k so i did a lil measuring today bc this has been buggin the hell outa me. the header on my ls vtec is the after market one from my old bolt on ls setup. its 4-2-1 and surprisingly gave the ls motor a kick in the pants over the 4-1 header i had b4 it. anyways i transfered it to my ls vtec setup n was wondering if it was robbin me of power over other b sereis vtec headers n this is what i found when measuring today.

the primaries on the non vtec header are 1 1/2" . the typical aftermarket b sereis vtec header has primarys that are 1 3/4" (thanks to a lil chat i had with john at lht about them). b sereis vtec race headers have primaries that are 1 7/8 " . so if anyone was wondering, YES there is a difference in the headers. also to be noted, some headers come with different size collectors (2.0" is usually the size to fit a stock cat). some of the better headers come with 2.5" collectors however. just thought id share with every one!!! i guess i can safely say that i definately am being robbed of some power up top, however i believe this makes a bit more tq due to the smaller primarys (correct me if im wrong). anyways...enjoy!