My Brison 2.4 would turn a Mejzlik 20x10 @ 7100 rpm - that's open exhaust (no muffler). A ZDZ-40 (rotary valve) with a B&B muffler (?) turns the same prop @ 7600 rpm. The point is that there is free power available if you can put a less restrictive muffler on these engines. Check your rpm open exhaust and compare to your muffler - you may be surprised.