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Why You Don’t Want Fans On Your Heat Exchanger

If you don’t want to hear facts that conflict with your opinions/assumptions/agenda, you like to be recreationally outraged, you’re easily upset, you like your safe spaces, you’re a special snowflake, you’re an unapologetic fanboy of XXXX heat exchanger or you’re simply looking for something to bitch and moan about……………..DON’T GO ANY FURTHER!! This isn’t for you!!!

The Cases For Not Using Fans And The Facts To Back Them Up In This Article

  • Fans will only move enough air through the HE core to replicate a best-case scenario of a 12-14mph vehicle speed. That’s absolute best case with every variable pushed toward the fan(s) favor. In reality it’s not going to be that good. We guess it’s more like 9-10mph. It’s irrelevant though. Anywhere in that speed range is not impressive even without factoring in the “costs”.
  • Fans only reduce water temps (when they’re effective under 12-14mph) 2-3deg best case. Which translates into a IAT drop of 2-3deg best case. Which is nearly nothing.
  • The fans hurt airflow through the core a minimum of 12% at 28mph and it gets worse exponentially as the speed increases.

Why We Wrote This

When we develop a product, we attempt to start with a clean sheet design and not take anything for granted. Especially market trends or market momentum. We don’t simply copy or modify the status quo. Who says the status quo is correct? History has taught us that a lot of the time, it’s wrong. So, in this case we went on a “deep dive” into fans and how they effect heat exchanger performance. That is what is necessary to put out the best product we can. Because of this deep dive we have good data/information on the subject. More than we have seen anywhere else…… combined. Why not post it all up in an article so everyone can learn?

Secondly, this article is pretty easy from a motivational standpoint. We are simply frustrated with the constant barrage of bad information out there about air to water cooling systems in general, and in this case heat exchanger fans specifically. We see this as a public service announcement. We care about the choices people make (and the money they spend) based on incorrect information.

Some may argue that we wrote this to support not offering fans on our soon to be released heat exchanger (2/1/19) for 2005-present Mustangs. That is 100% false. We chose not to offer a fan(s) on our heat exchanger because they are a waste of money for the end user (that’s you). It also allowed us to design/package it in a better way because we did not have to make compromises to incorporate a fan(s). Sometimes what works, and what the public at large perceives as “works” are two different things. And a lot of manufacturers will design something based on market pressure. Even if the “facts” those pressures are based on are wrong. We’re not that kind of manufacturer. We only deal in things that work.

We could have easily designed a fan(s) into our heat exchanger. It’s not exactly rocket science. That would of course driven the price up. And we’re not into driving prices up for “fluff”. We would rather lose some sales because some people just have to have fans (and bought something else), than compromise our “Design/sales morals”.

Disclaimer

We don’t contend that most of the manufacturers/vendors out there are selling heat exchangers with fans because of greed. We’ve never talked to one (and we have talked with a few) that has actually done effective testing (the key words here are “effective testing”, more on that below). As best we can remember heat exchanger fans started showing up in the early 2000’s with the Terminators and Lightnings. At first glance it seemed like a good idea (we thought it was a good idea at the time also). And we speculate that between not having any effective test data to work from and market pressure it turned into an industry wide game of “monkey see, monkey do”. We even played along for quite a few years with our own cars before we decided to get down to brass tacks and test them.

Abbreviations/Terms
 
We will be using abbreviations in this write up. Here is what they are and what they mean.

Ambient – Outside air temperature.

Anemometer – Basically a wind speed gauge

A2W – Air to water intercooler or intercooler system.

CFM – Cubic feet per minute (this is how fan flow is usually measured)
 
IAT – Intake Air Temperature. This is the air temperature measured after the intercooler right before it goes into the cylinders. This is very important.
  
IC – Intercooler. The intercooler is what cools the air coming out of the blower before it enters the cylinders. It is in the intake manifold and you can’t see it unless you have the blower off.
 
HE – Heat Exchanger. The heat exchanger is part of the intercooler system. It is the “radiator” that mounts up in the nose of the car and it sheds the heat that is picked up at the intercooler.
  
OEM – Original Equipment Manufacturer. This means stock. The parts, systems, etc the car came with.
 

