Entering is better. Exiting may cause excessive backpressure and blow the water line off and then you have no water at all. Besides restricting the incoming side retains a constant flow and that is what you want to see for the proper temp.
John
Actually just the opposite is true.You restrict the system outlet for better control and temperature regulation.
The cooling system has to stay full totally full of water to maintain control.The only way to maintain a full system is to have the outlet sized at the head smaller than the inlet.After the head outlet you don't care what happens to thw water to be dumped back into the pond.
If the outlet is larger than the inlet,especially on the bigger bore motors,you run the risk of the water flashing into steam in the water jacket and leaving voids in the head that get no cooling water at all.When you set up your inlets and outlets on your head make them 180 degress apart from each other.If the inlet and outlet are next to each other the water path will short circuit and no cooling water will reach the the opposite side of the head.
If you are concerned about over pressurizing the system just size the inlet to be a bit larger in area than the outlet.I.E. For a K&B 3.5 Outboard.....make the inlet 1/16" in diameter and the outlet .040" in diameter.
Notice on the K&B outboard head there is a divider dam in the water jacket separating the inlet from the outlet ports.They do this to elleviate the short circuiting I mentioned before.
The sizing of the inlet[at the rudder or water pick-up] and outlet ports[at the head outlet]is what determines the system pressure.
Remember,you can have pressure with no flow but you cannot have flow without pressure.
As a whole RC model boaters have "always" run their motors way to cold.
I know this is another subject but these two subjects go hand in hand.
There have been several discussions in this forum on motor "staging".Raising the motor operating temperature is the primary element in getting a motor to stage.
Stoichiometric or "perfect" air fuel combustion is what causes a motor to stage.That is when all the stars are lined up and mother nature [the laws of physics]are at their best.The higher the nitro percentage flashed in a confined area at the perfect temperature to allow this flashing to take place,the more power the motor will make.
On .21 outboard motors I believe it is impossible to get them hot enough to do any physical damage to the motor,no matter what style head they have on them.If they get to warm with their preset supporting fuel flow they just go lean and die but they don't hurt themselves.Just richen them up and go out do it again until you get it right.There just isn't enough combustion heat in a .21 outboard to overwhelm the ability of the motors aluminum mass to carry the heat away before any damage occurs.
