Big block cooling issues

DrCharles said

Still doesn't work that way. The only reason to restrict the velocity of the coolant is to avoid cavitation and turbulence issues. Not heat transfer per se. That is driven by the temperature differential between the hot side and the cold side. The radiator has to be large enough to transfer all the heat between a 180F coolant temp and (say) 100F ambient temp. The cylinder heads run a lot hotter than 180F so they don't need as much surface area.

In fact, if coolant moves TOO slowly then there can be hot spots and localized boiling in the heads - which also reduces the amount of heat carried away!

The only engines I have ever seen that don't run dead cold with the thermostat removed have a secondary disk attached to the moving t-stat element. Old Mercedes diesels, IIRC. This disk opens a bypass when the stat is closed - so the bypass is shut off when the stat is open. With the stat completely removed the large bypass stays open all the time and a big percentage of the coolant flow is diverted away from the radiator.

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You're wildly oversimplifying, as most people do. The cooling system is a complex system, and thermodynamics is not a science to be taken lightly. I have years of studying it and an engineering degree, and I'm here to tell you it's not so simple as you're making it. Even after all the time I spent studying it I would not consider myself an expert in it.

Yes, coolant can move too slow which can cause localized boiling which is bad. But that is also controlled by the pressure in the system. Unlike the radiator, which has a lot of built in restriction because of the size of the tubes, the passages of the engine are large. Transfer only takes place where the water is directly in contact with the walls, large cavities don't transfer heat directly to the water in the middle. Turbulence does effect heat transfer too, pretty much all of this stuff is designed assuming you have laminar flow. That's not always true, but that's the assumption and a lot of turbulence changes all of the equations. Cavitation would happen in the water pump, not in the block. The temperature differential between the block and the coolant is higher, which does improve heat transfer, but the surface area and the large cavities are a disadvantage.

The cooling system was designed originally with all of these things in mind. Including the amount of restriction at the thermostat. Yes, it's silly to say that water going through the radiator too fast is an issue. Mostly because of the design of the radiator itself and the fact that it's exchanging both coolant and air. But that also doesn't mean that the thermostat doesn't need to be there for more than initial warm up. It would be easy to make the thermostat orifice larger, it's smaller than the passages on both sides of it. And it is that way for a reason.