Tex,Yes, plenty of space behind the radiator. Getting to it is another matter.
The coolant lines are 3/4" I.D. And the radiator is a dual pass design.
There are a couple of things that I would do if it were my project. I would increase the coolant line diameters going to and from the radiator. The 3/4” I.D. lines that are adequate for a Rotax are too small for a larger displacement engine. I would use at least a 1 1/4” tube. More horsepower means more heat to reject. It may be difficult to replace these long tubes in a completed aircraft, but the only way to keep the temps in check is push a high volume of coolant thru the radiator. The fin dimension of the radiator in the S51 is 22”x18”x3”. Built by Griffin, it is a two pass radiator built specifically designed for the S51 with the inlet and outlet tubes being 1 1/2” diameter. This radiator can keep the coolant temps of a 600 HP engine controlled during a two minute static thrust check at takeoff power. If your radiator is of similar size, your coolant temp problem isn’t the radiator assuming that you can flow enough coolant thru it.
Note the bung at the top of the end tank. It is there to bleed air out of the system, both when filling the coolant system and during flight. A dash 4 line is attached and routed to the coolant expansion tank at the highest point of the coolant system. This line continuously “burbs” the air out of the system. Air bubbles are constantly being generated in the coolant system both by the impeller of the pump and by the mechanism of heat transfer. The pump impeller is like a “cuisinart” as it produces flow of the coolant. At higher speeds it can cavitate creating minute bubbles.
The other source of bubbles entering the system is the interface of the coolant cavities in the engine and the liquid coolant during heat transfer. Called nucleate boiling, it occurs when the surface temperature is hotter than the saturated fluid temperature. Think of heating up water in a pot on the stove. When the water temperature approaches the boiling point, tiny bubbles begin to form on the bottom of the pot, detach and float to the top. These bubble forming displace the liquid coolant. This is why the pressure in a closed system increases as the coolant temperature rises.
The method engineers employed to control the air in the system was to provide an area away from the heat source to allow the bubbles to escape out of solution. Up until the late 70s automobile manufacturers used a coolant expansion tank. This tank was on top of the radiator fins, the highest point of the engine’s coolant system. The radiator cap on this tank was a pressure relief valve preventing an over pressure situation which would blow a head gasket or a rubber radiator hose. Air bubbles in the coolant would float to the top of the liquid coolant and enter the expansion chamber (tank) on the top of the radiator. This is the reason why you should never fill a radiator completely up to the neck. If the pressure became too great the radiator cap would release the pressure to the atmosphere creating the classic cloud of steam coming from under the hood. Auto manufacturers in the 80s, having to deal with higher engine temps due to emission requirements and eliminating the possibility of puking a toxic fluid onto Mother Earth, began to employ coolant recovery tanks which dealt with the expansion do to air entrapment. These tanks are made of plastic and were connected to the coolant system at the radiator. When the coolant expanded it flowed down a siphon tube into the recovery tank where the air was removed from the liquid. When the coolant cools it’s volume decreases creating a vacuum which sucks the coolant out of the tank “recovering” it for use. These tanks must always contain enough liquid to keep the siphon tube covered in order for it to work properly. Cars equipped with recovery tanks rarely, but do occasionally, boil over causing a cloud of steam. The coolant expansion tank and coolant recovery tank do the same thing, except that the expansion tank will vent coolant overboard.
The cause of your projectile coolant vomiting was most likely due to an air pocket. When the surface of the coolant chamber where the air pocket occurred got hot enough, the liquid coolant “flashed” to steam. When the liquid flashes to steam, its volume expands 1700 times the volume of the liquid, causing an instantaneous pressure spike. This pressure finds the weakest link of the coolant system to vent to the atmosphere.
Got a little long winded here, but if was me trying to solve this problem, I would increase the size of the coolant lines, install a bleed line on the top of the radiator, make sure that the coolant recovery tank was half full when cold and fill the coolant system with the aircraft in a level attitude. Consider a vacuum fill system for eliminating the air prior filling with coolant.