No. But a lot of people still think so. We have come up with some explanations for the Doubting Thomas.
Debunking the I Can Have It Both Ways Theory
The water has to have "time to cool" argument is perhaps the most common one we hear. In a closed loop system if you keep the fluid in the heat exchanger you are simultaneously keeping it in the block longer. Unfortunately, the block is the part that is generating the heat and the radiator is the part that is shedding it. Sending hot coolant from your source (engine) through the heat exchanger (radiator) to the sink (air) will always transfer heat as long as there is a temperature difference between the source and sink. The more times you introduce the source to the sink the greater chance you have to shed the heat out of the closed loop system.
Debunking The Conscientious Electron Theory
We hear that the coolant has to stay in the system longer to cool but what is heat transfer really but conduction, convection and radiation of electrons. The fluid in your system transfers those electrons based principally on the source-sink differential and the exchange material's transfer rate. An electron moves at varying speeds - Bohr's model has it moving at 2 million meter/second and with a mere 11 million eV boost you can get an electron to 99.9% of the speed of light. Though they move at varying speeds physicists accept that electrons move fast - really really fast. Far faster than the flow rate of the water pump. Your engine coolant's electrons do not know (or care) how fast you send them through the system - they just knows that the source is hotter than the sink and... off they go.
Debunking Grandpa's Flathead Washer Theory
"But wait a minute, I know Grandpa used to put washers in his flathead to slow the flow and cool his engine." We know people did this too. They still do it but the cooling benefit is not from the slower flow but the increase in dynamic pressure in the block that builds from the restriction. Consider that Grandpa had two flathead water pumps sending twice the volume through the same size radiator core as the Model B 4 cylinder. Too much flow in this no pressure system results in fluid loss. Slowing flow rate with a pair of common washers helped prevent that. At some point Grandpa maxed out the throughput and began building pressure in his block. Increasing block pressure helps reduce the onset of hot spots on his cylinder walls and formation of steam pockets in his block. This is a real benefit and does help cooling but is only realized when throughput nears capacity or is at capacity. While these restrictions may make sense when your rpm is excessive or your flow rate exceeds your heat exchanger throughput, they do not make sense for most applications. If you still doubt this thinking then try this simple Ask Dr. Science experiment; clamp off the lower hose while you watch your temp gauge. Hopefully, you will debunk Grandpa's theory yourself before you experience vapor lock and melt your engine.
Flow restriction is not all bad if it serves to prevent cavitation. Cavitation occurs when a pump turns so fast that you generate lower pressure and air bubbles or vapor forms. These bubbles eventually implode and damage the engine block wall and impeller. Rapidly spinning the impeller can literally rip the air from water but may not actually move the fluid, it's tantamount to turning an eggbeater in a paint bucket. Restricting the fluid flow to raise system pressure in the block may help prevent cavitation at higher RPM but is it necessary for most vehicles? Probably not.
Most vehicles do not need to restrict flow because they do not reach or sustain high RPM. Additionally, thin aluminum radiators already restrict by design e.g. fewer rows of thinner tubes. Restrict it further and you may as well hose clamp the lower radiator hose and we know how that works out. When you face Grandpa on the track you may want your washers, otherwise, keep them in the toolkit.
Simply put, you have a far better chance of keeping your cool with greater flow rate through your heat exchanger and exiting the system than holding it in your heat exchanger while generating heat in your engine block.