Adding volume to a radiator via hoses and such.

i wanna parrot @zrx12man and reiterate that this explanation (of why having a thermostat installed might help the bike run cooler) is patently untrue. i'm not disputing that running without a thermostat might end up hindering cooling, just nitpicking this particular physics explanation.

anytime there is heat transfer between two materials, more physical contact area means more heat transfer. always. in the case of a solid/fluid interface, like the interior of a bike radiator, more fluid flow is usually the easiest way of achieving "more physical contact area".

the myth that there needs to be "sufficient time for heat transfer" hangs around because it makes some intuitive sense: if a particular droplet of water spends ten seconds in the radiator, it will certainly cool down a lot more than if that same droplet spent only a tenth of a second in the radiator! however, what's missing from that mental model is a full view of the rest of the system: if that second droplet spent only a hundredth of the time cooling down in the radiator as the first one (10s -> 0.1s) then it is also spending only a hundredth of the time heating back up in the engine - and it's performing a hundred round trips in the same time it takes the first droplet to make one full loop.

in that sense, it might look like everything would balance out and the two droplets should be equally efficient.. but the kicker is that heat transfer occurs more quickly with larger temperature differences. the longer that our ten-second droplet spends in the radiator, the slower it cools down, since its temperature is becoming closer and closer to the temperature of the metal it's touching. for the first tenth of a second, both droplets lost heat very quickly. by its last tenth of a second, however, the slow droplet is transferring heat much more slowly whereas the fast droplet is still entering the radiator at its full temperature and is thus still cooling down just as quickly. (you might - very correctly - note that the fast droplet still never cools down as far.. but something to keep in mind is that that doesn't actually matter, since it won't heat up as far either! if the slow droplet cools down 10x farther by spending more time in the radiator, it'll also heat up 10x farther by spending more time in the cylinder jackets.)

there are still many other explanations as to why having a thermostat would increase cooling system efficiency: cavitation and splashing inside the pump/pipes/radiator might result from too-fast flow, and that would dramatically reduce the metal/water surface area, for example. but the explanation of "faster flow == not enough time for heat transfer", in this scenario, is just plain incorrect.

<deep breath> ok i'm done nerding out for now

 

^ +1. good explanation.

 

Okay, for one cavitation is pretty much a non issue since the cooling system is pressurized and most impellars are fairly well designed these days.

Despite that long engineer type explanation the fact is, the longer the water spends in the radiator the cooler it gets and the more heat it absorbs from the engine. Move it too fast and it does not effectively cool the engine. Cooling systems that are designed with a thermostat tend to work better with either a restrictor in place or a thermostat.

 

Faster flow = more heat transfer. Which means more heat in and out of the radiator and hotter average coolant temperature.

 

Here’s a lot more info from a cooling system manufacturer.
http://stewartcomponents.com/index.php?route=information/information&information_id=14

My 66 Mustang used to spew water without getting hot if I left the thermostat out. That link suggests that the cap was being opening by the water pump pressure without the pressure drop from the thermostat. Funny and good to know. I guess I could have just bought a stronger cap instead of a bunch of other stuff.

 

With the rad being more efficient with a higher inlet temp, shouldn’t the avg temp be lower?

That kinda brings up another point. Does the temp gauge display rad inlet temp, outlet temp, or something else... that’d change someone’s perception of “faster flow = more heat transfer”. Of course, actual engine metal temps would still be lower no matter what the gauge says.

 

Yes if you can keep it from going up
On the bike in question pick up is on thermostat housing which is right after coolant exits head.

 

God darnit Mister Lamar, you use your tongue purtier than a twenty dollar whore!

 

Very basic statements like this are untrue.

There is a optimal saturation time, anything lower than or higher than will result in a loss in effeciency.

 

Typical pressure from water pump circulation is usually 3-6psi. Not 14-15.
If it was pushing water out it was from hot spots and it boiling....

 

I'd really like a physics-based explanation of this, because it does not at all jive with my not-tryna-be-arrogant-but-prettttty-solid understanding of physics, nor does it seem to jive with engineering resources like https://www.pdhonline.com/courses/m371/m371content.pdf .

There are many reasons why you wouldn't want an overly-high flow rate (nonlinear fluid effects like cavitation or splashing, structural effects like internal pressure limits or pipe vibration, etc) but some optimum "dwell time" for maximum heat transfer efficiency, where efficiency increases with time for some range, is absolutely contrary to everything i know about thermodynamics. if i've missed something - and i'm open to the possibility - then i'd love to get taught what it was.

