Cooling an Engine 101
 

Disclaimer: I'm no engineer, yet, so this is not a super-dooper-highly-accurate-with-nothing-left-out explanation. It's what my research and experience on the matter has taught me.

It seems there is much confusion on the matter of cooling. I will not cover the coolant reroute aspect as I have no experience with it and it's well documented elsewhere on this site. Rather I will explain how the cooling system exchanges heat to the air through the use of a radiator, shroud, and fan. This is not a how-to, but rather a summary of how the cooling system does its job. It seems there is a lack of understanding on this as recently several members have "upgraded" their cooling systems and immediately experienced overheating problems.

First, some background information so we are all on the same page.

Heat is always transferred from high to low. That means there has to be a temperature differential for heat transfer to occur. No delta T, no heat transfer.

Heat transfer can occur in three different ways: convection, conduction, and radiation. Convection is the biggie.

Our engine, like any other system, is not 100% efficient. The goal of the engine in our Miata is to convert fuel, which has chemical energy, into mechanical energy to turn the wheels. Our engine inhales an air/fuel mixture, compresses it, and ignites it in a controlled manner. When the compressed mixture is lite, it burns rapidly. In doing so, the chemical energy in the fuel is being converted into other forms of energy, mainly heat. Because we hold volume constant (sort of), the increase in temperature causes an increase in pressure. This pressure is what pushes the pistons down and creates a torque on the crankshaft. Then the exhaust valves open and what's left of the mixture is expelled out of the engine and the process repeats. However, some of the heat goes into the engine and must be drawn out to keep temperatures in check to allow for safe operation.

Specifically, about 1/3 of the energy from the fuel is converted into heat that travels into the engine. The purpose of the cooling system is to remove this heat such to maintain a safe temperature at all times. The 1/3 rule is pretty well accepted on most piston engines in cars.

The engine's cooling system circulates water throughout the engine to remove heat and keep temperatures safe. It's pretty easy to see that for the cooling system to do its job, it needs a constant supply of cool water. It can't cool the engine with hot water. So we have a radiator. This is our heat exchanger. It takes the hot water the engine spits out and cools it and returns it to the engine. That is, it transfers heat from the water to the air that passes through the radiator. The radiator's key method of heat transfer is actually not radiation, but convection. They would be more appropriately named convectors.

Simply put, the radiator is a row of vertical tubes connected by a tank on top and bottom. Water flows through the tubes and air is forced around the tubes. The heat is transferred from the water into the tubes and from the tubes into the air. If we don't have air passing through the radiator to cool the tubes, we aren't removing heat by convection. That's why our cars have fans on the radiator. These fans pull air through the radiator.

Here's a blueprint I made of my Koyo radiator. It's bound to be similar size as every other drop in replacement radiator for the Miata. The drawing is to scale. 1cm=1in.

Attachment 211340

Now when hot water pours in the top of the radiator, it is distributed to the tops of the rows. At first the water is very hot and the outside air is not. Let's say the water entering the radiator is 200*F and the ambient outside temperature is 100*F. So there's a 100*F temperature delta. It's easy to see that a lot of heat is dumped within the very few first inches of the radiator as the temperature delta is at its highest. As the water cools and travels down, it becomes harder and harder to remove heat as the temperature delta is lower. The coolest the water could possibly get is ambient. However, this isn't a Walgreen's commercial so it's probably going to be a bit higher than ambient.

Here's a blueprint with the fans installed directly to the face of the radiator, as in no shroud. The circles represent where the fans pull air. Without a shroud, the fans only pull air where the blades are at. There's a 5" circle in the middle that the blades affix to. No air moves here. The lines are the tubes. Air passes around the tubes and cools the water that flows through them.

Attachment 211341

So doing the math with 12" fans and no shroud….

Attachment 211342

So with our slim fans bolted directly to the radiator with no shroud, we are making use of almost 48% of our new radiator! Yeah! Wait, maybe that's why everyone "upgrading" to slim fans and no shroud is overheating?

Now I wish I could do the math and tell you how much heat the system can remove with no shroud vs. an ideal system but I can't. I'm in Thermodynamics I right now, but we haven't done much yet. Maybe I'll update this in a few months with some numbers to backup the theory.

