When redoing the exhaust on
"BIGBRRD" (our '78 TransAm) I first tried regular long tube headers. But after getting two sets that were screwed up from a fitment standpoint I went with shorty headers. At a local muffler shop recommended to me I had down pipes custom made to contact to the rest of the exhaust system.
I'm sure there will be someone who will supply a more technical reply, but my decision was based more on ease of installation and access below to oil filter and starter rather than performance.
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Jim Olson
"The Toys"...!!! Save the Wave!
Where I've been in a Corvette...!!!