i would recommend the 2035 associated or thunder tiger pipe for the ninja or the grp engines, it really wakes the engine up on the bottom and mid range, but still has all the top end you will ever need, if you have a novarossi 9853 or 053 that is also a good choice with a short manifold, and the mugen msr 1005 is awesome on these engines as well, i get 11 min run times with the 2035 pipe, 6.5mm restrictor, and os p3 plug, that engine dosent come alive till you get it around 220-230 its a different animal when u get it upto temp has good power everywhere