amid

hi boys ...i am the guy with the crx 91 from greece ...
i have change my stock filterbox with an itg froam cone freeflow filter .... what is beter for the crx the stock airbox with kn inside and ram air ? the simota topgun series with ram air pipe ?
or can i use my itg and just make a box to put it inside?
and about the pipe length ? is it ok if i weld half 2.5 inch and half 3 inch or 4 inch ? any trip for going the air faster and compresed ? .......