Well.. It's been a long time since I was building engines and race cars... back in the carburateur days to be exact. Here is what I know about exhausts.
You want some backpressure. Lower backpressure usually results in moving the powerband up on the RPM scale. Not a bad idea for a track car but low end torque gets you off the line and around corners on the street.
What sounds good to your ears may not be good for horsepower. I can't think of any professional engine/car builder that builds to a sound. They all build for power. What makes power may not be compatable to your ears. You'll learn to like what you hear.
If you are oing to make major changes to your exhaust, make sure to step down the ID of the pipes once, maybe twice, from the collector all the way to the tailpipe. As exhaust gasses cool they constrict. To maintain exhaust gas velocity you'll want to match pipe ID to volume. This helps scavange the cylinder of burnt gasses when the exhaust valve opens and it also helps suck the intake charge in during valve overlap.
I'm not familiar with the VQ engines in these Z cars (yet), so I do not know if these rules still apply, but they are somethign to think about.
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