I have took off the back box after trying different boxes and cracked it, replace with a 50mm round piece of exhaust pipe that's it. My tuning box had to be turned down from max 8 to 6 to combat flat spots , but wow what a sound . Nothing on tick over or driving about, but red line it through the gears and its sounds and performs a treat . just a £6-50 pence bit of tube and a bend connection £7 that's it , of course a k-n filter ,I swear down 0-60 - 8 seconds and 115 mph straight up to it pulling its balls off .