Looks the Biz' Glen, When I did mine, I had a Brother in law work out the area of two 1 5/8" = Big !!.That was the outlet pipe. The thing with two pipes like Reg's, You can have two silencers, So you don't have too much restriction, But I think Reg's would be too loud for racing in the UK, We all have to do whats best for us. I just bought a silencer for my road comet, I wanted a chrome one, No strawing!!, But it had a baffle right across the hole, The stainless one, Nice big hole !, So stainless it is !!, Not too loud at tickover gets louder the faster you go, But not too bad. All The Best, Bill.