Originally Posted by k-star
I also run big blocks with-out the by pass hose and have zero issues with it.. You do need the 1/8" holes in the t-stat..
One big reason for most cars that over heat is incorrect ignition timing... I would bet that the big blocks that have heating issues do not have enough timing on them.
Here's whats going on, or not, inside an engine with out any form of coolant return when the thermostat is closed. Coolant is not circulating! Therefore, the pump is in cavitation which aerates the coolant and causes vibrations of the impeller, it's shaft, bearing and seal that they are not designed for. This increases the potential for failure of these components, which you see as reduced life not so much a catastrophe like the pump blowing off the front of the engine or something exciting like that. More likely the seal suddenly leaks on a holiday weekend when you're a hundred and fifty miles from home. This might me more of an engineers than mechanics issue where engineers worry about things that might be where mechanics worry about things that are. Who knows? If your going to do this you probably should know whether your water pump has a metal or plastic impeller. I'd bet on metal putting up with cavitation for a longer period, but transferring more of the forces to the shaft, bearing and seal. Plastic probably absorbs more punishment without secondary transfer of forces, but won't do it for long. The second big issue with no by-pass during closed thermostat is no coolant flow in the block and heads. This means localized hot spots form around the usual super hot places which is the exhaust valves and seats. This forms steam pockets and steam is not a very good conductor of heat, therefore, the metal in these areas gets very hot and expands. Expansion against colder adjacent metal results in cracked castings for cast iron and will often allow the hard seat insert of an aluminum head to fall out, now that's a real mess. In any casting the valve seats will distort and the valves will leak gasses past these distortions. These results can be a bit expensive to repair.
These aren't just theoretical ramblings this is what's happening. The possibility of damage is a function of how often and how hard you push, all that does is establish when it does damage, nothing for if.
Now the fellow that started this thread stated he had no heater. The heater connection is also a coolant bypass when the thermostat is closed. So normal factory engines really have two bypasses, one being pump to block in the case of the BBC a hose to the manifold, the SBC uses a passage cast into the block and pump under the right side pump horn. The second is the heater core circuit which routes from various places on the engine to various schemes to the intake side of the water pump. This may be to the pump itself or to the cool tank side of the radiator or into a fitting in the coolant pump inlet hose. All of these are used by different years on different models. So if your system includes a heater, you also have a bypass beside the crummy hose between the manifold and the water pump.
Now I've had a few Ford FE engines in my life and owning one of them really makes you insensitive to the whinnings of BBC owners. If you want a bypass that's ugly and is a real pain to repair when it fails, get yourself and FE (332, 352, 361, 390, 391, 406, 410, 427, 428) and find out what real hardship is!