Yea I want to go fast I mean how fast is silver class? Maybe break 100 mph? Anyway when I first started this thread I was planning to use a thumper maybe the ys 110 or 115 if it was mentioned earlier. but, when foodstick mentioned a tuned pipe I knew I had to do a 2 stroke build. I remember seeing those things rip around when I was younger and they seemed super cool and fast. sorry I kinda turned this thread from what engine to use into my 2stroke build. But I am thinking a tuned pipe, larger carb, slight porting, polishing, raised compression, and maybe make a billet piston running 30% nitro will get me to 100 on a 6 pound plane if the engine holds up. If all else fails and it ends up sucking I'll chuck it and buy a ys. Or what engine would you recommend for this 6 pound p51 to break 100mph?