I am writing this thread in because “Whats a good Blow Off Valve” Is becoming a more and more frequent question. There are threads about Blow off valves and why to avoid them already but they don’t contain the description that I think many new drivers need.
In this thread I will attempt to explain what a Blow Off Valve is vs a Bypass Valve, why one is better for your car than the other and how you can get the Fast and Furious PSSSHHHTTT sound when you shift without hurting your engine.
I hope this thread can be helpful to Subaru owners of all sorts. Please feel free to link to it and refer to it in the future. As stated in my previous “What To Do First” thread please feel free to comment and make suggestions to make this thread better.
** This article is for all vehicle with tunes that rely on the MAF, All stock of mostly stock Subarus use this setup. If you have a speed density tune for example you can disregard this whole document, you know who you are **
Definitions to start:
Blow Off Valve - aka BOV, Atmospheric Blow Off Valve, Dump Valve - A Pressure regulating device present in turbocharged cars used to vent intake manifold pressure to atmosphere.
Bypass Valve - aka BPV, Compressor Bypass Valve, Recirculating Valve - A pressure regulating device present in turbocharged cars used to route intake manifold pressure to the lower pressure side of the intake path ( ie. turbo inlet side )
The key difference between a Blow off Valve and a Bypass Valve is that the Blow off valve releases air from the system whereas a Bypass valve moves it from a high pressure zone to a low pressure zone within the intake path.
Why you need a Valve at all:
When the engine is running exhaust gasses are running through the turbine side of the turbo. Under load the wastegate is closed and the exhaust is hitting the turbine wheel making it spin at a very high rate of speed, that rotation is transferred through a short shaft to the compressor side of the turbo. The compressor side of the turbo takes air in through the intake path (low pressure side ) and compresses it as it sends it to your intercooler and eventually to your intake manifold ( high pressure side ).
When you are driving with your foot on the throttle and the engine is at 4,500 RPM for example everything is great until you take your foot off the throttle, at that moment the throttle body valve closes and the compressor side will continue pushing air into the intake. The wastegate will open routing some exhaust gasses away from the turbine housing but the compressor wheel will continue spinning under the existing rotational kinetic energy, which will either force the intake valve open again or cause things like compressor surge, where the turbo creates more pressure than it can sustain and can cause bearing failure or in extreme cases total compressor wheel failure.
To avoid compressor surge we use a valve to dump the extra pressure in the intake manifold and re-route the exhaust gases around the turbine through the wastegate allowing the compressor wheel to continue spinning under low pressure until the throttle body valve is opened again.
Why Bypass Valves are better for your engine:
In many modern factory turbocharged cars ( Subarus Included ) the Engine Control Unit ( ECU from here on ) makes active decisions about how much fuel to send through the injectors based on a number of factors like throttle position, engine RPM and , most importantly AMOUNT OF AIR. The ECU gathers the amount of air in the system by reading values from the Mass Airflow Sensor ( MAF from here on ). As soon as air enters the intake at the filter is accounted for and the ECU knows about it. During normal driving air is sucked into the compressor side of the turbo and spit out at a much higher pressure, the now high pressure air moves into your intercooler and finally down into the throttle body and into the intake manifold.
When you take your foot off the throttle everything changes. The high pressure air that’s in the intercooler slams into a wall because your throttle body valve is closed. Because your throttle body valve is closed the turbo is pushing against an immovable object. To relieve that pressure we add our valve to the high pressure side of the compressor, usually after the intercooler so It is as close as possible to the throttle body. The Bypass valve opens and the pressure is released. If you have a stock ByPass Valve or an aftermarket Bypass valve that high pressure air is dumped back into the low pressure side of the intake, after the MAF but before the turbo compressor intake where it is still accounted for by the ECU. If you have an aftermarket Atmospheric Blow Off Valve then the High pressure air is dumped to the outside world where leaving the ECU expecting air that's no longer in the system. Because the ECU is still expecting that extra air it continues sending fuel through the injectors leaving the system with not enough air and lots of extra fuel. That condition is known as a rich condition, It’s bad for your engine because it does not burn correctly, bad for your catalytic converters because they have extra fuel to burn and bad for your shift recovery because your engine won't be running at peak power when you get back on the throttle.
