BOV or BPV? Can I use this Intake? MAF Sensor?

At COBB, we pride ourselves on the customer service we provide.  A major part of that service extends into technical support where we answer hundreds of calls and respond to even more emails each week.  These discussions involve a wide range of issues and topics but there are a few that we frequently encounter.  One of the more common conversations regards how a MAF sensor operates.  This usually comes up through enthusiasts wanting more information about running a Blow Off Valve (BOV) or changing the car’s Bypass valve (BPV) to vent to the atmosphere (instead of re-circulating back into the intake).  Other related questions generally involve intake compatibility with a certain map file.  With a straightforward explanation of how the Mass Air Flow (MAF) sensor operates, we’ll see why neither running a BOV or using an intake the MAF isn’t specifically calibrated for is optimal.

A MAF sensor’s job is to provide the ECU with a reading of how much air is going into the engine.  This works through some basic parts and principles.  Let’s first think back to high school physics and Ohm’s Law.  It states that I=V/R where I is current, V is voltage, and R is resistance.  The MAF heats an element in the intake stream to a specific temperature and works to hold it constant.  As air flows around this element, it cools.  The more airflow, the more cooling.  Based on the amount it cools, the resistance in the element drops.  As that happens, the current increases and is translated into a voltage change.  The car’s ECU uses Ohm’s Law to interpret this voltage change and calculates the incoming air.  It then determines how much fuel to inject based on this incoming airflow.


This is a sensitive setup that is specifically calibrated for the stock intake from the factory.  It’s so sensitive, even filters not shielded from the cooling fan can cause erratic readings.  Any differences in the filter element, changes in tubing bends, or different diameter piping can change how air passes over the MAF sensor.  This can cause an unintended voltage reading to the ECU resulting in an improper amount of fuel being injected.  If any of these changes are present, the MAF sensor must be properly re-calibrated to “see” those changes. If you run an intake for which the mapping is not properly designed, the car could potentially run dangerously lean(or rich) and stumble throughout the rev range.

Running a BOV or vented BPV can also result in a bad time on a MAF based car.  This is because the air escaping has already been measured by the MAF sensor.  The ECU determines a specific amount of fuel to inject based on the expectation of this air being present.  If this air is vented, the car will run rich.  This will happen every time air escapes through the BOV.  Running rich can result in fouled spark plugs, surging idle, backfires, premature cylinder wall wear, and other issues in the long run.

Many platforms supported by COBB use a MAF based tuning strategy in stock form.  These include all Subarus,  Mazadaspeeds, and GT-Rs.  Other platforms use a Speed Density based strategy which utilize a MAP(Manifold Absolute Pressure) sensor.  It is possible to change you MAF based vehicle to Speed Density which would allow proper use of a BOV or different intake.  However, it would require sensor changes and custom tuning.  More on that next time!

30 responses to “BOV or BPV? Can I use this Intake? MAF Sensor?”

  1. Hello Kyle,

    Will the front mount intercooler in the above image be available for the ’08-’14 STI. If so, would it possibly be making it’s way into the OTS stage 4 upgrade that is including the 20G turbo?

    • Hey Emil, we are working on intercooler upgrades for a variety of Subaru applications. It’s undetermined where we’ll fit them into our Staged Packages as some cars will need them earlier in the upgrade path than others do.


  2. Is there any way you guys would be able to make a tune for the AP that fixes the problem if running rich with a BOV?

    • Our Off-The-Shelf maps are meant to be used with a recirculated system. You can definitely have a custom tune created to compensate for some of the richness associated with running a vent-to-atmosphere BPV.


  3. You mention the OTS maps are meant to be used with a recirculating system to prevent fuel/air mixture problems. Does the new COBB ‘Subaru XLE BPV’ maintain a closed system? Will the engine run rich if I install it and use a OTS map – or should it be custom tuned like the BOVs. Thanks!!

    • Hey Austin, the new design is a fully recirculated design so there would not be any issues using one with our OTS maps. The previous XLE BPV did have a removable backing plate that enabled users to run a 50% vent to atmosphere.


  4. Is there a way to track the richness with the AP while using a BOV?I dont believe it is running rich but After installing the SF+airbox on 13 sti and ots map I am noticing some surge at WOT

    • Hey Shawn! You could definitely view fuel trims to see if they sway significantly during shifts. It’s normal to hear a bit of “surging” when getting into peak boost. Everything turbo-related is just much more audible with the intake installed.


  5. hi i have a 08 wrx with the stage 1 +sf and i have the new cobb xle bpv,i want to know if it is good for my car

    • Hey Mathieu, there is absolutely nothing wrong with using the XLE BPV on your car with it’s current setup. Install it and enjoy it!


    • We haven’t done any specific testing with a hybrid style BPV on the new DIT cars however it should be just fine as long as it is configured in either full recirculation or hybrid mode.


  6. Ok so after reading as much as I possibly could on this Cobb XLE BPV I am still unsure as to what it actually does to improve performance. So it does not add hp or tq, but does it reduce lag or improve spool time? Confused, I want it because it is shiny and says Cobb on it but if it helped turbo performance or cool sound in some way I will buy it.

