On my lasses old N/A the bung quietened it down with minimal loss to performance, it was a high revving honda B16a and it only seemed to loose a little at over 7500rpm.
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on my tubby the bung makes it less bassy so it doesnt sound as loud but it still is if you know what I mean, bass travels much further and through walls etc(like into your neighbours bedrooms when you are starting at 6am in the morning lol), but mine really seems to affect my performance, gets less boost, longer to come onto boost and just generally dulls it down.
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Its ok for those mornings when youre leaving early and dont want to be a willy but when youre coming home later on you can bet your last quid that you'll want to put your foot down for whatever reason and will wish you didnt have the bung in