I just installed a header that has a 63mm collector, it's just a little wider than the stock one, and lighter... But I still have 60mm exhausts, I tried with a friend of mine with an S2000.. we were identical...before and after installation of the header, we tried couple of times before and couple of times after, we also rolled around 20mph on 3rd to eliminate gear shifting.. I am ordering 70mm testpipe and exhaust invidia.. is it logical that I do not benefit from a header until I enlarge the exhausts ?!!

 

Of course .. research "Bottlenecking" until u open up the exhaust (especially switching the cat to a test pipe) dont expect to see gains of more then 5hp with a header .. unless you paid over $1000 for it..

also if you have just put it on, it could be your ecu has not adjusted yet.. and although its a non moving part i still feel everything has a certain "break in period"

plus unless you have a before and after dyno to compare you never will be able to tell.. especially the way you tried to test your gain.. Although you both have s2000's theres still 100's of variables as to why u havent noticed any increase in performance.