When i was at the NASCAR Tech in NC, one of the labs was to assemble a well used NASCAR Chevy 357 with various configurations of cams, intakes exhausts and restriction plates est and run it on the engine dyno.
One of the things that stuck to my brains is that a super free flowing exhaust can be detrimental to performance in the form of having far to much scavenging effect. Some cam profiles actually seem to prefer a slightly restrictive exhaust system, ones with to much valve overlap. This prevents the exhaust stream from sucking the new fuel/ air charge straight through the head and getting wasted. This has much bigger effect on fuel injection as the injector is timed with the valve opening to dump the fuel into the cylinder.
The reduced scavenging may account for the gains at the beginning but i'm sure it more that the engine was just warmed up.
Considering some of the crazy bends the high efficiency Eco cars have in their mass produced manifolds i'm not surprised a little hammer tweaking of a set of hooker headers will have no effect.
My supercharged legacy, inspired by a forum 1055 and I have been members of actually, ran better with the stock manifolds and 2” cat back exhaust. I reasoned that the stock cams overlap was longer than the super wanted and it would push the fuel right out.
My old 89 Mustang had BBK shorty equal length headers and i had to "persuade" to of the tube to clear the steering shaft even after new motor mounts and 1/4" shims. I loved that exhaust Great sound and big improvement in 3rd and 4th gear on the highway.
OK I have rambled and rattled on far to much and I will turn off my pretengineering now.
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-STu
There is no shame in defeat, so long as the spirit is unconquered
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