A .12 engine is normally more power as these engines are for race requirments and are more fully developed than .15 cause they are not ROAR LEGAL. tHE COST OF THE cv_r IS LIKE $95 FROM TOWER HOBBIES. Sometimes to musch power has negative effects on users who do not know throttle control like gunning it around corners and all they do is spin out all the time. Yes rotary carbs on these smaller enginges is better for low end throtlle transition and making smooth power.