RE: Header comment on the 6.1L engine
ive never taken a good enough look at them to know for sure, but lately so many performance cars are coming with an exhaust manifold so near the shape and flow pattern of a "header", that gains would be minimal...however, when you take into effect things like inside and outside powder coating, increased size, etc...it would make a bigger difference.
also, if the exhaust manifolds are similar to a "shorty" header, where the exhaust system comes up into the engine bay to meet them, then the exhaust travels through the "y-pipe"...you can often see some gains in going to a long tube header, which extends down under the vehicle, then into a collector.
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