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ok. most dynos i've found are for (a) headers, and (b) cat back (ie, both mid pipes and muffler). again, very few perform on paper.
if we look at the variation in cats between usa and row cats, we could assume that the secondary cats on usa cars are an afterthought (required for emissions) and were not considered in the initial tuning of the exhaust. so, we can return to the intended tune of the car by going to catless mid pipes (as well as reduce some lbs and improve sound - cool).
if that assumption is true, then the meaningful tuning comes from the headers and the relevant back pressure comes from the exhaust. i have a set of ebay 'knock-off' headers and would note that, while they are presumeably a copy of fabspeed or somesuch, they are very rough at the manifold flange and could really benefit from an old-school polish. as you state, varying results. a longer header will reduce the negative effect of equal length primaries at low rpm, but also reduce the benefits of tuning for low rpm. it gets down to fuzzy lines on inaccurate dynos. seat of the pants is your best measuring stick.
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