I have a 400 sport all orginal someone was saying i could get more H.P. by putting on a exust system and intake for a small price would like to know what kind would get the most H.P. and the cost and how difficult it would be to complete. I do my own work on old iron head harleys.

 

Get ahold or Rick Ritter, he can do alot for these machines.

 

Good performance gains are made with just a good pipe and matching clutch kit.
The only ones to think about are Aaen,HPD,RcR,HSP,and my fav Dynoport.
If you still have a stock piston get it out of there before doing any mods.
And if you want to really turn on the HP get the cylinder ported t the pipe you get.
A good set of reeds helps too.The list goes on and on but remember the more you build it the les reliable it becomes.