i Dont know what your mate is thinking, he may just be baiting you into being made a fool of.
Atmospheric Blowoff valves are ILLEGAL ON THE ROAD, and they sound pathetic.
For those that dont know, a blowoff valve is primalily used to dump waste pressure from an exhaust system, or induction system, it can be used like a waste gate. AND THEY DONT HAVE TO SOUND LIKE THAT, most blow off valves are factory items that plumb the blown off gases back into the induction.
No point having a blowoff valve unless you are producing mass amounts of boost into your induction, blowoff valves, they are designed to get rid of excess boost and are commonly spring adjusted to control release pressures.
Example. I hooked a blowoff valve onto the intake of my Toyota 2GG (washing machine type) supercharged Holden v6 to prevent the intake from blowing off the motor when i took my foot of the throttle and the throttle body shut. The boost has to go somewhere.
If you want to get a little device to make everyone think you are turbo charged or supercharged, every one will laugh at you. DONT DO IT. SAY TO YOUR "MATE" 'I NEED A TURBO/SUPERCHARGED ENGINE FIRST DONT I?' your mate is having some fun with you, do your research.