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The "goofy" longitudinal g chart is a direct representation of the torque curve (upside down) if done proper. I've confirmed this on the dyno many times, and you can clearly tell when the torque dip is gone. I have dozens of dynos and hundreds of datalogs that confirm this....simple math if executed properly. The said header will get dyno'd when we get time.
Now as to why the a proper header gets rid of the dip, it comes down to scavenging and runner lengths. Yes, you do move it to some extent, but when you have good collector, correct primary length/size, you end up making more HP overall so when you move the "dip" to another area, it simply becomes a global increase with the gains focused on the low end. The smoothness here is all important. Opposing runners are bad, and unequal is just fine as long as the runners from each bank are similar lengths.
The tune then becomes all important because the exhaust cam timing is directly calibrated to the scavenging of the stock header. So here we will see how much more we can make on the dyno as the g meter is only good for curves not fine tuning. I suspect it will be quite significant since the stock cam profile "gives up" in the dip, and with proper header that means we have room to shove some more air/fuel in there.
I doubt the cat itself is the issue. At least until you put it on a racetrack and it melts...
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