I just put a new cat on after running almost a year without it yesterday. After driving around for about an hour I began to notice what sounds like either detonation or worse,a rod knock on acceleration from a stop. I'm leaning towards the detonation side and hope it will fade off as the ECM adjusts to the new cat but with over 138,000 on the car I'm wondering what if. Any thoughts?

------------------
Mongoose-NCTGA-1990 Pontiac Formula/305 TBI-700R4 with Edelbrock Performer TBI intake,TES headers,and STB,K&N,MSD-6AL,Holley laser shot coil,Accel Extreme 9000 wires,Jet Stage 2 chip,TCI shift kit with corvette servo,Richmond 3.73 gear with Auburn 3 series posi,no cat,SLP 3" cat back,Eibach sportline springs,Energy Suspension poly throughout,Lakewood LCA and adjustable panhard bar package,SSM subframe connectors,IROC wonder bar,Tokico 5 way adjustable struts and shocks,Autometer electric fuel pressure gauge,Alpine,JL Audio,MB Quart,92 GTA taillights,92 GTA aero wing and hatch,Steve Spohn LCA relocater kit,MacEwen White face gauges,84 T/A cold air induction air cleaner,Grant Formula GT wheel.

 

A cat should have no effects on a rod. Possibly it is just the exhaust system "bangin" around when under load or you have an exhaust leak.

Get underneath the car and shake the exhaust system a little to see if you can notice any knocking sounds. Next look for "black carbon" around the connections of the cat to the exhaust system. Hopefully it is just one of those two possibilities.