[William]

On the standard muffler system the tailpipes were not welded to the mufflers.

The ZL1 cam probably magnifies the limitations of the chambered system - 4 mufflers, lots of bends and small pipe diameter. If your Flowmaster system was 2.5" it may not have been much better than stock or other changes in fuel volume/timing may have been required to bring up the power. One of our customers did a COPO clone that he raced and I recall at one point the car just shut off at about 1000 feet - the engine was using fuel faster than the pump/lines could supply it. Intake/exhaust is a delicate balance and a change in one generally requires a change in the other.