OK, I have the 2.5 MSA exhaust but bought have a vintage ANSA rear muffler. I'd like to use that one but the inlet is 2 inch. Motor has headers, bigger air flow meter but otherwise stock. Going down from 2.5 to 2 inch- is that big deal?

Absolutely not.

Do not overestimate how much air you actually need for a mostly stock Z car. I am currently running an MSA header with a 2.5" outlet, that stays 2.5" all the way until the rear suspension, then necks down to 2.0" so it can go to my ANSA. I can promise you, I would not pick up any power at all from replacing my 3 feet of 2.0" pipe with 2.5" pipe.

By the time the gas reaches the rear of the car, the gas has cooled significantly, and thus has a lower volume given the same mass than when it comes out of the cylinder. So going to a smaller tubing toward the rear of the car would probably actually help keep the velocity of the gas up without effecting back pressure.

How loud do you think the ANSA muffler is going to be compared to the MSA Magnaflow? I dont have the center muffler for the ANSA, just the rear.

hmm, not sure, but I had a turbo style muffler with my 2.5" all the way back and my MSA header, and it was louder than my ANSA with a resonator at cruise. But at full open the ANSA just wails like an old Ferrari

The ZedOne sounds GGOOOODDD!

THanks!