GFB Blow Off Valve Trumpet w/Filter MountIf your BOV vents to atmosphere for sound and you drive in dirty environments, the 5703 trumpet is for you. It is designed to enhance the blow off sound like a regular trumpet, but features a 38mm (1. 5) section onto which you can mount a suitable filter such as our 5711 (available separately). This ensures your BOV is protected from dust, sand, mud and other debris, whilst giving a much louder and more unique sound than using a regular hose barb