to maintain a MAF based file.

Best information I have ever heard on this subject.

kristokes
03-28-2011, 04:33 PM
There is a lot of pertinent information missing from this thread regarding the advantages of either system

Much gratitude for the added information b7kevin [up]

I wanted to keep the original post short, sweet, and straight to the point so I excluded many in-dept details (especially about BT applications). The original post's sole intention was to eliminate many of the newb questions threads: "can I use a BOV and which one.."

caspar
12-15-2013, 10:53 AM
If someone could told me when the DV return the air to the intake and that air passes through the MAF in opposite way if that air is metered too?

mec
12-15-2013, 11:17 AM
If someone could told me when the DV return the air to the intake and that air passes through the MAF in opposite way if that air is metered too?

when air is returned from the DV it does not pass through the maf again. All air that enters the engine goes through the maf once. The Dv just recirculates it from the charge pipe back to pre-compressor area.

caspar
12-15-2013, 11:23 AM
I know that but if you have higher boost is it possible in some case to pass through it in opposite way?