No Cats + Smog Pump can = A PoP.
Especially if it's in the high revs off throttle. Add in a minor exhaust leak with it and you can get a BooMinG PoP. 1st time I had what I'm talking about I thought it was a ShotGun blast in my car! I had my buddy
Once over the exhaust and bypass the pump and could not get it to do it again.