On what track are you planning to use the engine? If it is a tight technical track, I would try the R5R. I have seen the 21-4 on a medium sized track and honestly, was not impressed by it. As said before, the danger in using offroad engines is the temperature, as more than often, to be able to stay close to the faster engines, you will end up leaning the engine out too much and overheating it. If you use an offroad engine, put minimally an onroad pipe to it (2068 for example) and try a short manifold. Use an onroad plug as well, OS RP7 or even RP8, with 25% Nitro. This is what you can do with offroad engines before you start modifying the internals. If the engines are new, then there is something you could try out without actually taking a dremel to alter the sleeve...