I have a 1995 Z28 with a Comp#501 cam,52mm tb,K&N intake,pocket ported rebuilt heads,Flowmaster cat-back system,Blaster coil,de-screened maf,and under drive pullies.This car has barely passed smog for the last six years.It had a new cat when I bought it, didn't have a cam or rebuilt heads,but it was hard to pass smog checks.It did have a minor internal coolant leak that blossomed into failure years later.What is news to me is that an internal coolant will make the cat converter lose its ability to function (contamination).With a new CA legal converter, my HC took a dramatic drop,allowing me to pass in the middle range of HC,nox, and co2. It seems that most muffler shops know that head gasket leaks ruin converters. Thru this and other Chevy forums,I have searched and not found this kind of knowledge.