The coolant will typically only build to 16-18 PSI, due to expansion up to 200°F. However, if the engine does overheat due to external factors, the pressure inside the cooling system could reach as high as 28 PSI. Once the radiator cap has opened and vented coolant, the engine will not cool down until it has been turned off. The radiator cap is basically a "safety valve", so always use the highest pressure radiator cap that the radiator will tolerate. If you are unsure of the pressure rating for your radiator, check with the manufacturer for the maximum recommended operating pressure.

 

I don't quite understand their statement about the engine not cooling down after venting until you turn it off.  I would think that reducing power and therefore heat into the coolant would cause the problem to subside - assuming you have any coolant remaining {:>), perhaps their statement "..and Vented Coolant" means ALL the coolant.

With the pressure cap in the system between the radiator and the recovery bottle, any coolant that gets by the cap and into the bottle cannot be recovered until the engine has cooled to ambient, or close to it. Only then will the system pressure drop below ambient and coolant in the bottle will be at ambient, and thus at a higher pressure, so it replaces the coolant lost by the system. Not all of it, but close, because it takes some pressure to open the return valve in the cap. The make up bottle has a level marked on it so you can fill with coolant to that point. Notice also there are marks for cold and hot coolant. About a quart apart on the bottle. This is so there is no chance that a system cooling down will ingest air from an empty recovery bottle.

 

 The water pump is mounted very high on the engine, and a big dose of air will prevent the pump from circulating coolant, with tragic result. Notice that in the "car" system, that coolant is full into the cap well on the radiator. When the system heats up, it is in hydraulic lock and builds pressure up to the relief setting on the cap. Only then does the cap begin to bleed off coolant to maintain the rated pressure. The cap well on the radiator is not the very best way to remove air from a rotary engine. If you take the time to run through several heat cycles after you have the system open, then you will get most of it out.

 

The early Rotary powered cars had a plastic bottle on the firewall, and that bottle had the pressure cap on it. There was a cap on the radiator used for filling it but had no pressure element. Mazda was not alone in this practice. The advantage here was that the engine didn't have to cool all the way down to recover coolant from the recovery bottle. Small volume changes from changes in heat load would cause fluid exchange from the pressurized bottle to the top of the radiator. Coolant entered the bottle from the bottom, so a slug of air forced out of the radiator would pop to the top of the recovery bottle and when power (and heat) was reduced in the engine it would recover only water from the bottle.

This, because the pressure in the bottle would be (for a few seconds) higher than the coolant system pressure. I keep my pressure bottle about 1/2 full. I have added a Shrader valve so I can pressurize the system before I start the engine. Both systems work well, and most people use the "car" system. Because they are not aware that there is a (in my mind) a better system. And there is little to fear from using the car system.

 

The advantages of the old Mazda system are: it will return all of the coolant in the bottle to the engine in case of a leak. You get nothing back from the "car" system until the plane is on the ground and cold. This system is much better at removing air from an operating system. The weights are about the same for both. You can still find them in the junk yards but you can make one if you want. You can mount either system anywhere. They don't need to be higher than anything to work. The car system is just a little less well off when mounted low but not a problem, but it is open to ambient. The Mazda bottle can be mounted anywhere so long as it is upright. And "anywhere" can be a real handy location.

 

Lynn E. Hanover