Originally posted by tuxsavvy
There are many ways to make the blow off valve sound more evident but none of them would make the blow off sound more louder. There are many considerations which can affect the outcome and it all depends on various factors.
Some donor turbos yield more amount of times the blow off valve going off (notably with the boost gauge filling up all the way), but it may not increase the volume of the blow off valve.
The sound appears to be coming more from the exhaust pipes than from where the engine actually sits. This is apparent when say you are in front engined car with exhaust pipes running all the way to the rear of the car. You use roof/bonnet view in that instance and look backwards as you change gears, you can hear blow off valve going off than from the front. I think the "blow off" sound may sound a little louder if say for example the car is midship (mid engine mounted for instance) or maybe even rear engine mounted. Technically you could also relocate the engine to these points in a bid to hear a slightly louder "blow off" sound.
Of course this is just a simple one-off case, your mileage may vary. Again I must stress that none of these methods are guaranteed methods to gain louder "blow off" sound. At this stage there is nothing much you can do about it.