I used to race a Competition Jet boat all over the Southwest, 460"BB, Doug Thorley upswept headers and a 4-2-1 collector. They ran great but never saw a dyno or did any comparisons so who knew? The boat won two national championships. At that time like you said everyone knew they lost some power upstairs. (Well, what is upstairs anyway?) In my view what everyone knew was really that anytime you made a given header longer you would lose power upstairs. (If it was long enough to begin with.) Put those collectors on a Shorter primary header and voila. This is exactly what I did back in 1995 for the Cup restrictor engines and Voila.NewbVetteGuy wrote: ↑Tue Oct 31, 2017 2:11 pm
The traditional rules-of-thumb / advice that 4:1 headers have huge advantages over 4:2:1 at high RPM but give up a lot in mid to low RPM torque and do nothing to prevent reversion as overlap increases & that 4:2:1/Tri-Y headers are for low to mid RPM torque improvement, reduce reversion and are largely appropriate for low RPM truck and street motors and fall on their face at high RPMs seem to be increasingly untrue as you look at the newer improvements to both designs. (Old street designs like the Doug Thorley 4:2:1 Tri-Ys come to mind immediately, advertising that they're better than 4:1s "up to 4,500 RPM"; note: I'm in no way bashing these, simply highlighting an example of a Tri-Y design clearly in the "improve low end torque at the expense of high end HP" camp. )
Length protects your bottom end power and gives up upstairs.