I have a 86 blazer wiht what looks like it maybe the original cat on it. I have always had trouble with it running in colder weather. It would start fine, but if you tried to accelerate in 2nd or higher gears it would bog down until it got warmed up. This problem never happened when the weather was warm, even if I had just started the vehicle. I recnetly swap the original 305 for a crate 330 hp 350. WIth the addition of the new motor the problem got worse. It doesn't want to go up on high idle on its own. I assume this is becauses of the lope at idle. IF you start it and rev it manually for a second and then let off it kicks up on high idle fine. Also when you rev it manually it acts like there is a blockage in the exhuast and when it has blown it out the engine sounds and runs fine. Is this cause by the cat being blocked or defective in some way. Also does anyone know who makes/sells high performace cats. All I have found so far is dynomax and those things are kinda high. I figured I would at least compair some prices before I bought one. Besides that no one local carries Dynomax around my place. Thanks for the help.