 

Just to throw in my understanding on this. Higher temperature will increase heat transfer and therefore the longer the dwell time the LESS the gradient will be (leaving the radiator) and thus the less heat transfer. I would imagine that it is an exponential function so you'd imagine that the "optimum" dwell time will just be until it drops off like a rock.

Just reread your post and there is no way that efficiency increases with time. The efficiency is always approaching zero where there is no heat exchange going on.

 

As I understand it the transfer of heat from water to the aluminum in the radiator doesn't take very long but you still have to exchange that heat from the radiator into the air. The only things that will lower temperature is more surface area over which to exchange that heat or more airflow through the radiator.

 

the radiator is basically two solid-fluid heat exchange interfaces, working in opposite directions: water flowing through the pipes transfers heat to the aluminum, and the aluminum transfers heat to air flowing through the fins. hotter water in the pipes results in faster heat transfer to the aluminum, resulting in hotter aluminum, and hotter aluminum results in faster heat transfer to the air.

actually, i may have just unintentionally stumbled onto the source of the confusion here: faster water flow does result in "hotter water" - at the radiator outlet. less time in the radiator obviously results in less time for it to cool down, so the difference in inlet vs outlet temperature is smaller and the average water temperature in the radiator is higher - which is actually precisely why it's more efficient. however, faster flow does not directly result in a hotter average water temperature overall, because the water also spends less time in the hot engine block and so has less time to heat up as well.

faster water flow results in smaller temperature variations in inlet vs outlet temps across the various heat-exchange interfaces in the system (radiator and cylinder jackets), and that is why the system becomes more efficient: more of the system is operating at its designed (and high-gradient) temperature.

 

We could also discuss the effects of air temperature and altitude or air density on the rate at which the heat can be transferred out of the radiator into the air. Those are all factors in how well the system can do its job.

 

absolutely. there are many, many, many factors that go into it. i just got completely nerd-sniped into arguing very specifically about coolant flow rate and its effect on cooling system efficiency via heat transfer rates.

 

And that is exactly why moving the water too fast can be a bad thing. It can happen.
Think of it this way, the coolant is a medium used to transfer heat(ie energy) if it enters the engine at 190deg it is only capable of absorbing x amount of energy and carry it to the radiator where it sheds itself of said energy. Same thing goes for the radiator side, if it doesnt spend enough time there for its temperature to drop thenit still still carries the energy with it. Resulting in a less effecient system.

All of the science I can find on it is based on steady state transfer wihout the fuid moving through a system and coeffecient charts that represent a hot object dropped in a cool fluid and measuring the heat exchange. That leaves out a lot of different aspects such as boundry layer and its effects on heat transfer as well as the possibility of it changing relative to the velocity of the coolant.

Have seen multiple times on different race cars where the coolant had to be slowed down to cure a heating problem. Dont have all the answers as to why, but damned sure dont beleive a theorist who says he does while looking at it from a simple perspective.

 

How about someone with a flow meter and a few thermosisters end this. Thermostats in a street motor are about efficient,clean combustion, not sustained full power. (Not that road racing is full sustained power, lol.) You are modifying a system that was made for emissions compliance for race use. Every race designed system I have worked on didn't have a thermostat or restriction, it was designed to remove maximum heat at peak temps, period. Because if it didn't, you were fucked. There wasn't enough cool down to worry about after that, before the cycle started again.

Optimum emissions cylinder temp and optimum power cylinder temp are very different. If you expect a system designed for one to work best for both, you are a fool.

Thermostats are used to try and maintain a cylinder temp irregardless of usage to a specific desired range. When designed right, they work ok over a extended range. If a system doesn't need an extended range then flow and cooling can be calculated based on average heat loads and it can work even better.

Stop try to make duck taste like chicken.

 

This might hold if the rate of transfer in the radiator was the same as the rate of transfer in the engine block. It also might be true if the amount of time in the radiator was the same as the amount of time in the engine block. Since neither of those variables is known in this particular case, the argument/explanation is speculation at best. Since we are now down to speculation, I think the amount of heat transferred is based on either magic or alien technology.

 

Hmmm..... good timing, he just made this new vid today