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I was watching a Tom's Turbo Garage video on YouTube where he was talking about (at about 10:30 in the video) having a blow off valve (BOV) which is recirculated. This means when the BOV activates it allows the excess pressure to be run back through the intake instead of dumping to atmosphere. I also noticed the BOV is dumped between the MAF and the turbo (not before the MAF).

Questions:

  • Why choose one over the other (recirculated or dumped to atmo)?
  • What are the pros and cons of either method?
  • Why is it dumped between the MAF and turbo and not before the MAF?
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  • Isn't the MAF between the turbo and the throttle body? If so, isn't dumping between the turbo and the MAF "before" the MAF?
    – 3Dave
    Jun 2, 2017 at 16:38
  • Unless you've got a full Group A competition car with anti-lag, chose a recirculating dump valve. May 21, 2018 at 11:51

4 Answers 4

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I'm not a fan of these answers, and some of the information is flat wrong. I appreciate that I am super late to the party here, but decided it still may be helpful to contribute (additionally, I have also watched Tom's Turbo Garage channel :) )

Answers to the questions:

  • Why choose one over the other (recirculated or dumped to atmo)?

Typically it's not a choice, but rather one should follow what the car manufacturer used. A MAF setup will use a BPV - Bypass Valve (or, a BOV set to recirculate). This is because the MAF meters the air, and uses this information to determine how much fuel to supply. Venting air to atmosphere removes the already measured air from the system, but the computer still thinks it's there, so will command more fuel - the car will run rich.

Conversely, an SD setup (Speed Density) doesn't have a MAF. It can be found on some older factory cars; though just about every factory car these days feature a MAF setup, SD is still used on aftermarket conversions as well as utilized with various standalone ECU offerings. Since SD doesn't measure the air like a MAF, you can vent to atmosphere all day long and it won't affect the fueling (since fueling is calculated differently). In these cases, a BOV works just fine. One could recirculate on a SD setup, but it's not necessary, and most don't because they typically want the BOV sound (also it's easier, no additional piping needed to recirculate the intake air). Additionally, BOV's came into mainstream use by factories about the same time as SD was being phased out. There may be some old factory cars out there that had a BOV set to vent to atmosphere, but if so it's pretty rare. For SD, BOVs are added aftermarket.

Some people opt to use a BOV venting to atmosphere on their MAF cars. There is a big debate on whether it actually hurts the car or not. I'm firmly in the camp of "it's not a good idea." But that's a different discussion. To answer the question here, the reason they do this is simply for the sound of the BOV dumping to atmosphere. There is no performance gain.

  • What are the pros and cons of either method?

It depends on what system of air metering the car is using.

If it's a MAF, then...

BOV Pros: none really, unless you count the sound as one.

BOV Cons: you are tricking the computer into thinking it has more air then it actually does. This can lead to rich spikes, fouled plugs...or maybe no problems. Is it worth the risk? Your call.

BPV Pros: it's how the factory designed the car, so everything continues to work properly. And depending on your intake, you can still hear a BPV "venting" back in.

BPV Cons: none. Unless you consider it not being loud enough as a con, then there's that.

If it's a SD, then...

There are no cons to either set up. The system literally doesn't care whether you vent to atmosphere or recirculate it. So you can do whatever you feel is best for you.

  • Why is it dumped between the MAF and turbo and not before the MAF?

This is critical. If it's dumped before the MAF, then the MAF will read it as NEW air coming in - even though it already read it. Fuel will be commanded accordingly...but there really isn't that much air. So it will create a rich spike, just as if you venting the air to atmosphere.

Additionally, the air cannot be recirculated AFTER the turbo, as this doesn't fix the problem of surging. The air in front of the turbo is LOW pressure. The air after the turbo is HIGH pressure. The BPV wouldn't work if it was dumped after turbo - it's the same "pipe" where the BPV is located, so same pressure. The only proper place therefore is between the MAF and turbo - where the air was already read, and it's a low pressure zone so the air will actually vent.

