all i did was fit a metal plate to the top of the panel filter...after a bit of trial and error found out that if you drill 19 10mm holes in this plate it will restrict the air flow to approx low 130kw at the wheels... this was tested on a dyno and i wrote up a special sheet listing all this information listing the normal power output of the skyline and then stating my new power output and the weight of the car etc then i showed some calculations and got the owner of the dyno place to sign and stamp the form etc etc made it all official and sent it into the insurance company..