Basic Topics/items You Need To Understand/House Cleaning Before Going Further

Effective testing

This is a pet peeve of ours. The word “testing” is thrown around in the hot rod world a lot. Rarely is anything we see an actual test. We don’t think we have ever seen anything even remotely close to the scientific method. We’ve never even seen a A-B-A test. Which is the most basic of tests. In most cases what we see are sample sizes of one (which is not a test). And most, if not all of those don’t have consistent control of the variables for the before and after. Which really makes it a non-test. We understand why almost nothing is actually tested. It’s hard, time consuming and very expensive. Eliminating and/or controlling variables takes considerable effort. We also understand that most of the results we see are not proof. We almost never see a test where we immediately don’t think, “but you didn’t control for X, Y and Z”. You should too. We’re not saying that what you’re seeing is useless (in some cases is it though), only that you should question it. You should even question this article. Additionally, a lot of “testing” in this industry is just badly masked advertising. Keep your eyes open.

The reason this section is titled with testing in quotes is because even though we go to great lengths to remove variables when “testing” (we go a lot further than just about anything we see in this industry) most of the stuff we have tested has the variables minimized. Not completely eliminated. We don’t have the time or money to test in the truest sense. The case of the heat exchanger fans being an example of “almost a test”. It’s more than we have seen anywhere else, but our results are still not proof. Anyway, that’s our “testing” disclaimer. We’ll use the word testing (not in quotes) from here on out. What we mean is “almost testing” though.

Testing Variables

The biggest and first variables that has to be removed when testing heat exchanger and component performance (in this case the fans) is the intake manifold and intercooler. And that right there eliminates every heat exchanger or fan test we have ever seen. You can’t use IAT’s in the manifold to collect data on HE’s. The intake air temperature is made up of:

  • Ambient air temp
  • Supercharger temp
  • Intake manifold temp
  • Cylinder head temp
  • Engine valley/vee temp
  • Engine compartment temp
  • Throttle opening
  • Time idling
  • Erc, etc, etc.

That is a LOT of variables going into a IAT reading. The cylinder head temp can fluctuate 30deg in a matter of seconds. And because the manifold is bolted to the heads the manifold has gigantic temp swings too. Same for the blower. You will see more extreme temp changes in engine compartment temp and ambient air temp (they effect IAT’s less though). The water temp change in the HE is very small by comparison (more on that below). A 30deg intake manifold swing is larger than the swing you will see at the heat exchanger. It’s absolutely ludicrous to think that you can judge HE performance by measuring temperatures inside the intake manifold. You can try and make the argument that at the end of the day IAT’s are all that matters though, so why not take the temp there. That would be wrong. If you can’t tell exactly what is happening at the heat exchanger how do you know if it is helping IAT’s or if it’s simply just a swing in another variable? The answer is, you can’t. That’s why you have to test heat exchanger performance by measuring water temp. At a minimum where it exits the HE. Better yet, where it enters and exits the HE. All of our testing of HE’s and fans over the years was done by sampling water temps. As far as we are concerned any HE test data that involves the use of IAT’s is simply put, junk data.

Water Temps In The Heat Exchanger As They Relate To The Use Of Fan(s)

For this bit we’re going to assume the HE in question is of decent size and performance.

One very large misconception that we see commented on, usually delivered as a retort. Is “But radiators need fans”. Or the smart-ass version, “Well, I’ll just remove the fan from my radiator then smart guy!”. The “logic” these retorts are based on is incorrect. An engine cooling system is not a A2W IC system. They operate in completely different temperature ranges, very different “time under heat loads” and you can run a much more effective fan on a radiator because you have more room.

Engine cooling systems run at much higher water temperatures over a given time period than a A2W system does. You’re only putting significant heat into your A2W system while making boost. The engine is putting significant heat into its system all the time. A heat exchanger will have water temps in the 5-20deg over ambient range most of the time (on a 75deg day, 80-95deg). Your engine cooling system is at a minimum of 160deg (if you have a really cold thermostat) and is usually running in the 180-225deg range. Because heat transfers faster the bigger the temperature differential a fan on the radiator is far more effective. For example:

If your radiator water temp entering the radiator is 200deg (on a 75deg day) and a fan can remove 30% more heat than no fan you’re looking at a 37.5deg drop in water temps as the water moves through the core due to the fan. This is assuming you’re driving slow enough to need the fan/the fan moves air fast enough.

