Radiator Fans - Technical Information
 

Figured I'd follow up the oil cooling and the charge air cooling posts with a radiator fan primer. Most people overlook this fairly crucial system of the cooling system in exchange for space, but for cars that idle or are operated at slow speeds, fans that flow well are a necessity!

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Synopsis: Radiator fans are a necessity for properly cooling vehicles during all conditions. They are a very simple component but are often overlooked and miss-understood. This post will go more in-depth with radiator fans; when/where they are beneficial, how they work in the overall cooling stack, common misconceptions, things to look out for, and more. In this post, we will be focusing on electric fans as that is what is typically used for sports cars and late model cars in general.

When are radiator fans helpful? This is dependent on vehicle and cooling stack thickness, but generally, electric radiator fans are only needed up to 20-30MPH road speed. What can influence this is grill opening, cooling stack thickness, outlet ducting, and the fan’s characteristics. After this speed, air speed and pressure is sufficient enough to out-flow an electric fan’s capabilities.

How do electric radiator fans operate? Electric radiator fans have a pressure vs. flow curve that governs its performance. This curve is dictated by the motor and the aerodynamics of the blades.

Any manufacturer that states a single flow rate without a pressure reading is trying to sell something. To take this a step further, stating the flow rate of a fan at zero static pressure is completely and utterly useless in understanding the fan’s true performance! A fan’s job is to pull air through something, this something will ultimately create a restriction or a pressure drop. A fan will always have to deal with a restriction. Below is numerical data of a SPAL fan showing what is called, in the cooling industry, a fan curve.

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So what is the difference between slim fans and regular fans? Typically the aerodynamics of the blades is very similar if not the same, but the motors are much less powerful. Due to this less powerful motor, the fan cannot pull through high static pressure. Below is this illustrated for the same 12” fan, but one is a slim fan, and one is the high-performance variant. As can be seen, the low profile unit drops off under higher static pressure, significantly. The high-performance unit continues to pull reasonable amounts of airflow even after the low profile unit stalls, pretty impressive.

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What is a typical static pressure? A typical radiator core has around 7-15 mm H2O of static pressure when the fans are pulling air through them depending on its thickness. A thicker radiator will inherently have a higher static pressure unless some trickery is done with fin heights and louver details. We’re going to assume standard cores, fin heights, and louver details though as very few core manufacturers have the ability to vary the fin geometry. What compounds this pressure drop is thick charge air coolers, which block airflow further.

So what does this pressure drop look like and how do I figure out how much airflow my fans can pull through my radiator? That is most certainly a difficult question to answer. We’ve created a graph, and while it is indicative of real-world behavior, it is a made up scenario for this specific case. Heat rejection, in general, is still a bit of a dark art to most, as there are a lot of factors most people do not consider. Below is a graph of what a 27mm Denso core (what we use in our radiator) would typically flow vs. a given static pressure, and a more standard used 56mm core.

https://static.wixstatic.com/media/b...562422~mv2.png

There are some neat things we can learn from this curve.

1. Low profile fans pull dramatically less airflow through the radiator cores than a high-performance fan.
2. Looking at the 27mm core, a low profile fan would pull around 650 m3/hr of airflow while a high-performance fan would pull around 1100 m3/hr. That’s a significant difference in airflow and one that will most certainly make an impact on low vehicle speed cooling.
3. We can clearly see that the 27mm Denso core flows more airflow for a given static pressure. This makes sense, as the core is thinner. From past experience, we know the 27mm outperforms the 56mm core in heat rejection as well, which is pretty impressive given that it is under ½ the thickness.

Major Take-Aways:

• Fan ratings at 0 static pressure drop are useless for determining actual operating performance
• Low profile fans dramatically reduce cooling at low air speeds while reducing size significantly
• Fans are truly only beneficial at road speeds under 25-30MPH

As always, feel free to ask questions that are not fully answered.

Thanks for your interest and time,
Eric

I wanted to follow up on this thread and see if anyone had any questions I left unanswered.

Thanks,
Eric

Eric: imho shouldn't hurt to add specs of stock fans and nfo about how on twins fan regulation by ECU is done and what specs/features fan might need to work well on ours (for example - does it need to be w. PWM support or not and so on). +maybe link to this thread.
Absence of questions may also mean that your post was well written enough :)

No one has specs of the stock fans to my knowledge. Do you? I wouldn't mind looking at the fan curve, but I couldn't find anything when I went looking.