What I can tell you is you are wasting your time upgrading fans if you don't build a shroud for them to pull air evenly across the radiator. On my 99', the stock cooling fans are anywhere from 1.25" to 1.75" away from the radiator, depending on where you measure from as the blades are curved toward the radiator. I'd say the average spacing is 1.5". That's probably a pretty safe number to go with when constructing a shroud. However, more is better as it will allow the fans to pull air more evenly across the radiator. Of course sealing the shroud to the radiator is important too. Pulling air through the fans doesn't cool the water. Pulling air through the radiator does.

Some may be space limited when constructing their shroud because of a sway bar upgrade or something else. In my case I squeezed an intercooler between the radiator and condenser. When I get ready to install my Koyo, I will have to move the bottom radiator mount toward the rear of the car about an inch or so. I'll have to space the top out too. So in my case I'll be space limited in building my shroud as the sway bar will be in the way of making a deep shroud and my radiator will be 1" or so further back than normal. I may end up building an angled shroud, one that is say 1" deep at the bottom (arbitrary number, I have no idea what this will be) , but angles to 2+" at the top. That way I'm getting more even airflow vss having the entire shroud 1" away.

I was gonna post this in the Archives but it wouldn't let me. It's definitely something all the newbs need to read as recently there has been a slew of overheating issues.

Funny, I upgraded to slim fans with no shrouding and had no undertray and I never overheated.

You forget the fact that you only need the fans when you are sitting idly anyway. The amount of airflow through the radiator at speed is many times what the fans can recreate.

Also, if you are using Scott's recent try at putting the ebay fans into the stock housing as an example I can tell you exactly why that didn't work. The tips of the fans did not come close enough to the housing. Also the housing is not shapped the same as the tips of the fans. The fan is then unable to pull as much air through. Same principle as a turbo compressor wheel. It must be a certain distance to the housing, any more and it become ineffective. I told Scott this before he started, but he wanted to go ahead and try anyway, can't hurt to try.

The simple matter is that ebay fans are useful for thicker than stock radiators to clear the sway bar. Also the blow more air in a smaller package than stock. At speed the fan is more of a restriction than a cooling aid, PERIOD.

Did you have A/C? 95-100*F temps? I know the fans are for when you're not moving much. I say this because several people have recently replaced their stock radiator and fan with a big radiator and slim fans and went from not overheating to overheating.

Also yeah I thought the same thing about Scott's fans.

I haven't seen any of these "I bought a bigger rad and slim fans and now im overheating" threads.

Fwiw, a buddy of mine may have saved me some time/trouble.
I recently upgraded my rad, and had a set of ebay slim fans ready to go in at the same time. Reason? Because I thought I'd have fitment issues with the new rad/stock fans, AND "y not theory".

I was considering making a shroud, but wasn't able to come up with something I thought ideal...his suggestion, see if the stock fans fit..they are already shrouded well.

I did this, ragged on the car hard today for 2-3hrs, and had no overheating.

I'd suggest everyone trying the stock fans with their new rad first to see if it fits. I have a GS rad, and although stock sway bar, seems to me its tight but fits and has a lil room to spare..

Just my 2cents

Pat,
If you have a properly implemented air flow plan and are in constant motion, then fans aren't necessary and in fact are an obstruction to flow. Of course, real world driving means traffic and red lights. The fans should only be necessary when at low speed, stopped, or other times when there is no airflow (like tail gating).

I know someone who overheats with a big ass Koyo radiator, 2/3 Cobra shroud, and dual spal fans while at speed. Why? My guess is that the shroud and fans limit airflow at speed. Meaning more air would flow without them there. I'd like to see him remove the shroud and bolt the fans directly to the radiator.

There's a reason why race cars don't have shrouds.

Race car's don't have A/C condensers. Race car's don't stop at redlights or stop signs. Race cars are different. That is understood.

I seriously doubt there's that much "flow" through the radiator with an intercooler and condenser infront of it. Sure there's air moving, and more than the fans can pull at speed, but people who think there's 10x the air moving through the radiator at 100mph are wrong. That's not happening. Air will just build up infront of the car and go around.

While yes the fans are of some restriction to flow, it's likely negligible.

Neo- look around. There are people overheating.

has anyone raced with slim fans bolted right to a gs/miti/koyo radiator and over heated? cause I realize that while idling you may, but the theory is that once you start moving you're fine, but cruising down the freeway with plenty of airflow is much different than racing. while you're still moving, you're also creating much more heat. I have no ac, an intercooler, a miti rad and ebay slim fans. any theories as to whether or not I'll overheat at the track?

race cars are not different. they stop less, but they still stop. they go to the pits. they get stuck in (race) traffic. so they don't have condensers, so what. they still have intercoolers and oil coolers.

i've had many conversations with too many racers who all say the same; you want as little fan as necessary.

