Before I built my headers I did a heap of research on this and spoke to some local header builders on the subject too, they all said length is irrelevant when you're using a collector (moreso if a choke collector) but what is important is to promote "Swirl" in the collector by tuning the firing order into the collector. Clockwise or counter clockwise didn't matter, provided that it swirled. Anyhow, I followed that logic and my 1.875" primaries on my 400 seem to work really well. Car runs a reasonable mph for a 400 and is still pretty good on the street.
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'71 Holden HQ Monaro - 3850lbs race weight, 400c/i - 11.4 @ 120
'66 Pontiac GTO - 389, 4 speed street cruiser
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