The 3 blade 21 inch should fit with just a 1/2 inch clearance around the OEM shroud! If hutch is correct in our 5.9's needing around 2100 cfms pulling thru the radiator, I would say this 21 inch fan capable of 4300 cfm should be able to pull 2100 cfms across the radiator then?

 

I would agree it probably should. When you see anyone publishing numbers that have something specific related to motor size, such as they do, they've probably done some testing and know what it's capable of doing. Not sure what they are charging for the fan though, and if it's 300+ you should look around a bit for something cheaper. My push is going to be to get the industry to start advertising fans at .3-.6, so you'd know that 4300 cfm fan is capable of 2100. Industry is a joke right now.

I have 2 really good industry insiders that work for major aftermarket manufacturers. They've tipped me off that only about a third of the industry does true testing, and a lot of the crap that comes in from china is knock off stuff that no one has ever measured. Have one of those import fans waiting for me to get testing in my possession already and expect it to flow a 1/3rd of the advertised numbers. Already have one manufacturer inquiring about pricing for me to start working on their fans.

I won't be able to get seriously into my airflow testing until next weekend because my power supply didn't get shipped on time, so hoping I can uncover some quality fan choices in the next couple weeks. While I do this for a living, used to dealing with power sources that range from 24 volt to 480 volts, so I don't have a small enough power supply. Since I'm making myself liable to suit by saying brand 'x' blows, want to make sure everything is tested properly.

edit: related to the 'size' of the fan. Blade count/pitch/surface area/rpm are fairly generic factors related to actual airflow. You can have a fan blade of 5 fins outperform one with 9 fins if the design is better. A lot goes into fan design and it's not really easy to look at a blade and know it's better then another. I test at least 500 fans a year and really don't know how they'll perform in their installed environment until I dig out the test equipment.

 

Something to keep in mind. I don't know what the towing rating of the Durango is, but if 7300lbs falls into the factory rating, then it's not relevant to fan sizing because it would already have been figured into the specs off the initial chart I posted. If it's outside of that range, let me know how much and I'll do some digging for a better cfm number.

Towing tends to be thrown around a lot with trucks and the fan rating, have to admit I don't know why. Motor doesn't know the difference of having an extra load on the backside outside of running at higher RPM's. Unless you are getting crazy with your loads, and running at higher rpms at very low speeds, shouldn't really make a difference.

Feel free to rip me apart on that if I'm missing something though. I don't tow that much. I work in airflow testing and engineering, that's my expertise and pretty much all I know...lol

 

I have the upgraded cooling package on mine.

5.9L, 46RE trany, 2 trans coolers 11", 392 gearing, also cooler fins on everything else.

The problem here is that it doesn't list the REAL truck packages and their intended use. I am one to bet the farm that if the clutch fan is engaged fully and your pulling 3500 rpm with a full load that CFM with the fan I have, it's gotta be in the 4000+ range.....IMHO

Now not engaged 1500 or 2000 or so. But a fan like what I have moving at that RPM?

 

Problem with that is no one publishes real fan ratings for the factory fans. It's hard to measure what a fan is doing at 3500 rpm because at that higher rpm, you obviously are moving and the fan is getting 'boosted' by the air entering the radiator. When I measured my truck, couldn't really get more then 700-800 cfm at idle, and when I applied fan laws to get it up too 2000 rpm, that's how I figured I needed 2k before really finding the info.

Understand your fan is only really working at lower speeds and rpms, and once you get moving, the air moving across the radiator is doing most of the work. The air volumes being provided at higher speeds are way beyond what a fan could even think of doing.

Look at the factory tow rating of your vehicle, and if it's under the 'max' rating, you don't need to worry about getting more then the 2k through the grill specific to towing.

 

Hutch

It sounds like you measured your fan without the fan clutch engaged for those numbers and not fully engaged/

Also do you have the same fan and radiator as I do?

 

Believe you have a different sized radiator/fan, and the fan was engaged. Numbers seemed low to me at first as well, until I started to see what others were rating for airflows, then it seemed to verify my measurements.

From what I've read on dimensions between the radiators, I don't see there being a huge difference. Again, after market numbers seem to verify this since they are specifying the same airflows for your gen Durango. Also looks like your tow rating is right around what you are towing, so probably won't be a huge issue with fan sizing.

If you want to add cfm because of your motor mods, or future mods, go for it. Worst it can do is pull more amps and kick off/on more then needed. Then it's a matter of knowing your charging system can take the hit. If you decide you want 2500 to 3000 cfm of airflow, that's your choice, just make sure you get a manufacturer to tell you their fan will move that amount of air at .5" s.p. If you buy a 3000 cfm fan at 0' s.p., you'll probably be back down closer to 2000 cfm anyway.

Had a fan rep tell me the most common question they get is 'how much cfm' for their vehicle. When they say x amount, they have a fit because the box on their fan said 6000 cfm, which is ridiculous. One more thing I'd say on this, thinking the cfm's get thrown around, 2000, 3000, but you probably don't know how much that really 'is'. For reference, if you live in a house with a single furnace, most it can flow is about 1700 cfm.