I have a set of edelbrock headers with air pump provisions on them. My first question is do smog legal headers flow as good as non legal ones? And 2nd, If I plug them with block off plates will it hurt performance at all?

 

in comparison to headers without fitting and ones with I really dont think there is that much of a difference I just blocked off the egr fitting on mine, if your really worried about it take a hack saw and chop off the fitting then form a mask over the hole and weld it up.

 

Not too worried about it, it just looks ugly. Just wondered if it hurt performance in any way with the air trying to escape through an extra channel.

 

As long as your blockoff dosn't leak, no. The void will become pressurized and act as a wall to guide the air past it. If you have a leak though, it could cause turbulance and sacrifice some flow on that particular primary. I doubt it would be enough to even notice though.

 

Does this also occur, when a cut out is closed? The same effect?

 

Yes, same principal.