I think maybe the classes will be engine displacement grouped. The airplane manufacturers engine displacement recommendations will be the guideline for max displacement for their particular airframe. Kit or ARF will be subject to this rule. Non production will fall into the X-open category. No restrictions on brands of engines,pipes or fuel for any class. You may run a production airframe with an over spec engine,but it will automatically fall into the x-open class. Airframes that deviate from production form will automatically go into X-open. I am not even going to consider a tech inspection on the different engines to determine if an engine is stock or not. Most of you guys wouldn't go for it anyway. We will just have to depend on each others honesty and integrity.