I ran about 3 tanks after break in with the O.S. engine and it performed well. Didn't seem to bad to tune either but it's more temperamental to the outside temp than the SH engine. It could be that I've gotten better at tuning though. I haven't noticed much of a difference in acceleration between the two either. Again, the only reason why I made a stink with Redcat about the OS was because I planned on getting the electric starter before I even bought the buggy and it doesn't work with the OS engine due to no backplate available for the OS engine. Now the list the Backdraft 3.5 as coming with a ".21 Big Block", which IMO is not misleading anymore but plain wrong. If someone wants to buy the starter they won't know about the backplate until it's too late.