just fitted
 

a hks sqv dump valve on my uk99 took it for a spin and there was about 4 times when i heard popping coming from the exhaust ive never heard this before on the car ive got no centre cat but i still got the down pipe cat, can this popping damage the engine in anyway

cheers

It wont damage the engine, its caused by the maf measuring the air in the inlet manifold. When you back off with a vta dump valve the air that the ecu knows about from the maf measurments is lost, but the ecu is still fuelling for the amount of air it used to have. So you get a very brief over fuelling happening, its this extra fuel you hear popping in the hot exhaust.

cheers bruce. would it damage the cat on the down pipe

Not sure on that one as my WRX has been decatted for some time. I fitted the VTA dump valve after the decat.