6000 rpm can be done, but most likely there will be no advantage in doing so. It's been discussed here before, and is well worth the read, but piston speed is what'll getcha. If you exceed the critical speed of your piston's material- it comes apart. Also, you might spin to 6k, but horsepower and torque will taper off before you get there. Right now my engine makes peak HP @ 4250 rpm, and holds at 5200. After I correct my intake airflow issues I too hope to be capable of 6000 rpm, but I betcha my peak HP is still under 5000.