I've seen headers with a choke point or venturi built into the collector. I'm not sure if it's a race application thing or if it helps throughout the RPM range but it might be another feature to explore.
Formed Collectors, hand built Custom Collectors, Street Exhaust Hookup Cones, Megaphones
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-Mike
2007 Ford Focus ZX5 - 91k - SGII, pending upper and lower grill bocks - auto trans 
1987 Monte Carlo SS - 5.3/4L80E swap - 13.67 @ 106
2007 Ford Focus Estate - 230k - 33mpg - Retired 4/2018
1995 Saturn SL2 - 256K miles - 44mpg - Retired 9/2014
Cost to Operate Spreadsheet for "The New Focus"
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