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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?
  • 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 '17 at 16:38
  • Unless you've got a full Group A competition car with anti-lag, chose a recirculating dump valve. – Steve Matthews May 21 '18 at 11:51
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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. :/

  • I just realized who asked this question - hopefully that wasn't too pedantic. – 3Dave Jun 2 '17 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.

  • 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 '17 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.

  • 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 '17 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 '17 at 20:26
  • If the valve is open under vacuum, it's a source of unfiltered air. That shouldn't happen. – 3Dave Jun 8 '17 at 22:04

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