I'm using the term "partial pressure" to distinguish that a portion of the system is running at a pressure above the radiator cap rating. Without a restrictor or a t-stat this won't happen and the boiling point of the coolant, even though elevated due to the cap pressure rating, will be lower than with a restrictor or t-stat, and localized boiling will occur. To run without a restrictor means that the cap pressure will need to increase, and the rest of the system is probably not up to that greater pressure.
