This unique system is commonly referred to as “volume control” or “adjustable volume”, which allows you to change how loud the venting sound is, from completely silent (recirculated venting), to extremely loud (full atmosphere venting). Whilst blow off valve volume adjustment is a handy feature, there’s a lot more to the Venting Bias Adjustment system than just more or less “whoosh”.
Adjustable Blow Off Valve Sound
What is venting bias?
Venting bias is the term used to describe the ratio at which the air is split between re-circulating, and atmospheric venting. The OEM valve your car came with is 100% re-circulating, a full “atmo” blow off valve is 100% atmosphere venting, and most dual port valves are more or less 50/50 split between the two.
How can I change my venting bias?
This is only possible on GFB valves with the Venting Bias Adjustment system. It uses an internal rotating sleeve, that allows the ports to be opened and closed depending on its position. Rotating the sleeve all the way in one direction will close the atmospheric port, and fully open the re-circulation port. The other direction will do the opposite. If the sleeve is in the middle, both ports will be opened.
How does this benefit me?
Well, the most commonly recognized benefit is that it can allow you to set how loud you want your blow off valve sound. The valve can be set any where between a full-noise, Fast and Furious style “whoosh”, and OEM stealthy silence. This can all be done in seconds either by rotating the sleeve manually, or electronically depending on your valve.
Another benefit that is less well-known, but potentially even more important is its ability to adjust the venting bias to suit cars with sensitive ECUs.
On some vehicles equipped with a MAF sensor, venting too much air to atmosphere can cause tuning issues as the ECU has already measured the air being vented, which adds extra fuel whilst the valve is venting. In some cases this can cause the car to backfire or stall.
Let’s use the example of a 2013 WRX. These models have a tendency to backfire when fitted with atmosphere-venting blow off valve, even more so with an exhaust. That rules out using a full atmosphere venting valve, and sometimes even a 50/50 dual port can be too much. That’s where GFB’s venting bias adjustment system can help. By allowing fine-tuning of the venting bias to suit the vehicle and its other mods, you can still vent a portion of air to atmosphere for the sound, but without the drivability issues.