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May 30, 2018

Vented Blow Off Valves on a Mass Air Flow Meter Nissan Skyline

OEM Nissan Skyline recirculating valves also called blow off valves (BOV)

Vroom, pshhhh. Vrooooommmmm, pshhhhhhh.

The OEM R32, R33, and R34 GT-R all have a dual recirculating compressor bypass valve configuration. These are most commonly referred to as  blow off valves, or BOV.  These are recirculating valves, as they are post MAF(mass air flow meter).  The air that has passed through the MAF, needs to be put through the engine, or you will cause running issues.

These issues with a BOV to atmosphere include stalling after you let off throttle, and idle instability.  Basically the engine expects there to be a certain volume of air, and will match fuel for that amount of air. Since that air is released, the car will run rich.Here are some ways to fix the issue on a Nissan Skyline GT-R.

1) Put it back to stock. You won't get your pshhhhh sounds, but the car will run better.
2) Put the MAF after the BOV. A little difficult in a dual MAF setup like a Skyline GT-R
3) Go MAFless, or use a MAP (manifold absolute pressure) sensor. You can do this a number of ways, but most common is an aftermarket ECU.

The MAF does a really good job of measuring air entering the engine. A MAP sensor is a much less precise device as it takes pressure, along with temperature to try and figure out how much air is entering the engine. On a ITB(individual throttle body) engine like a RB26, you also have to think about running hybrid fuel control. Alpha N, and MAP to give the car near stock driveability.

RB26 MAF : Nissan Skyline GT-R Mass Airflow Meters : Troubleshooting and Repair

A compressor bypass valve (CBV), also known as a pressure relief valve or diverter valve, is a manifold vacuum-actuated valve designed to release pressure in the intake system of a turbocharged vehicle when the throttle is lifted or closed. This air pressure is re-circulated back into the non-pressurized end of the intake (before the turbo) but after the mass airflow sensor.
A blowoff valve, (sometimes "hooter valve" or BOV) performs the same task but releases the air into the atmosphere instead of recirculating it. This type of valve is typically an aftermarket modification. The blowoff action produces a range of distinctive hissing sounds, depending on the exit design. Some blowoff valves are sold with a trumpet-shaped exit that intentionally amplifies the sound. Some turbocharged vehicle owners may purchase a blowoff valve solely for the auditory effect even when the function is not required by normal engine operation. Motor sports governed by the FIA have made it illegal to vent unmuffled blowoff valves to the atmosphere.[citation needed]
Blowoff valves are used to prevent compressor surge, a phenomenon that readily occurs when lifting off the throttle of an unvented, turbocharged engine. The sound produced is called turbo flutter (the slang term "choo-choo" is sometimes used). When the throttle plate on a turbocharged engine closes, the high pressure air in the intake system is trapped by the throttle and a pressure wave is forced back into the compressor, the resulting collision of pressure waves creates an effect similar to cavitation producing the unique noise.

Recirculation piping hotside RB26
OEM RB26 Recirculating Valve or BOV inspection and testing

In the case where a mass airflow sensor (MAF) is used and is located upstream from the blowoff valve, the engine control unit (ECU) will inject excess fuel because the atmospherically vented air is not subtracted from the intake charge measurements. The engine then briefly operates with a fuel-rich mixture after each valve actuation.

The rich mixing can lead to hesitation or even stalling of the engine when the throttle is closed, a situation that worsens with higher boost pressures. Occasional events of this type may be only a nuisance, but frequent events can eventually foul the spark plugs and destroy the catalytic converter, as the inefficiently combusted fuel produces soot (excess carbon) and unburned fuel in the exhaust flow can produce soot in the converter and drive the converter beyond its normal operating temperature range.

An alternative method for utilizing both a MAF and a blowoff valve is to have the MAF located down stream between the intercooler and the throttle plate. This is known as Blow-through rather than the traditional Draw-through set up. Care must be taken as to the position of the MAF to prevent damage to the sensitive element. For example, on a SR20DET engine, the MAF must be at least 12" (30cm) from the throttle plate, and the blowoff valve must be 6" (15cm) from the MAF sensor. By using a blow-through method, the MAF won't be affected by the blowoff valve opening as the pressure is vented before the air reaches the MAF.

One approach used to mitigate the problem has been to reduce the boost pressure, which reduces the required venting volume and yields less charge over-calculation by the ECU. The air can also be recirculated back into the intake, a typical stock setup for cars with an upstream MAF sensor. The situation can also be corrected by switching the fuel metering system over to a manifold absolute pressure sensor, a conversion that usually requires a compatible aftermarket ECU or piggy-back fuel controller. The MAP sensor monitors the absolute pressure in the manifold at all times and will correctly detect the change that occurs when the valve vents, allowing the ECU to reduce fuel metering accordingly.
From Moates
Strengths of Mass Air Flow

  • Extremely accurate fueling and spark delivery across a diverse range of engine conditions (at least while in steady-states): the holy grail for engine management. A properly set-up MAF system can adapt to changes in weather and altitude with ease.
  • Minor changes to engine equipment (i.e. headers, minor camshaft changes, intakes that do not significantly alter the placement of the MAF) do not require recalibration of the ECM.

Weaknesses of Mass Air Flow

MAF systems are known for having these issues:

  • MAF systems are extremely intolerant of vacuum leaks.  Any leaks between MAF sensor and engine generally cause all manner of odd problems, running lean in most cases due to un-metered air making it into the engine.
  • MAF sensors can be extremely sensitive to how they are “clocked” – merely rotating the sensor at a given spot in the intake tract can be sufficient to significantly change its output.
  • MAF sensors require laminar flow to read 100% accurately.  True laminar fluids do not exist so this introduces some degree of inaccuracy to MAF sensor readings.  Placing MAF sensors near bends, size transitions or obstructions where flow is less laminar greatly magnifies this issue.
  • A MAF sensor can be a flow restriction in cases where the MAF housing is the smallest portion of the intake system.
  • Hot-wire MAF elements are very fragile.  Debris can destroy delicate wires easily.  Dirt and oil deposits can build up on the sensor element, adversely affecting readings.
  • MAF systems have a relatively poor response to transient conditions, such as sudden throttle changes.  This is explained by the time it takes air to move from the MAF sensor where it is measured to the cylinder where it can be involved in combustion.
  • MAF sensors are not “one-way” sensors – reversion from a camshaft with large amounts of overlap can cause air to be metered on its way in to the engine and then again on its way out resulting in an artificially high MAF reading.  This can almost always be fixed by placing the MAF sensor sufficiently far from the throttle body, however doing so comes at the expense of making transient response even worse.

It may seem like there are a lot of weaknesses of MAF systems, but it is truly hard to emphasize just how amazing and important the strengths are.  It is no secret that the majority of OEMs today are implementing MAF systems as the primary control strategy.  There is a good reason for this, namely that engines can be controlled much more precisely (with the goal of meeting stricter and stricter emissions standards) with a MAF system than any other type of control strategy.

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