I have a motor on my dyno.i built, making 675 to 680 hp with muffled dyno headers at 5900/6000, around 625 tq. Makes 698 hp at 6300 with mufflers off.
having seen a modest torque and hp gain with eddie marine manifolds vs my imco power flows on stockish 500 efi last year i decided to use eddie marine manifolds with 36" long dry to tip tails on this build. Going to bolt them on dyno unmuffled and make a couple pulls here soon. On 500 efi i dynoed before the eddie marine manifolds made more torque than headers to tune of 10 to 15 foot lbs. Those manifolds had 19" tails i think.
that motor was peaking at 5400 though and only making 520 hp give or take 10 or 15 snd 570 to 590 ft lbs tq (depending on exhaust). Yes, we've all seen the famous manifold test where keith eickerts shine but that was at 480 hp level. What are we going to see at the 680 hp level, post your guesses and ill post results in next few days, Smitty

Good test!! 647hp, 610 torque. The long dry tails will help. Knock off another 20-25 hp mixing water into the exhaust?

What kind of headers are these Smitty, street car headers ? or jacketed marine

The headers i normally use are dedicated sinhle wall dry headers, i don't test
with wet exhaust often as it is a wet mess BUT my dry, race car dyno headers MUFFLED are close in power at lower hp level motors to manifolds ( 10 to 15 ft lb tq gain w manifolds, within 10 hp)

2.25 primary equal length ? If so I'll take the headers for 5-7 hp gain @ 6k

The emis have slightly smaller ports than these afr 300 heads, open gasket up to match heads, now im going to open manifolds

up to match heads

Kudos to you. Something I preach all the time but no one does. From here, that match is not too bad. Worst offenders I've seen is some raised port exhaust SBC's with dports and the manufacturer leaves exhaust port bolts in OE location. Should be checked on all performance applications because who knows with core shift . fabrication/or etc issues.

Larger than heads...not a big deal....smaller and thus blocking the exhaust...big deal. Not that big of a deal with intake+heads..you figure.

question, are the collector/ie pipe lengths going to be the same with these exhausts being tested ?