If your intercooler system water temp entering the radiator is 85deg (on a 75deg day) and a fan can remove 30% more heat than no fan you’re looking at a 3deg drop in water temps as the water moves through the core due to the fan. This is assuming you’re driving slow enough to need the fan/the fan moves air fast enough.

This is assuming that heat transfer is linear, which it is not. In reality the bigger the temperature differential, the faster the heat is transferred. The heat transfer rate in reality is skewed even more toward the engine coolant/radiator.

You can run a far more effective fan on a radiator too simply because you have more room for much deeper blades. The stock radiator fan on your 2005-present Mustang moves about 3600cfm at an air speed of 29.3mph. That is effectively double the air speed you can get out of a HE fan.

There is no doubt that a HE fan(s) “works” under a certain vehicle speed (details/numbers on that below). But if you’re looking at only getting a 3deg drop in water temps (which is actually what you will see for a maxiumim), is it really “working”?

That 3deg water temp drop number in the HE by using a fan is a good number. We have tested this extensively. How much does that 3deg drop effect your IAT? If it effected it on a 1:1 ratio, which is best case scenario (it will never be as good as best case), you’re looking at a 3deg IAT drop. Which is insignificant and also impossible to accurately read at the IAT sensor.

Fan(s)

Fans are the main focus of this article and not nearly as black and white as you would think. So. this section will be quite detailed.

Fan Rating & Real-World Performance

Fans are generally rated as cfm (cubic feet per minute). The first thing to get out of the way is that fan manufacturers measure their own products flow. This should already make you suspicious. There are many, many ways to rate/measure/test the flow of a fan. Is the number based on open flow which is just the fan moving air, no restrictions such as a heat exchanger to pull or push through? We suspect that most are rated as open flow. It will give the biggest numbers for advertising purposes.

Some questions you should have in the back of your thoughts when digesting fan flow ratings:

  • What does that fan flow when it’s attached to a HE?
  • What if the HE is 1” thick?
  • What if it’s 2” thick?
  • What if it’s 3” thick?
  • What if the HE core is very dense?
  • What if the heater core is very open?
  • Will fan blade design “A” lose flow at the same rate as fan blade design “B” when faced with a restriction?
  • What voltage was the fan rated at? 12v? 13v? 14.2v (where most 2005+ Mustangs run the alternator voltage)? Or 16v? Different voltages will give different results.

There are no standard testing parameters that the fan manufacturers all use as a “zero”. So, you can have two fans that advertise the same flow, but one will flow more than the other once you get it mounted up in the place it will be getting used. Long story short, you want to take fan flow numbers with a grain of salt. If their ratings are anything like water pumps (they probably are are) their actual “in use” flow numbers will be quite a bit different than advertised flow numbers. And the same flow numbers from two different manufacturers can be completely different in reality.

Fan Testing

We tested four 10” fans to get an idea of how they stacked up in real life and to find the best performing fan to give the argument for fans its best chance.

The fans were all tested for “open flow” (they were not on a HE). Then tested on our test heat exchanger which has almost zero restriction to the fan(s). The restriction is really small at about 1/4mph. That’s in the margin for error. In this test the HE was not a factor. All fans were tested at 15v which is the only voltage we could get to stabilize consistently. Your 2005+Mustang runs about 14.2v. All of these fans moved a bit more air at 15v vs 14.2v, but it was so small it was inconsequential. All fans were tested using an Anemometer (wind speed gauge). We tested the wind speed gauge against a known wind speed (hung it out of the truck window at 40mph and 50mph) and it reads accurately.

This is the information you will see for each fan:

  • Advertised cfm rating
  • Average measured air speed across the fan face. The air speed across the face of the fan is not the same everywhere. At about 25% from the fan edge you will see your highest peak speeds. From 25% out to the tip of the blades the air speed drops about 25%. From 25% in from the edge of the fan blade to the fan motor the air speed decreases about 50%. And behind the motor the air speed is quite slow at about 20-30% of the max air speed seen toward the end of the blades. We had to eyeball average air speed and make some judgement calls as to the average across the face. Plotting everything out would have taken forever. Our seat of the pants average “calculation” was pretty much spot on with the advertised cfm of two fans, the ones that made their advertised air speed. Conclusion, our testing model and results are good. Here is a quick model of the different air speed zones across the fan. Orange is 25% less than peak speed. Red is peak speed. Yellow air speed drops from the outside edge toward the inside edge. Blue is the “dead zone” where the motor is.

