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I don't know what the primaries do on various Cobra headers when they get to the collectors, but on OE Viper headers (a few years ago), the primaries scrunched down to half their diameter when they clustered into the collector. BAD DESIGN! Talk about hitting a WALL at the collector. Viper operations had various aftermarket exhaust & headers on hand at the engineering garage, & I looked at a set of headers with undistorted primaries where they dumped into the collector - NICE design!
As far as Cobra mufflers, avoid creating a flowpath obstruction - you want a smooth transition all the way downstream. Hitting a 1/4" edge will create a jolt to the flow. As far as needing a 30" overall muffler, if you completely eliminate the bushing (neck) at the inlet end, you will be hitting a "wall" when entering into the flowpath of the muffler. You must leave a funnel of sorts to transition smoothly into the muffler. If your collectors are 3", all you have to do is cut the inlet bushing (neck) at the proper spot for a snug fit to the inside of the collector, then TIG weld the muffler to the collector.
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