This was meant more as a primer as well. Not specifically for this chassis but applicable as most people are modifying and changing their fans.

Thanks,
Eric

I supercharged my gt86 and changed the radiator to Jackson Racing dual Radiator + oil cooler v2
I also added Mishimoto low temperature thermostat valve.

The temps are OK and well controlled 78-94°even under spirited driving but the moment I get into heavy traffic or idle it creeps up relatively fast to 100 degrees c°
So I thought it's not a brainer, definitely the fans are not pulling enough air through the main radiator, Air-conditioning radiator and SC intercooler.

I need your help on what are the best fans + shroud combo to ensure perfect cooling under lower speeds / idle.

Thanks in advance for your kind help

Unreal

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Just geeked out to this...
 

Wow... that was an awesome post. I wish they had fan curves on the box when I buy them. Fun fact: the low profile fans and high performance fans have different curves because they utilize a different fan blade type. The low profile fans are called "forward curved" fans and the high performance fans are direct drive plenum fans.

Forward curve fans operate well at steady state speeds but are less efficient and experience flow seperation at higher rpm and eventual stall. The plenum fans are able to hold higher pressures and also typically have airfoil blades which also help with efficiency

Nick
AM2 Racing

Go with the largest, most powerful fan you can fit in there. Ensure it is ducted to the radiator.

You may have a misconception that more cooling is better. That is incorrect. 100 deg C should NOT be considered a problem. In fact, that is a good temp to be running. If you keep your engine below 100 deg C, water can accumulate in your oil system. Having your system hit 100 degrees regularly should be your goal. If you are regularly getting above 125 deg C, then you may have a problem, but not before.

Hello Eric,

I'm sorry for bumping this up after almost 18 months...

The overheating issue turned out to be due to a partially stuck Mishimoto Thermostat.

First- I changed the thermostat back to an OEM one, and it solved the temperature creeping up.
Second- I installed Mishimoto aluminum fan shroud with two SPAL VA10-AP70/LL-61A12V fans over Jackson Racing 2in1 radiator + oil cooler v2.0. I plugged them directly into the oem fan sockets. And they're doing a terrible job cooling the car while it's in low speed or stuck in traffic.

- My questions are,

1- why I'm I getting poor fan performance much worse than oem?
2- Do I need special relays in order for the mentioned fans to perform at full capacity?
3-I was think that NG of buying your quad hood vents + the radiator duct. I'm just worried the latter won't be compatible with JR 2in1 Radiator.

My goal is to have a stable coolant temp around 88-90C° in all different driving situation and with ambient temperatures of over 45C°

Thanks in advance for your kind help!

BR,

Abdalkarem

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I'm not sure what is going on with your specific case but that should not be difficult. Our radiator and fan combo were able to cool my BRZ to those temperatures in 130 degree ambient heat in Arizona with a large front mount intercooler.

Is the radiator ducted well? Are the fans turning on fully? Is the radiator a quality unit?

I know with our install, the fans need a relay as the fans pull 25 amps, and the factory circuit is rated for 15 if I recall correctly.

The Mishimoto shroud+fans flow worse than stock. That is the issue.

Edit* saw you were using SPAL. However the shroud is still part of the issue.

Thanks for your reply Eric...

1- what temperatures did you achieve in Arizona? Because the heat in Riyadh is comparable to Tucson, AZ.

2- I didn't use a dedicated relay, and I believe that's the culprit. How can I calculate the needed relay for the mentioned Spal fans?

Thanks



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[emoji53][emoji17] Are you sure the shroud is part of the issue? I paid a lot to get yet another shi**y Mishimoto product on my car... First their rubbish cold thermostat that got stuck in less than 1.5K miles and now you're telling me their shroud is also problematic?

I'd be very thankful if you could explain to me how a mere shroud can negatively affect my cooling system capacity?

Thanks again for your help.

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As I said, 130 degree ambient.

Look up the SPAL fan you are using, see what amperage the fan pulls, base wiring, relays, and fuse off of that.

If the shroud is designed poorly, it can reduce airflow through the radiator at speed.

Everything is ducted well in front of the radiator correct?