We felt it was about time to deal with the issue of BOVs (pronounced as if you were saying BOVril)

Similar to Bovril, an atmospheric BOV is just as useless on your modern vehicle.

What is a BOV, it's an acronym for a Blow-Off Valve.

Other names include:

• Atmospheric dump valve
• Dump valve
• Dumb valve (? lol, more common than you think)
• Pop-off valve ( again, eh?)
• Also Popof valve and Popov valve (hmm)
• Diverter valve
• DV

What nobody calls it:

• Choo choo freedom gate
• Timothy

With the slang out of the way, it's worthwhile explaining what it's intended purpose is before getting into why using it on your car is not a great idea (in most cases).

Let's break out MSPaint for this scientific explanation, under normal operation, as the turbo spools, air is drawn in, through the Mass AirFlow meter (MAF), the mass of air (in grams/second or equivalent metric) is metered, that air is compressed by the turbo, makes its way through the boost piping, through any charge cooling and into the engine.  The Diverter valve is closed (held closed by boost pressure, and a weak spring). Boost rises, the ECU sees the correct readings from the MAF and all is well.

When the throttle closes, the air moving through the piping, pushed by the turbo, builds up against the throttle blade, causing a sharp pressure spike in the boost pipes. On the engine side of the throttle, a vacuum forms as the throttle cuts off air to the motor. The vacuum (in the case of a vacuum actuated DV) causes the reference line to open the DV. The air that was building up behind the throttle, now reverses and flows backwards out of the valve, relieving the pressure spike. This operation prevents off-throttle compressor surge, protecting the turbo. In the OEM configuration, this air enters the intake AFTER the MAF sensor. This is important.

If your car has a MAF, it is a closed, mass-balance system. The fuelling, timing and all ECU functions relating to load, are dependent on the reading that the MAF sensor gives. In all operations of the turbo and diverter valve, the air stays BEHIND the MAF sensor.

When you install one of these rocketbois, or any atmospheric blow off valve:

There is no recirculation function, when the BOV opens, that air returns to the atmosphere, meaning that the mass-balance is now inaccurate along with the timing setpoint, and more importantly, the fuelling setpoint.

The fuelling balance is as follows (simplified of course):

Correct Air Mass (metered by MAF) / Desired AFR = Correct Fuel Mass = Good

When the Diverter Valve recirculates that air after the MAF, the mass balance is accurate and the correct amount of fuel is added. When an Atmo-boi / BOV is used, the amount of air metered is MORE than the amount that enters the engine, as that air is metered and then leaves the boost system. The equation then becomes:

Incorrect Air Mass (Actual = metered by MAF minus(-) Mass of air lost to atmosphere) / Desired AFR = Incorrect Fuel Mass = Richer AFR = Bad

Every time the DV dumps, metered air is lost, however the ECU has already received the MAF sensor reading and has added the amount of fuel that it is commanded to. As that air is no longer there to burn all of the fuel added, the mixture becomes richer than desired. The result is that in a closed loop lambda controller system, the fuel trims are continuously negative, as the ECU attempts to correct for the incorrect sensor reading. Other negative effects are:

• Poor fuel consumption
• DV Errors (Pressure drop diagnostics)
• Limp mode / CELs
• Annoying BOV sound
• Headaches
• Tuner aggression

The exact same principle applies to DV spacers, or splitter DVs that vent a certain portion to the intake and a certain portion to atmosphere. Because who doesn't want just a little bit of crappy fuel consumption. Hmmm lovely.

There are acceptable setups on which you are more than welcome to install the chromest, most annoying, whistly BOV you can find, and that is if:

• Your vehicle does not use a MAF, and instead uses Speed-density, to calculate load and subsequently fuelling/timing etc. An example of this is the Mk7 GTI, 8P/V RS3 and others, that use a MAP (manifold absolute pressure) sensor to calculate manifold air density. In this case, using a recirculating DV or atmospheric DV is irrelevant as it does not affect the pressure that the sensor reads.

• You use a Dicktator (lol) or any other aftermarket MAP based management system that does not use a closed mass-balance/MAF system

• You're JDM AF BOIIIIIIIII.

Don't arrive at your tuner of choice (us) with a BOV hanging off the boost pipe of your MAF-equipped car.

We're not mad, we're just disappointed..

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Cheers,