But that PSSHHHTTT sounds so cool:
I agree, there is a high school boy somewhere in me that loves the sound of an atmospheric blow off valve. I have spent a great deal of time working with film sound designers and I can assure you that sound is often copy/pasted onto any scene where there are import cars racing for the same reason there are gun cocking noises every time a police officer opens a door. It sounds cool to the general public.
I still think it sounds cool:
If you want more sound from your stock or aftermarket ByPass Valve I would start by smoothing out your intake path. The stock intake path has sounds reducing baffling that helps keep the car docile feeling for people interested in a reliable sports sedan. By replacing your stock rubber baffled intake with a smooth aluminium or rigid polymer intake you will inherently make the intake louder and when the stock ByPass Valve opens you will hear it dumping into the now smooth the intake . Short ram intakes are particularly loud because they have fewer bends to capture ByPass Valve release sound.
There is a middle ground:
If you are dead set on having a louder Bypass Valve there are companies that make “Hybrid” or “Adjustable” Bypass Valves. These valves vent some of the air to atmosphere and send some back into the intake path. When installing a valve that is hybrid you will still cause a rich condition but the severity of said condition will be controlled by how much you choose to vent them to atmosphere. More vent to atmosphere = more volume of air lost, louder sound and more rich condition.
For hybrid valves that dump a predetermined percentage to atmosphere there are ways to safely tune the ECU to handle the venting pressurized air. These tunes would need to be discussed with your tuner and they would require you to tune for each position of the valve. The safest way to tune for a hybrid valve would be to take it to your tuner and have them run a road tune where they can compensate for the conditions that the ECU is seeing. Making adjustments after the fact for any changes ( like turning that valve for more or less noise ) would be detrimental to the system. These tuning options exist for fully atmospheric blow off valves as well but for reasons discussed earlier we will assume you want to keep that atmosphere in the system to be reused.
TLDR - Blow Off Valves are bad for cars that rely on the Mass AirFlow Sensor to manage air fuel ratios. If you want more noise try an intake with an appropriate tune or talk to a reliable Subaru tuner for more options.
In this thread I will attempt to explain what a Blow Off Valve is vs a Bypass Valve, why one is better for your car than the other and how you can get the Fast and Furious PSSSHHHTTT sound when you shift without hurting your engine.
I hope this thread can be helpful to Subaru owners of all sorts. Please feel free to link to it and refer to it in the future. As stated in my previous “What To Do First” thread please feel free to comment and make suggestions to make this thread better.
** This article is for all vehicle with tunes that rely on the MAF, All stock of mostly stock Subarus use this setup. If you have a speed density tune for example you can disregard this whole document, you know who you are **
Definitions to start:
Blow Off Valve - aka BOV, Atmospheric Blow Off Valve, Dump Valve - A Pressure regulating device present in turbocharged cars used to vent intake manifold pressure to atmosphere.
Bypass Valve - aka BPV, Compressor Bypass Valve, Recirculating Valve - A pressure regulating device present in turbocharged cars used to route intake manifold pressure to the lower pressure side of the intake path ( ie. turbo inlet side )
The key difference between a Blow off Valve and a Bypass Valve is that the Blow off valve releases air from the system whereas a Bypass valve moves it from a high pressure zone to a low pressure zone within the intake path.
Why you need a Valve at all:
When the engine is running exhaust gasses are running through the turbine side of the turbo. Under load the wastegate is closed and the exhaust is hitting the turbine wheel making it spin at a very high rate of speed, that rotation is transferred through a short shaft to the compressor side of the turbo. The compressor side of the turbo takes air in through the intake path (low pressure side ) and compresses it as it sends it to your intercooler and eventually to your intake manifold ( high pressure side ).
When you are driving with your foot on the throttle and the engine is at 4,500 RPM for example everything is great until you take your foot off the throttle, at that moment the throttle body valve closes and the compressor side will continue pushing air into the intake. The wastegate will open routing some exhaust gasses away from the turbine housing but the compressor wheel will continue spinning under the existing rotational kinetic energy, which will either force the intake valve open again or cause things like compressor surge, where the turbo creates more pressure than it can sustain and can cause bearing failure or in extreme cases total compressor wheel failure.