  • Actual cfm rating. We got the actual cfm rating by calculating it using an online calculator. The calculator is pretty accurate. Two fans measured just about dead on in relation to their advertised rating. And two, which we thought had ridiculous cfm claims from the start, measured way off. Long story short, measured wind speed numbers accurately translated into accurate cfm numbers. Here is a link to the calculator:

https://www.engineering.com/calculators/airflow.htm

The Results:

Derale #16910

Advertised cfm – 500

Average measured air speed – 10mph

Actual cfm – 475

Perma Cool #18120

Advertised cfm – 1450

Average measured air speed – 10mph

Actual cfm – 475

Perma Cool #19120

Advertised cfm – 2350

Average measured air speed – 12mph

Actual cfm – 575

Spal #30100435

Advertised cfm – 802

Average measured air speed – 17mph

Actual cfm – 815

Clearly there is some funny business going on with fan ratings, we’re not shocked one little bit. It’s nice that two performed pretty much right on target because that verified that going from air speed measurements to cfm using the linked calculator works. It also verifies all of the findings later on in this test are sound because the measuring instrument and calculated numbers are solid.

For the rest of this article we will be using the Spal fan as the benchmark for “best fan” giving the “for fans” argument its best chance.

Understanding CFM ratings & How They Work In Reality – They’re Deceiving

The fans flow rating can be very deceiving too. The rating is cfm, that is pretty easy to understand (cubic feet per minute). But, 1000cfm with a 9in fan is a lot different than 1000cfm with a 12in fan. Sure, they move the same amount of air. But, the air speed will be slower with the bigger fan. Equal air volume through a bigger hole equals a slower air speed.

Here are some examples of how cfm and fan size can get confusing. These are the top performing fans that we could find in these diameters that are thin enough to be used as a HE fan. If you are able to run a thicker fan, and therefore deeper fan blades, those cfm numbers go up significantly for a given diameter. But you only have so much room to work with when putting a fan(s) on a HE so this is what your able to use.

We used an online engineering calculator to calculate these MPH ratings. And we use the same calculator for other bits of data throughout the article. It could be argued that the model this calculator is not an exact representation of a fan. We did check actual air speed on a few fans and compared it to the online calculator and the results were so close they were in the margin of error. Conclusion: The online calculators results are solid.

https://www.engineering.com/calculators/airflow.htm

NOTE: These fan ratings are based off of advertised numbers, not what we have tested them at (except the 10” fan Spal we’re using for all future 10” fan data).

The results are surprising:

10in fan – 802 cfm -17mph

11in fan – 844cfm - 14.5mph

12in fan – 909cfm – 13.2mph

16in fan – 1300cfm – 10.6mph

Interesting huh? The cfm rating doesn’t translate directly into more airspeed. This is because even though the fan blades are getting longer, they can’t get “deeper”. And a “deeper” blade scoops more air.

Be careful with trying to equate air speed with cfm ratings. There are a lot of variables in play before you can get to a mph through the core result. Which is what matters to you. It’s really kind of irrelevant though. After you finish this article you will see that even if fan driven air speed through the core was doubled (which it won’t, because there are no such things as fans which will fit that move air that fast), it still isn’t enough air speed to justify the use of fans. Especially when you take their penalties into account.

Effective Fan Area

This is a very important factor. And a little complicated. There is going to be some ideas/math in here that is important to understand when it comes time to analyzing the results.

The fans don’t pull air through the core evenly everywhere. The fan pulls air through the core in front of the fan blades out to the inside edges of the shroud. Not through the entire face of the HE. Here is an example of the effective fan area:

The “face” (the front side) of this HE is 22.5”wide X 11.375”tall for a area of 256sq in. That is the total amount of area that air can pass through. The effective fan area of those two 10” fans is 157sq in. That means 39% of the HE is not getting air pulled through it by the fans. We feel the only fair way to rate how fast fan driven air moves through the HE is to average it across the entire face. An extreme/impossible example to explain the reasoning is if you had a 1” fan that moved air through the core at 100mph you couldn’t honestly say that the fan moves 100mph through the entire core now could you? That would be ridiculous. If the fans move the air at 17mph (the best performing 10” fan) your average air speed through the entire core is only 10.4mph. Or in plainer terms, the fans only replicate 10.4mph in real world air speed through the entire core. We used the online calculator linked above to come up with this number.

