Prob a good chance the stellings were leaking though fouling out a plug before the dyno pull!

like SB mentioned it s not only the smaller headers but also the collector is very important the KE have the inner pieces of the runners come together in a point and most of the others or all have the pipes stop equal and then even put a big elbow on it wich squeeze 2 pipes ,a friend of mine built a new collector on a pair older headers whit the point like the KE and it make about the same power as the KE so there is much to gain by other manufacturers if they put some better collectors on them .