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There are several ways to determine the "best" spot to put it but I believe on the C4 you are fairly limited by the clearance around the trans. You should probably put it as close to the headers as you can, the effectiveness gets decreased as you get farther from the headers.
From: Good health is merely the slowest possible rate at which one can die
St. Jude Donor '04-'05-'06-'07
After a drive to get everything up to operating temps, put the car up on stands. Then while running the engine, measure the temperature of the system every few inches with a non-contact thermometer. The hotspot will be readily apparent. One small area will be about 100 degrees hotter than anywhere else. This is where to put an X pipe or crossover tube. You can also put paint on the system before heating it up. Where it burns off first will show you the optimum area too. I did this on my PE system and the difference in torque and exhaust note (smoothness) was apparent.
I have been laying under my car now for two days trying to figure out where everything should go. I didn't want to put the cats up next to the trans so I put them just into the tunnel and the x-pipe will go behind them. By the time I get the cats and x-pipe on I will be right near the turn of the tailpipes. Do you guys think this will work? I have fabricated the entire true dual exhaust from 45 & 90 degree bends and I see no other way to do it.