Without a turbo what is the most horsepower that i could gain from bolt on part for my 2000 civic si?

 

There's no definite answer to this question. Could could certainely port and polish and bore the head and work intensively on the head, but the power you ultimately get will not be worth it compared to just getting a turbo. However, if you built the guts of the engine, you can boost more later...

 

honestly, what's your budget? If you're looking to gain as much power as humanly possible with amodest budget, don't even bother with all-motor; it's just too expensive to make power with (that's not to say you can't make power staying N/A, but the parts are really expensive compared to the power gains). If you have a pretty good sized budget and are just building an all-motor car to prove a point, I say go for it.