Originally Posted by RWD_cossie_wil
on some dumpvalves, you can make them stay half open and whistle if you get the throttle just right, which indicates to me that the spring is weak,
I think that has more to do with the fact that you have a lower pressure from throttle plate to engine than you do from turbo to throttle plate
I can get mine to do it for short periods (usually on a slight uphill) and have actually checked it on the RR, it was not leaking at full chat. Only 20-something psi though.