    • Hey Andrew, sorry for any confusion in the article! The XLE BPV itself will not provide any additional power or increase turbocharger performance. The main advantage is for users who are looking to target boost levels beyond what the factory bypass valve can handle. The design of the XLE BPV allows it to hold shut under higher loads. It’s also shiny and makes cool noises, bonus!


  7. Hi Guys,

    I have a ADM 2015/16 WRX STI. I have fitted a S+ Airbox and Map. I was thinking of fitting a XLE BPV but would like to know 1. Does it fit my car. 2. What benefits would I get 3. Is there any issues I might get. 4.Would I have to do any tuning for the BPV. 5. Is there any extra noise with it. I am also looking at fitting your Turbo Back Exhaust so would this still be suitable.

    • Hi Ken! You should not have any fitment issues with our BPV on your vehicle so long as you are using the factory top mount intercooler. The main benefit of an upgraded bypass valve is increased pressure capacity over the factory unit. No tuning is required when installing our XLE BPV aside from dialing in pre-load on the adjustment screw to best fit your vehicle. You wouldn’t notice an increased amount of noise, but the tone the valve emits will be slightly different than the factory unit.

      Let us know if you have any other questions!


  8. Hi Guys,

    Ken Coomber on February 15, 2017 says:

    Hi Guys,

    I have a ADM 2015/16 WRX STI. I have fitted a S+Intake, Airbox and Map. I have just ordered your Turbo Back Exhaust system and can’t wait for it to arrive. My question is what Map should I use once I have fitted the exhaust system.

    Ken Coomber

    • Hi Ken!

      We do not currently offer a map for that setup. The fuel system is limited on the newer cars and can only support either an intake or a downpipe upgrade. A while back we put together a write up about how the modification path for the 2015+ WRX STI would differ slightly from that of previous model year vehicles. You can read up on that here : 2015 STI Development : Staged Power Package Evaluation.

      In short, you will either need to have the car custom tuned for the intended setup or purchase some fuel system upgrades if you wish to utilize an Off The Shelf Map. I hope that helps! Let us know if you have any other questions.


      • Thanks for the reply Marshall,
        I was a bit disappointed that you do not have a Map for the Turbo Back with s+ Intake & Airbox setup but fortunately I ordered your 750cc Injectors and Fuel Pump as well. I intended to fit the Exhaust system myself and then take it to a Pro tuner to get the Injectors and pump fitted and then custom tuned. So is there a Map that I could use to safely drive the car to the ProTuner after I have fitted the Exhaust.


        • Hey Ken!

          If you promise not to go WOT, I would recommend to retain the Stage1+SF map in that case. This will ensure you have proper MAF scaling for the intake however you may see a CEL due to the downpipe. As long as you are driving straight to the Protuner you should be just fine.


  9. I have a 16wrx and I have a turbo xs hybrid BOV paired with a Cobb big SF instake and a stage 1 big sf 91 oct OTS map. My car at idle should be around 750 rpms but shortly after installing the BOV it goes down to 400 and 300 and nearly stalls the car. Is this a washer or a tuning issue ?

    • Hey Nikolas, the symptoms you are describing are fairly typical when installing a valve that vents to the atmosphere. Are you able to check to confirm there are not any leaks at idle? Does swapping the factory bypass valve back onto the car solve the issues?


  10. Hi, I have a 05 WRX wagon and im considering the SF intake. Is it safe with the factory MAF sensor calibration? Or will i have to re calibrate it. Can you re calibrate with the AP?

    • Hi John,

      Luckily for you, the SF Intake for the 02-05 WRX can be installed on the factory tune. Most of our intakes do require very specific calibrations but not in this case.
      – Adam@COBB

  11. If I am understanding this correctly, it is safe to run a vent to atmosphere bov with a good pro-tune?
    Second question are hybrid bov safe to run without a pro-tune?

    • Hi Marcel,

      It’s not about how good the tune is. Even a 50/50 BOV will cause rich conditions when air is vented. You’ll need to change to a speed density tuning strategy in order for the escaping air to not cause any issues. Contact a Protuner near you for more details:

  12. I have a 2013 Subaru WRX Hatch. I had just replaced the stock BPV with a Turbosmart atmospheric blow off valve, just to try out for a bit (perhaps a stupid decision) I did not have any tuning done either.

    For the first day, the car ran good, with a few back fires when reaching boost. But, today, (2 days later)
    Ive noticed, very delayed throttle engagement: When Starting from complete stop, I will press gas as normal, but the throttle will not engage for like 2 to 3 seconds. Like I could seriously take the pedal to the floor at this time and nothing will happen for those first few seconds, and then all at once the throttle will activate.

    As well when I get into boost and shift around 5-6k RPM, the rpm’s shoot up about 1000 in between the shift (Sometimes throwing over redline)

    I just switched back to the stock BPV today and it is still proceeding to do this.

    Is there something I need to do to recalibrate this issue?

    • Hi Chandler,

      It’s tough to say what the issue might be but it sounds like a mechanical problem. If that is the case, it isn’t something tuning will overcome. I’d recommend taking the car to a mechanic to see if they can help identify the issue.


  13. Hi

    I have a 2011 wax and was planning on getting the stage 1 + sf intake and accessport. I was wondering if I could run the Cobb lf bpv without having it run rich or with any problems.

    • Hi there! Short answer – yes! Our LF BPV for your car is a direct replacement for the factory BPV and does not require any tuning changes.


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