what size are the header tubes?
a motor is a air pump, cubic feet of air in and same cubic feet of air out.
When you try to make max hp/tq you need to look at your carb, air cleaner, intake, heads, headers and sidepipes. All must work together.
Everything could be matched but the air cleaner and you will lose hp/tq.
Take a look at Classic Chamber
Classic Chambered Exhaust Inc. - Cobra
I bought a pair of 3" i.s. pipes that were 4" longer than the ones I have. More hp and quieter.
I have a 354 c.i. motor in my Cobra with 401 rwhp / 416 rwtq.
A 155 hp lost is a lot.
Keith Craft told me that he typical saw a 20% lost in a Cobra from engine dyno to chassis dyno for the same motor.
Part of the fun is looking for 10 more hp.
Dwight
when they dyno motor they use air horns, largest headers, no mufflers, no alternator, electrican water pumps and other tricks to make max hp for the motors they sell.
the chassis dyno is the real story.