Real Colector lenght doubt

[BrazilianZ28Camaro]

Hello Gents

I have a doubt about the best performer colector lenght on a tipical SBC 400, because everybody knows the magic number is 18" but I also heard that this lenght must be measured after the header primary tube end /collector junction ,meaning you should subtract the header colector lenght for the extension.

This way the extension itself would be much shorter than 18".

I'd like to hear your opinions and experiences.

Dyno/drag strip experiences would be great.

Thanks

[1980RS]

I have run extensions form 6" to 3' and the only reason the long extension worked I think was the header had a collector that was way too big for the engine. When I switched over to a merge collector the car ran the same times as the long pipes and no extensions.