To avoid compressor surge we use a valve to dump the extra pressure in the intake manifold and re-route the exhaust gases around the turbine through the wastegate allowing the compressor wheel to continue spinning under low pressure until the throttle body valve is opened again.
Why Bypass Valves are better for your engine:
In many modern factory turbocharged cars ( Subarus Included ) the Engine Control Unit ( ECU from here on ) makes active decisions about how much fuel to send through the injectors based on a number of factors like throttle position, engine RPM and , most importantly AMOUNT OF AIR. The ECU gathers the amount of air in the system by reading values from the Mass Airflow Sensor ( MAF from here on ). As soon as air enters the intake at the filter is accounted for and the ECU knows about it. During normal driving air is sucked into the compressor side of the turbo and spit out at a much higher pressure, the now high pressure air moves into your intercooler and finally down into the throttle body and into the intake manifold.
When you take your foot off the throttle everything changes. The high pressure air that’s in the intercooler slams into a wall because your throttle body valve is closed. Because your throttle body valve is closed the turbo is pushing against an immovable object. To relieve that pressure we add our valve to the high pressure side of the compressor, usually after the intercooler so It is as close as possible to the throttle body. The Bypass valve opens and the pressure is released. If you have a stock ByPass Valve or an aftermarket Bypass valve that high pressure air is dumped back into the low pressure side of the intake, after the MAF but before the turbo compressor intake where it is still accounted for by the ECU. If you have an aftermarket Atmospheric Blow Off Valve then the High pressure air is dumped to the outside world where leaving the ECU expecting air that's no longer in the system. Because the ECU is still expecting that extra air it continues sending fuel through the injectors leaving the system with not enough air and lots of extra fuel. That condition is known as a rich condition, It’s bad for your engine because it does not burn correctly, bad for your catalytic converters because they have extra fuel to burn and bad for your shift recovery because your engine won't be running at peak power when you get back on the throttle.
But that PSSHHHTTT sounds so cool:
I agree, there is a high school boy somewhere in me that loves the sound of an atmospheric blow off valve. I have spent a great deal of time working with film sound designers and I can assure you that sound is often copy/pasted onto any scene where there are import cars racing for the same reason there are gun cocking noises every time a police officer opens a door. It sounds cool to the general public.
I still think it sounds cool:
If you want more sound from your stock or aftermarket ByPass Valve I would start by smoothing out your intake path. The stock intake path has sounds reducing baffling that helps keep the car docile feeling for people interested in a reliable sports sedan. By replacing your stock rubber baffled intake with a smooth aluminium or rigid polymer intake you will inherently make the intake louder and when the stock ByPass Valve opens you will hear it dumping into the now smooth the intake . Short ram intakes are particularly loud because they have fewer bends to capture ByPass Valve release sound.
There is a middle ground:
If you are dead set on having a louder Bypass Valve there are companies that make “Hybrid” or “Adjustable” Bypass Valves. These valves vent some of the air to atmosphere and send some back into the intake path. When installing a valve that is hybrid you will still cause a rich condition but the severity of said condition will be controlled by how much you choose to vent them to atmosphere. More vent to atmosphere = more volume of air lost, louder sound and more rich condition.
For hybrid valves that dump a predetermined percentage to atmosphere there are ways to safely tune the ECU to handle the venting pressurized air. These tunes would need to be discussed with your tuner and they would require you to tune for each position of the valve. The safest way to tune for a hybrid valve would be to take it to your tuner and have them run a road tune where they can compensate for the conditions that the ECU is seeing. Making adjustments after the fact for any changes ( like turning that valve for more or less noise ) would be detrimental to the system. These tuning options exist for fully atmospheric blow off valves as well but for reasons discussed earlier we will assume you want to keep that atmosphere in the system to be reused.
TLDR - Blow Off Valves are bad for cars that rely on the Mass AirFlow Sensor to manage air fuel ratios. If you want more noise try an intake with an appropriate tune or talk to a reliable Subaru tuner for more options.