I don't know the exact decibel level, but it can be pretty loud from 50 ft away - see my recent video of K-Man S running a slalom with his Capristo recently.

You can close the valves and it is relatively quiet - until you hit 3000 rpms, then the valves open anyway. I'm not sure, but you might be able to temporarily disable the valves if you are going to certain tracks with more stringent sound restrictions. You might want to PM K-man S and ask him.

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2006 CS Tip (Daily Driver & Part-time Track Toy)
2005 Ford Expedition (to haul the CS)
2006 Lexus RX400h (wifey's)
Gone, but not forgotten: 2002 WRX, 2004 MINI