Pat, you have great ideas in many of your threads. A common theme in them however is your unwillingness to take advice from others with experience. You certainly seem to be above the average in common sense and intelligence, but you need to be willing to accept that a good idea isn't always correct.

I agree with you that stock fans will work on these rads, I disagree that slim fans are causing people to overheat, I disagree that the lack of a shroud as you describe it is causing overheating.

Ben I agree with you 99.5%. Race cars need as little fan and shroud as possible. Race cars have heat exchangers too, but most will stager them so that every one has ambient temp air moving across it. Not really arguing with you on any accounts.

Well, you're right. I'm pretty hard headed at times though I try not to be. I look at things from several perspectives. I also look at real world situations, and what others do and what racers do. I see Ben's point he makes and I agree. I've seen many track cars with little or no fan. Seen many drag cars that way too. Have one small fan and an electric water pump to cool the engine down after a run. Seen circle track cars with one small fan with no shroud. Gotta have it for red flags, etc. But circle track cars get cool down laps too before stopping. They are the same in many regards, but different in others.

I look for trends. I don't think I've seen anybody on this forum who has ripped out their A/C, added an IC, upgraded radiators, and installed slim fans with no shroud overheat. Again, I think those guys are fine. Seems so. It's the people who have an IC, condenser, and Radiator with no shroud that overheat at idle. My car stock will overheat at idle after 20 minutes and won't cool back down. That's with stock cooling system though and the stock fans. But get up to 20 mph for 30 secs and temps drop back to normal and it's good for another 20 minutes. It just can't quite keep up in 90*F heat at idle forever.

And yes I could be wrong about all the shroud stuff. But to me it makes sense. Maybe after a semester of Thermo I I'll laugh at this. I'd love to hear your educated opinion on the matter of fans and shrouds.

I think sealing between the bumper opening and the radiator will bring much more worthwhile results.

I just have to add that on my trip from Northern California to Oregon during 108 degree weather if I went below 60 or above 70 I would start to overheat. I have A/C, an intercooler, and slim fans without shrouding. I bypassed the temp switch so that the driver-side fan was on at all times, but still overheated.

That little bit of anecdotal evidence shows that going faster does not add enough air flow across the radiator to adequately cool after a certain point.

Granted, I was going up over multiple 6% grades, and pulling a trailor.

you have to have a plan to trap air in the mouth and force it to go through the radiator. unfortunately a stock miata does not do this very well; there is too much pressure in the engine bay. extraction needs to be as much of the plan as anything else. it's the key.

I was stuck just behind a severe accident on the interstate Friday evening. About a half dozen cars and 1 semi involved, all damage was major, and injuries were significant. My car didn't move for nearly an hour as one of the injured parties was evacuated by life flight. I allowed the car to run with the a/c on for the entire duration, and my laptop was logging. The car had no problem maintaining standard operating temp with the a/c running at idle on a sunny 90F day, packed onto blacktop with hundreds of other cars. I'm running a Griffin radiator and, for the moment, nearly 2 decade old stock fans.

Something to possibly look at would be the radiator design. I'd image my Griffin allows airflow much more efficiently than a China cooler.

I agree with Pat. I did specifically as he describes and got the indicated results.

I replaced the stock radiator and fans with a Godspeed with slimlines on the surface and now can not run the A/C without over heating.

There was no change on other areas of shrouding or ducting. I have run without a bellypan for years and have only the stock shrouding in the mouth to try to direct air.

The change came about strictly with the change of stock fans and radiator. I doubt that the radiator is that inefficient, but will find out when I put stock fans back on.

It also helps when you get a real 1600CFM fan....I wish I had a way to measure, but from the ones I've experienced so far, it's doubtful they are.

Brad still has issues with overheating with his Godspeed and (1) 12" ebay fan. We added a second yesterday. The new one blew less than the first, measured on a maghenellic gauge.

I have a funny hunch that the real issue is just sourcing the correct fan for the job...this doesn't mean a $20 one...

Bruce, make sure the fans' blades are set to pull and not push.

But I agree with Scott here. There's a reason why a single Spal fan costs as much as 10 chinese fans.

FWIW I am running the stock radiator with two 11" Spal fans, wired in parallel, no shroud. Yesterday I was out playing on switchback roads in the Texas Hill Country and I was running hard enough to get the brakes fading. I did not have any overheating problems.