Honestly if you want a good motor(not saying all the others mentioned are not good). I would suggest the P5X. It has very usable powerand good duability. I have 8 gallons through it and I can't turn it over by hand. Jp1 pipe goes good with it ,although I just purchased the JP3 pipe , and it might like that one too.BUT I also just put a gallon of fuel through the IRC Radical 621P motor,and I would have to say that I like it better than the P5X. It has more bottom end than the P5X and the same top end of a 421B. Im gonna see how it works with the JP 3 pipe.Im sure it will like it. later

PS Both engines were modded by RB Mods. Rick really knows what he is doing.If I were you I would go with an RB Mod. Check out his website.