Whoever told you that lied. Or they don’t know and they were repeating the lie. It can’t be they don’t know because it’s been proven many times it’s wrong.
You want to know what makes good headers and exhaust not make more power, or even lose power? I’ve been harping on this for decades.
When the LSA is too wide for the application the engine will be numb to any and all exhaust tuning. The wider you get from optimum the more numb the engine is.
So the defacto catalog 110 LSA is usually 3-4 degrees too wide. Usually. Some of the junk these guys are grinding is on a 112 and that’s HORRIBLE for the low (relatively) compression ratios most guys run. In fact, it makes the engine essentially numb to most tuning.
When you open the LSA wider than it should be, you kill overlap flow, and that kills power everywhere (IF your tune up is correct) and…think it through…it usually opens the exhaust valve LATER as you open the LSA.
If (its a HUGE IF) you have an incredibly efficient exhaust port you can delay EVO and get more work on the power loop and as long as you can get to atmosphere or close to it at BDC (and again, IF the port is efficient) you won’t lose anything on the pumping loop side.
If you’re running a crap exhaust port (most SBM exhaust ports are crap on a cracker) and you delay EVO you not only screw up the pumping side you kill blowdown.
So saying a 3 inch exhaust kills power over a 2.5 inch exhaust is at BEST based on poor engine building philosophy and worst case scenario it’s just nonsense.
EVO is more critical than most people think. So IVC is most important and EVO is second in what matters. A close third is IVO and EVC is fourth, but three and four control overlap. It’s all critical.
So consider this. When someone advances their cam from straight up (where the overlap triangle is centered where it should be) and they gain power and driveability they think it’s from opening the intake sooner. I suggest it’s opening the exhaust sooner which gives more blowdown time and reduces pumping losses.
It’s not just a matter of bolting **** on and then saying it doesn’t work. It’s a system and every system affects the other.