Test Data & Results

We devised a test to collect more data on how the HE works at speed, how it works at speed with the fans on it, etc. With our professional stunt driver behind the wheel, driving in our top secret private test facility, I rode in the bed of my truck standing up (helmet on), looking over the cab, so I could take readings behind and around the HE I put up there. We found out some interesting stuff. We wish we could test this faster but 40mph was the limit for how much room we had/safety/etc. It would have been nice to have access to a wind tunnel that day!

We used a HE with one of Griffin’s high flow 1.5” single tube cores. Basically their “race core”. At 1.5” thick it’s quite thin compared to all the big heat exchangers out there. So, this is going to be best case scenario for core resistance to air flow.

We used a top end Spal 10” cfm puller (better than a pusher) fan. “But some HE’s have 11” fans that move more air” you say? We just went over that above. A larger diameter fan doesn’t necessarily mean it moves air faster through the core. In fact, most of the time the larger fan moves the air slower through the core.

Air Speed

Air speed in front of core – 38mph
Air speed no fan behind core – 26mph
Air speed fan behind core – 21mph

Air speed in front of core – 28mph
Air speed no fan behind core – 19mph
Air speed fan behind core – 15mph

One thing to bear in mind is that the HE was placed right on the cab of the truck (the aero of the truck was not giving false readings at these speeds). Effectively the HE was out in the open. The air could spill around it pretty easy. When the HE is in the front of the car there is a “pocket” of air formed in front of its face. The faster you go the bigger/deeper the pocket gets and the more air is forced though. In theory this will be on a curve going up. We can’t test this though. In theory with the HE mounted in the car you’re going to force more air through the core. And the faster you go, the more efficient it will get. There are of course a ton of variables in here though that we also can’t test. Like the aero of the grills in the bumper cover and how they effect airflow through the core as one example.

Clearly you get more air speed through the core when you don’t have a fan behind it. The fan absolutely does hinder air speed. The data shows that it hinders air speed going as slow as 28mph.

Cutting the data in half:

Air speed in front of core – 14mph
No fan behind core – 9.5mph
Fan behind core – 7.5mph

We know that data isn’t going to pan out because the fan moves the air at 10.4mph through the entire core. So that is going to be the minimum air speed. We can look at this data and extrapolate that fans moving the air through the entire core face at 10.4mph (which is our best-case scenario) is roughly replicating the vehicle moving at 14mph because the fans are moving the air through the core at almost the same rate as 14mph of vehicle speed (10.4 and 9.5mph respectively).

Core Flow Restriction Due To Blockage From Fans

So far, we have looked at air speeds that fans generate and by extension at what vehicle speed they’re effective to. 14 whole miles per hour best case scenario. What we haven’t looked at yet is the flip side of the coin. That is, how much do fans hurt flow through the core? There are no two ways about it, the fans simply being present restrict airflow through the core up past the speed at which the fans move the air. Any restriction in front of or behind the core effects air speed through it. And the fans are a really big restriction in regards to surface area. Then you have to factor aerodynamics and its nature in. Let’s looks at the data from above again.

Air speed in front of core – 38mph
No fan behind core – 26mph
Fan behind core – 21mph

Air speed in front of core – 28mph
No fan behind core – 19mph
Fan behind core – 15mph

Already at a vehicle speed of 28mph the fans simply being present are hurting flow through the core to the tune of 4mph (the difference between fan and no fan behind the core). This represents a 14% drop in air speed through the core. At 38mph the fans are hurting flow through to the tune of 5mph. This represents 15% drop in air speed through the core. Because of the way aerodynamics works this percentage will keep going up as vehicle speed increases. And it’s not linear, it’s exponential. What is it at 60mph? 75mph? 100mph? We would be guessing. But a 14% penalty at 28mph is already a big hit. What if it’s 30% at 75mph? It could be.

