Tuned header?

[Blue68Camaro]

I am having trouble finding a header that will work with my brodix track 1 heads. I found THESE (http://www.jegs.com/webapp/wcs/stores/servlet/KeywordSearchCmd?storeId=10001&catalogId=10002&langId=-1&N=0&Ntt=737-h8047&Ntk=all&Nty=1&D=737-h8047&Ntx=mode%2Bmatchall&Dx=mode%2Bmatchall&searchTerm=737-h8047) but is says they are not "tuned" headers. What does this mean?

I am looking for headers with "D" ports and that will work with angle plugs and these are all that i could find.

[woodsman]

I think what they mean when they say tuned headers they refer to the equal length of each tube all the way down to the collector.