Other Notes: there are different BOV/BPV designs, but they typically only have 1 nipple, which is the source from the intake manifold. The pressure differential that makes it work is typically supplied by where the piston is, on the intake. Some designs might have two nipples and thus require two different sources, but this is not very common.

The sole purpose of the BOV/BPV is reduce or eliminate surge on the turbo compressor wheel, especially in high(er) boost applications. When one is doing a pull, max acceleration, and then lifts off the gas to shift (or an automatic transmission does this behind the scene), the throttle plate slams closed, and all that pressurized air from the turbo now has no place to go. So it stacks up, and pressure waves back in the other direction...toward the turbo. This is really bad because the compressor wheel is still spinning in a specific direction, and now incoming pressure waves arrive in the opposite direction - and impact the compressor wheel. This is like sticking your hand/fingers up in the fan blades of a ceiling fan that was spinning but has just been shut off. The fan blades react poorly to the introduction of your fingers - as does the turbo compressor wheel. It creates surge, and if bad enough can damage the turbo. To alleviate these pressure waves, when the throttle plate slams shut, this triggers the BPV to open, which gives that air that previously had no place to go...someplace to go. This means little to no pressure waves heading back to the turbo, no damage to the compressor wheel. And also has the added benefit of letting the compressor wheel continue to spin unimpeded, so when you are back on the gas it's ready to spool right back up. With no BPV, the pressure waves will SLOW the compressor wheel down even more as they collide - meaning more time needed to spool back up.

Edit: I updated a bit about SD usage based on some good feedback received through comments.

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  • Welcome to Motor Vehicle Maintenance & Repair! I'm glad you're here and a correct late response is always welcomed :o) One thing you might want to clarify is your statement about SD applications. When delving into the performance realm, most engine builders get away from MAF and use SD due to the restriction a MAF can put on the system. When you start talking about 3+ Bar, a MAF isn't going to be able to hang. My point is, SD isn't as dead as you are letting on in your post. Just a thought. Jul 14, 2023 at 11:29
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    Thanks for the feedback, and you are absolutely right. Additionally, many standalone ECU options feature SD. I will update my answer accordingly, thanks :)
    – Scott M
    Jul 15, 2023 at 3:48
  • The aftermarket BOV setups I've worked with have vented to atmosphere upstream of the MAF. It won't affect the MAF reading.
    – 3Dave
    Aug 13, 2023 at 0:30
  • @3Dave Where is the MAF situated - pre or post turbo? If the MAF is placed between the turbo and throttle body (aka blow through) then sure, vent right before the MAF. But this placement of the MAF isn't super common - sure people are rolling their own turbo kits in which case put the MAF where you want to and tune, but there are a couple good reasons why the placement pre-turbo is preferred - and what modern OEMs use. If what you are describing instead is the MAF is pre-turbo, and you are venting before the MAF, then your BOV is useless and not actually doing anything.
    – Scott M
    Aug 24, 2023 at 20:27
  • @ScottM You might notice that this discussion is from 2017. I sold the car long ago. But, that was using an aftermarket turbo kit. The MAF was between the intercooler and throttle body, with the BOV before the MAF in the circuit.
    – 3Dave
    Aug 24, 2023 at 20:29
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Recirculating wastegates dump the exhaust that's bled by the gate into the exhaust to avoid toxic fumes and noise.

Recirculating BOV's or bypass valves are typically installed by the factory to reduce noise, since many people that buy a turbocharged 2.0L Cadillac don't want the "pssst" or "pop-pop-pop" noise that a BOV generates when vented to atmosphere.

Although, since the purpose of the BOV is to reduce pressure in the intake, I'm a little confused as to how venting back into the intake reduces pressure. :/

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  • I just realized who asked this question - hopefully that wasn't too pedantic.
    – 3Dave
    Jun 2, 2017 at 16:37
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The MAF measures the amount of air going into the engine, to calculate the amount of fuel to be added.

