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I use an Audi 710n diverter valve, manufactured by Bosch. Was able to get it for 25 euro second hand. Prefer it over a much more expensive Forge BOV that I tried before it.
Maybe try a motorcycle prefilter sock or just a uni foam style filter over the bov, or build a shield with sound absorbing material on it to direct noise down? Or switch to a bypass valve. Which technically will improve spool/throttle response. Or a hybrid valve. All bov's make noticable noise so I don't think it's worth switch it out for another.
My MSM came with an HKS SSQV (?) vented to atmposphere with a purple insert that was supposed to accentuate lower frequencies. It was LOUD and the GT2560R spins up quickly so it was easy to startle pedestrians and annoy anybody in the right lane with their window down. One of my first modifications was to cut the center out of the plastic insert and smooth out the remaining lip using some JB Weld. The sound is of course louder than recirculated but sounds just about as smooth. It no longer screams boy racer, look at me. If there the airflow exit isn't smooth you might take a Dremel sanding drum to the inside and if that's not enough, think about recirculating it as nothing will be as quiet as a recirculated BOV.
Last edited by Jesse99James; Aug 14, 2021 at 09:21 PM.
Reason: typos + clarification