Now to be 100% consistent we have to factor in that the fans don’t cover the entire core face like we did when we factored it for fan effectiveness measured as an average through the core. Here is a picture of the HE we have been using as an example without fans.

As you can see to mount the fans there is more blockage to the core than just the fans themselves. And this is one of the better ones. Many HE’s have more blockage on the back side. We also need to consider that because of the way aerodynamics works there will be more “blockage” than the size restrictions represent. The air has to move around those blockages which increases their effective size. We’re not going to go into a whole “thing” analyzing this. It’s pointless. It’s not like the argument for having/not having fans is even close to going the way of “have”. Even at best case. So, we’re taking an educated guess based on our “eye-crometer” for this one. The fans cover 61% of the HE face. We’re guessing another 29% is compromised by the brackets/mounts/aero/etc for a total of 90% (see what we did there? Easy math). Take a look at the pic of the HE again with the fans on it. How much free flow area does it have? It’s not much. 10% is probably just about right.

 


If you take the total restriction area and math it out over the entire face of the HE with the restriction caused by the fans you’re probably looking at a total average net restriction across the core of 12%. And that is at only 28mph. It’s going to get worse as you go faster.

Almost all of the high-performance cars out there have flaps that open at speed across the face of the radiator fan shroud. The manufacturers didn’t spend all that time and money putting them in there because they don’t work. They put them in there because they dramatically increase airflow through the radiator at speed……because they reduce the restriction on the back side of the core. You can’t go without a radiator fan on your car so they developed the flaps. You can go without fans on your HE.

Here is the bottom line, up past a best case of 12-16mph vehicle speed fans simply being there are hurting you cooling performance considerably.

Conclusions – Some Opinion – Questions - What Did We Learn?

-Fans will only move enough air through the HE core to replicate a best case scenario of a 12-14mph vehicle speed. That’s absolute best case with every variable pushed toward the fan(s). In reality it’s not going to be that good. We guess it’s more like 9-10mph. It’s irrelevant though. Anywhere in that speed range is not impressive.

Additionally, keep in mind that we used the most effective fan at generating air speed for this test (10” Spal). Simply moving to an available 11” fan will give you a best case of a 15% drop in air speed through the core. Some HE’s have 11” and 12” fans on them. They will perform significantly worse than the examples in this article.

-Fans only reduce water temps (when they’re effective under 12-14mph) 2-3deg best case. Which translates into a IAT drop of 2-3deg best case. Which is nearly nothing.

-The fans hurt airflow through the core a minimum of 12% at 28mph and it gets worse as the speed increases. 12% is a LOT of percents!

-Even if you were to use fan(s) that were twice as good (which isn’t possible) you’re looking at them only replicating a best case of 28mph vehicle speed. That’s still not great.

-Does the best case scenario of a 2-3deg drop in IAT’s with the use of fans under 14mph offset the damage they do at every speed above that? We think no.

-Does the best case scenario of a 2-3deg drop in IAT’s with the use of fans under 14mph offset the cost of fans? We think no.

-Does the best case scenario of a 2-3deg drop in IAT’s with the use of fans under 14mph offset the amperage load on your alternator? We think no.

-Does the best case scenario of a 2-3deg drop in IAT’s with the use of fans under 14mph offset the noise they make? We think no.

-Does the best case scenario of a 2-3deg drop in IAT’s with the use of fans under 14mph offset the HE size and or design considerations that need to be made to fit those fans in there? We think no.

Cost/Benefit

At the end of the day it comes down to a cost/benefit analysis. In a perfect world you would have HE fans that didn’t cause a degradation in performance up past 12-14mph (absolute best, best case scenario). But that is impossible. So, a cost/benefit situation arises.

What are you gaining by having fans?

Best case you’re gaining a 3deg drop in IAT under 12-14mph. That is fantasyland best case. We’ll go with that though. That is all you have as far as gains go. And for what? Your IAT’s under 12-14mph don’t matter one bit. The only time your IAT is important is when you’re in the boost. While cruising around your IAT’s can be 225+deg and it’s not hurting anything. In fact, you get better gas mileage. What you see for in boost IAT’s have very little to do with what you’re seeing while just cruising around. You’re in boost IAT’s are made up of ambient air temp, the temp rise due to boost, the water temp coming out of the HE and the water speed. Very, very little of your in boost IAT is made up of what the intake manifold and blower temps are. “Heat soak” of the hard parts is a very small fraction of the heat making up your in boost IAT.

