Exhaust sizing help needed for 454HO with EFI
I'm confused over how and why to size an exhaust system. My Beast is an 86 Suburban 1/2 ton 4WD. Power is from a GMPP 454HO (425HP@5250, 500FT-LB@3250, hyd roller cam with Lift: .510" I, 540" E. and Duration @ .050: 211º I, 230º E., has rectangular port heads and 2.19" intake valves and 1.88" exhaust valves with 118cc open combust. chambers and 8.75 compression). I've also added Hooker super comp headers and custom 2.5" dual exhaust with H-pipe. I also added a Holley Commander 950 multi point fuel injection kit w/30LB injectors.
When I initially set up the exhaust system, I bought some Dynomax headers for it, but as soon as I opened the box I realized they were smaller diameter than the exhaust ports on my rect. port heads so I sent them back. I learned the Hooker super comp headers were 1 7/8" diameter so I went with them since that closely matched the exhaust port size. At that point I had a local shop bend up the rest of the dual system using 2.5" pipes, Magnaflow mufflers and two cats. I also had a H-pipe put in ahead of the cats.
But I keep reading that smaller pipes yield more torque down low (and sometimes mileage?) at the expense of HP at upper end. OK, but does that mean you should use something smaller than the exhaust port size? I'd always heard you wanted to reduce back pressure, not introduce it. So what gives?
Also, the system I've got is NOT mandrel bent. Would replacing it with a freer flowing mandrel bent system hurt the lower RPM performance? Or would the difference be so small I probably wouldn't even be able to tell a difference?