How To Choose Between A Bov And A Recirc Valve:

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We Will Be Going Over:

  • Bov and recirc valve function. 
  • Which is better for my vehicle
  • How to choose the right spring setting for my engine. 

 

Bov and Recirc Valve Function:

What is a Bov?

A bov or blow off valve is a device used on forced induction setups that relieves the excess pressure in the system between shifts or after letting off of the throttle. Blow off valves are hooked up to a vacuum source (normally the intake manifold/plenum) so that when you let off of the throttle the vacuum from the engine assists the valve in opening. It then vents to the atmosphere so that the pressure in your piping doesn't cause dmg to your turbo by fluttering. The choo choo choo sound a lot of us have heard in videos of high hp supras spooling the turbo up and letting off the throttle is actually caused by the lack of a blow off valve. With no bov to release excess pressure, the pressurized air in the intercooler pipes flutters between the shut throttle body (butterfly valve) and the turbo compressor wheel. While it sounds really cool, it actually causes damage to the bearings inside of the turbo. If you continue to run a turbo setup like this, that flutter will cause premature failure, and we definitely don't want that.

What is a recirc valve?

A recirc valve does a very similar job to a blow off valve, the key difference is that a recirc valve recirculates the excess pressure back into the intake, instead of out into the atmosphere. This, in turn, helps your turbo setup retain psi level better between shifts, and gets rid of turbo flutter. 

 

Cool But Which Is Better For My Vehicle:

 

Both bov and recirc valves can work on your turbo setup, that being said the general consensus is that a bov should be used on a car that has a map sensor (map sensors essentially calculate the air within the system), while recirc valves are normally reserved for vehicles that us a Maf sensor (Maf sensors measure the air that passes through a point in the intake). 

Ok, but can I use a bov on a Maf car?

The short answer is yes, but one thing to keep in mind is that because your car is measuring the air coming in, and cant measure air escaping the system post-Maf, your vehicle will run slightly rich between shifts. This is because once the maf measures the air, it tells your ecu (the brain of your car) that you have "blank" amount of air in the system. Your ecu then increases fuel to compensate for the increase in air. BUT when your bov releases a bunch of air post maf, the ECU has no way of knowing that and still sends the same amount of fuel required as before. hench running slightly rich after a shift. 

What about recirc valves on a Map setup?

The cool thing about map setups is that they measure the pressure within a system (Manifold Absolute Pressure). Which allows a bov or recirc valve to expel air or recirculate, with the ecu always knowing how much pressure is in the system.

 

How To Choose The Right Spring Setting For My Engine:  

 

You may not have even know that blow off valves had springs in the first place, but they need a spring as well as a vacuum source in order to open once you let off the throttle. Most companies that make bovs have specific spring charts showing what spring stiffness is best for your engine. They calculate the correct spring pressure by the amount of vacuum/hg your engine makes at idle. If you get a spring that's too stiff, your bov may not open to let the pressure out. And on the opposite side of the spectrum if you get a bov with too weak of a spring the bov will stay open at idle, letting a bunch of extra air in that wouldn't have come in otherwise. Here is the Tialsport bov spring chart for reference. 

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Written By: Ocean Nickel