If the air would be released to the atmosphere, the MAF reports more air than actually goes into the motor, leading to a too rich mixture.

If instead, the air is feed into the intake behind the MAF, it reports exactly the amount if air going into the motor, and the mixture is fine.

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  • Good answer, but not true for all configurations. My BOV is between the turbo and the MAF - actually, before the intercooler - so venting to atmosphere has no effect on MAF accuracy.
    – 3Dave
    Jun 8, 2017 at 19:20
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Background:
The purpose of the blow off valve is to vent pressure in the turbo to throttle body hoses and intercooler when the throttle body is closed. If you do not have a blow off valve, when you close the throttle body, the turbo is still spinning and pushing air. With the engine only consuming a tiny bit of this air, the pressure builds up in the intake hoses and slows down the compressor wheel. This is also said to be bad for the bearings. I believe this is known as compressor surge. Venting this air allows the turbo to stay spooled and decreases wear on the turbo.

MAF Issues:
Some times this is an option, other times it is a requirement. It depends if you have a MAF and where your MAF is located. If your MAF comes before the BOV, then you will need to recirculate the air back into the system, after the MAF. There are 2 reason for this. Under some circumstances, the BOV can be open while driving. This could allow unmetered air to go in through the BOV and not go through the MAF. This would cause a lean condition. The second reason is when the BOV vents boost pressure, that air has already been metered through the MAF. Having it metered but not used would cause a rich condition. The whole point I am making here, is if you vent to the atmosphere, the MAF reading would no longer be accurate.

Other reasons to recirculate:
Aside from issues with the MAF, there are other reasons to recirculate. It is said that pushing the air back into the intake (before the turbo) helps keep the turbo spooled up. Most of the time that the BOV is being activated, it is because you are shifting to the next gear. You want the turbo to be ready when you get into the next gear. Another reason would be that since the BOV can pull in air, the air from the intake would already be filtered by the air filter. Recirculating is also much quieter than venting to the atmosphere. This is preferred in cars that aren't designed to be sporty and show off the fact that you have a turbo.

Venting to the atmosphere:
Venting to the atmosphere is not commonly done in production vehicles, but is very popular for aftermarket and modified turbo systems. Venting to the atmosphere is much simpler as you can leave the BOV open, or put a filter on it directly. To recirculate, you would need a pre-turbo hose with a port for the recirculation, a BOV that is made to output to a hose, and a hose to go between them. This would need to be designed vehicle / engine specific and would be difficult on custom, or modified systems using generic parts. You also cannot do this if you put a cone filter directly on the turbo, or if your BOV is not designed to be connected to a hose. Some BOVs have more of a horn design. This brings me to my next point, noise. A lot of people with modified turbo systems love the sound of the BOV. Venting to the atmosphere is louder.

Side note
I am trying to remember the circumstance in which the BOV can be open and allow air to be sucked in. The BOV has 2 ports - a pre TB port and a post TB port. The pre TB port is usually on the base, or it will be labeled as boost only. When the pre TB port measures X psi higher than the post TB port, the BOV opens. Many aftermarket BOVs are adjustable. Weaker settings allow it to open faster, but it may not be able to hold as much boost. Stronger setting are the opposite, more boost, slower opening. If the difference required to open is less than the idle vacuum, it will be open at idle.

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  • Under some circumstances the BOV can be open while driving The valve should only be open if its under pressure, which means it's not sucking air in. If the valve is open under vacuum, the valve or spring isn't doing its job properly. A BOV should never be open under vacuum.
    – 3Dave
    Jun 8, 2017 at 19:22
  • I've heard of it happening. If the spring isn't strong enough, the valve will open at idle. stronger spring = slower release. The 1G DSM BOV would open at idle and could hold 18 psi unmodified, or 30psi+ if you disconnected the boost reference port.
    – rpmerf
    Jun 8, 2017 at 20:26
  • If the valve is open under vacuum, it's a source of unfiltered air. That shouldn't happen.
    – 3Dave
    Jun 8, 2017 at 22:04

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