hey rc,ers im a newbie here but i have 2 nitro x-cellerators i picked up off of ebay.they i think are awesome for what i payed and they run great when i dont break anything,but i have a question.i got a sirio .21 with the green head brand new for free and was wondering how fast that car will be with that engine.

maybe 40 or 50 on a good day

Hard to guess actual speeds in actual MPH. Probably pretty quick, perhaps more than you'd want. Not sure if you could get a big block .21 to fit in a 1/10 scale vehicle like the X-cellerator. At the end of the day, it doesn't matter how sweet of a power plant your RC has when its wrapped around a tree because you couldn't turn quick enough. ='(

yea not knowing the setup of the x-cellerator can't say if you can even strap a big block in
SOME 1/10ths its relatively easy it would literally just bolt straight in with the rght mount, some it can't be done period.

but IMO all that engine would do is destroy the drive chain and drink tons of gas on that truck. Most 1/10ths that can take them need hardened gears installed to handle the big blocks cause the stocks shatter under the stress.

well last night i installed a rrp gear that is aluminum # 1513 in the diff. from what i hear they fit in the x-cellerator and with in a minute with the stock engine it stripped so i guese with the sirio .21 i would have to modify a different diff. for it so if any options let me know. thanks

honestly you would probably be better off auctioning the one off as a roller and buying a used 1/8th buggy, truggy, or even a savage(if you want a Monster Truck) roller and putting the radio, servos, and sirio motor in that. Instead of trying to reinforce the XTM's diff. Trust me trying to fit diffs from other vehicles in can get expensive fast been there, and done that.
but thats just my opinion