I think that the idea that too fast flow through the radiator causes overheat comes from an entry in a book that has been around in its current form since the mid 90s
updated and added to, new pictures, new corporate names since then, but the same old text. Its a Good Read
Larry Shepherd in his how to build mopar small block engines book goes into details about how to reduce pump speed for high rpm engines, stating that for ideal cooling the pump must run at nearer to standard speeds, or you will overheat when you convert your small block into High rpm race engine. Gives part numbers etc to get ideal pulley gearing for RPM range, doesn't explain why, just states that all that science is for cooling system experts. This prompts the idea that too fast is bad, seems to imply that its the flow speed of the coolant through the system that the problem, by the way the subject is covered.
I took this book and this section at its word and assumed speed of flow was the issue not something else.... i presume others did as well, and i think sections of that book were lifted from previously published articles, so the ideas around at least slowing the pump had been around for a good while. The explanation as to "Why" you slow the pump was lacking
It did not explain that a pump that produces a higher pressure in the system could be a problem if the pressure exceeds that of the cap
It did not explain that a pump that is spinning faster than it was designed to, might cause cavitation or may indeed become completely inefficient and actually pump less causing very little flow and an overheat via a different route.
some explanation as to why, in articles that potentially have been around since the 70s or 80s may have helped us all....
In a cooling system where the pressure vented filler cap is positioned at the point in the system with the highest pressure. Basically it sits directly after a hot cylinder head which is directly after the high pressure output of the pump. standard pump, standard rpm, standard fan and radiator with a standard cap and good tune...no problem...... BUT if you change things enough,
A situation could exist where the heat flow into the coolant and the action of the water pump can conspire together to produce a pressure higher than the cap vent pressure, gonna be linked to load, rpm, and ambient temperature. That venting may just be of vapour but over a number of journeys your coolant level can slowly decrease to the point where you properly overheat. so when a guy puts in a high flow/high volume pump or "pumps the coolant faster" i.e raises the pressure differential between inlet and outlet of the pump, the first indication of an issue is a full on "steaming overheat" and maybe not on the hottest day, or the day you sat in traffic for 45 minutes, because the overheat has been caused by the slow loss of coolant over a good number of previous trips out, due to the slight lifting of the vent in the cap when engine heat, and pumping action of the water pump, cause the pressure in the top tank to flutter around the cap vent pressure level often leaving little or no evidence.
you could combat this with a no loss system and a return cap but keep an eye out for your bottle getting unduly pressurized... it would be a band aid not a solution to your problem
modern radiators have their filler cap/plastic bottle plumbed in to the lower pressure side of the system before the entry into the water pump at the colder end of the radiator....
probably for good reason. your modern car will be like this
I learned this stuff the last time we had a row about this
I may still be wrong, i was last time..... but i felt i'd learned enough about the errors in my previously held view, to have a newer different view of the situation
Dave
