What is your definition of HUGE. The question itself is vague.
Is 4-5% huge on a 300hp engine,or is it 10%, likely unattainable. That's the issue. Stock manifolds will not compete with even the junky 1 5/8 header when the carb and timing are tuned properly... which the magazine DID NOT do in that 360 manifold article. More camshaft than that ittty bitty stick and the gains increase across the board. It was swap item for item and run it. I bet the headers would show even more legs than the test results displayed. Headers make more power pretty much everywhere which is another benefit.
Basically if you had a car that was well sorted out with 300-310flywheel hp. it should run the the 12.85-12.90 range at 105-106
If headers were to provide about a 4-5% increase in power 12-15 hp, the car would run 12.60-12.65 at 108ish mph. If that isn't what you consider a huge gain, then pass on headers.
