The best way to improve exhaust scavenging is with a properly designed exhaust header. Why? This allows the use of the exhaust pulses exiting the cylinder head to create a "partial vacuum" at the collector, which then pulls the next puls out (and repeating the process ad infinitum when the engine is operating at the speed for which the header is designed.) The only real problem with this is that you have to know the optimal operating speed of the engine to design the header - but most of the work is done for you. Borla, Banks, and probably PaceSetter and Hooker have all done the engineering for you, and I haven't taken any measurements yet but I think the OEMR header you already have done a good job.
If you want a quick and easy increase in exhaust flow, start by opening up the restrictions you already have - the catalytic converter and the muffler. I replaced my exhaust parts (when the time came) with "universal" parts that were listed for use with Big Block Chevvy engines, and that was worth a notable increase as measured with the Butt Dyno. It's been a while, so I don't remember the BD readings, but between the cost of the parts, the percieved increase in power and response, and the nice exhaust note I got, I'm not complaining. Cost? About $70 for the converter and $20 for the muffler, and about anothe $20 in pipe and fittings (plus $5 for the muffler shop to weld the old converter flange to a piece of pipe for me, since I'm too far out of practise to weld well...) I'd do it again.
Bear in mind that anytime you allow the exhaust gas to cool off in the pipe, it slows down and that results in lower scavenge rates. Want to increase scavenging? While you are changing parts belowdecks, use some Thermo-Tec wrap between the cylinder head and the catalytic converter to keep temperatures up in the pipe!
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