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We had custom headers built and the dyno difference was almost 50HP between the ProParts and custom..
The install is in a 71 Duster, built 440, Tunnel ram, runs high nines..
As mentioned previously check out the major tube length differences of the ProParts....

Just my $0.015..






Same collector design and dimensions? Do the flanges mate up to the ports the same?
What I'm asking is, are the only difference between the two sets primary length?



Not complicated..
Flanges, ports, diameter and collector are virtually the same.. Basic difference was that all tubes were of equal length(50").
As mentioned previously check out the mismatched length of the Pro-Parts tubes, now go check out the Hookers or others..

On a full race built 440 the mismatched header tube lengths will cost alot of hp..

Just my $0.015...



Here's my $.020...it all depends on the motor and header I guess. A few years ago we did a header test on a PROVEN dyno. A mild 440, 12:1-ish, small (.650 x 270) solid roller, iron heads, M1/Dominator...very basic bracket moder. The best header was a custom (standard under chassis design for B/E bodies) step header...to the tune of 40-something peak HP and 12 ft/lbs!! BUT the OLD 2" Pro Parts headers were 17 peak HP and 8 -or so ft/lbs better than 2" Hooker fenderwells. the only diff in pipes was the PP heads had a slightly longer collector than the hookers. This was without touching the moder...who knows maybe there was a tune up each header liked differently.