[911nut]

I have a bypass pipe and a sport muffler also and backfires too but it's a mild popping rather than a big boom. Have you checked your mixture and is your O2 sensor functioning?

Bruce Anderson writing in the "911 Performance Handbook" stated that the only way he found to eliminate backfiring on throttle closure was to add a premuffler in place of the bypass pipe. Sounds like you have a CIS/mixture related problem though.