You could argue that the 3deg lower water temp while cruising with fans matters when you get into boost. And you would be correct a little. That could turn into a 3deg lower in boost IAT for the first couple of seconds that you’re in the throttle, you’re under 12-14mph and the first time the water circulates through the system. If you’re making a 0-120mph run that takes 11.5sec how much of that run was helped by the fans? 1-2 seconds? At what cost? That’s below.

What are you losing by having fans?

  • You’re losing a bare minimum of 12% of your airflow by 28mph. How much is it at 60mph? We can only guess based on some math and peripheral data, but another 12% for a total of 24% is not unreasonable. This loss of airflow is happening exactly when you need it most, when under boost. We were not able to effectively test this to the level of “brass tacks” but when we took all of the findings, data and observations we have amassed it’s reasonable to say that by 45-50mph your water temps are 3-4deg lower in the HE if you don’t have fans. Why would you give that much temperature up when you need it (under boost/moving) to gain a little airflow with fans when you’re cruising at low speeds and IAT’s don’t matter?
  • You’re also losing packaging/performance advantages. Most HE’s are right up against the back side of the bumper so fans can be fit in behind them. How much airflow are you getting THROUGH the bumper and then through the HE? Not much. What happens when you can pull the same size HE 2.5-3” away from the bumper because it doesn’t have fans on it? The answer is a lot more flow through the previously shrouded part of the HE.
  • You’re losing the cost ($$$) of the fans.
  • You’re losing the silence you get from not having two fans spinning their guts out (most of them can be heard in the car).
  • You’re losing the lower amperage draw on your alternator. Which ultimately translates into HP.
  • There are actually a lot more subtle losses due to heat transfer rates in the system but the background needed to go over heat transfer rates and their effects is quite complicated and would need its own article.

In conclusion, the choice for/against HE fans is a cost/benefit analysis for the end user. There is not going to be a one size fits all perfect solution for this. Our opinion is that the costs of HE fans far outweigh the gains in 99% of situations.

Other things of note to chew on:

The only situation where we can see having fans would help is at the drag strip for cool down between runs and through the burnout and staging. We would bet money that it would be more effective to run the pump with the car off in the pits to cool down though. You can make the argument to run both. But you will smoke your battery like that. You would need a generator, a place to plug in and a converter to go to 12v to pull that off. How many people/cars are we talking about now? Not very many go to the strip. And how many are going to be able to “plug in”? At that point just bring a damn fan to the track that runs off 110v and aim it at the front of the car.

We ran the at speed tests with the fan on and off. The fan being on or off didn’t change the air speed enough to be detectable (standing in the back of a truck). Maybe in a static test we could nail down a .5mph swing or something. We think it’s irrelevant though. As the air speed through the core goes up past the fan speed it starts to free wheel. It’s possible at 100mph that the fan has some drag just freewheeling. We can’t test that though.

A trick we learned with our 2007 shop car was to seal the HE to the A/C condenser and use the pull from the radiator fan to move air through the core of the HE during slow speed operation. That’s what the radiator fan is there for right? Slow speed cooling? And the radiator fan is a BEAST. Why add more fans to the car to cool the HE when you can simply seal the HE to the A/C condenser and get the same effect? You get all the advantages of having a HE fan(s), with none of the downsides. We have set up our soon to be released HE to work just like this. It uses the radiator fan to move air through the HE at slow speeds.

We have been doing a lot of heat exchanger, fan and water pump testing with vehicles for almost 10yrs now. We’ve known for a long time by tracking IC system water temps that HE fans are nearly useless, and a lot of the times damaging. We have seen hours, and hours and hours of data on the dash in many different situations to base our findings on. But, to really answer the question we had to go backward and “bench test” a few key things. Usually you would bench test first. Again, we already knew fans weren’t the way to go. But after we did the rest of the testing and put all the data in one spot did we realize just how bad of an idea they are.

We can’t imagine anyone looking at this data and while being intellectually honest make the claim that fans are a good idea.

We hope this article shed some light on heat exchanger fans for you.

Thanks!

Department Of Boost

 

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