Usually higher nitro works better with a more cone shaped combustion chamber where low-mid nitro content works better with a more bowl shaped combustion chamber. With lower nitro fuel you also generally want a wider squish band with a slight angle and for higher nitro fuel a narrower squish band with a steeper angle. Note - this is only what I've read, I have not tried it myself but it makes sense. My Jett's are setup to run 15-20% nitro and have a cone shaped chamber, whereas an Italian SuperTigre has a bowl shaped chamber which is setup to run 0-5% nitro.

Do the Moki/Mark engines use a button head or one piece head?