Yeah, but different fanclutches have different rate of lockup to rpm ratio, and have different total percentage of lockup. If they make an electric fanclutch with 100% lockup we could run, it would be magic.
Thats the only reason I didnt jump onto the ford one ak diesel driver found and is using and waiting for Leroy's testing to be completed on the actual clutch system.
When overheating down the hiway and getting hot, the idea of downshift for more rpm helps drive up fan speed to pull more air(and more coolant flow). While it is a bit of a problem for gmt400 trucks, hummer/hmmwv airflow on freeway is WAY worse. Loosing 500 or more rpm on the fan is huge.
I havent mentioned it much because it doesnt apply to this forum much at all, but in the fleets I wrenched on and ran, many class 7&8 trucks were there, as well as heavy equipment. My ase master certs were auto, med& heavy duty rigs, truck equipment. So yeah, quite familiar with slip style fan clutches in rigs. I still have multiple master keys for Pete's, KW, frieghtliner, Sterling, etc. Life long friend is head of a republic services fleet, and keeps me aprised of many of the headaches as they arise just in general conversation.
They are a neat concept for some areas of the country, with rigs that have a properly oversized radiator that does not need the fan engauged much down the hiway. When the radiator btu/h is sized too close to engine btu/h output like gmt400 and hummers- not as good. After loosing many engines from overheating in big rigs, many fleets in the southwest, including republic services, has had to retro fit air fan clutches with 100% lockup in place of the slip style clutches.
The other huge factor for heat transferance is humidity. Having been born in Tallahassee, and visiting there and many areas over the years, I know a touch about humidity, besides what we learned in science class. Humid air on the radiator increases it's thermal convection at an amazing level. Think how good does an air cooled engine shed heat vs water cooled. Radiator is no different. Dry air is an insulator.
When an OTR rig goes through dry& hot southwest, the drivers has to back down and take it easier to not overheat. They hit areas with more humidity at the same temps and it's hammer down. Wont overheat near as easy because raditor starts working twice as well.
Gmt400 inital hot climate testing was not done in the current dry climate area they use. That is simply why they goofed up. Ford 6.9 & 7.3 did their testing in dry Arizona and quickly beefed the radiator for the trucks vs the gas engines. GM stuck their head in the sandand was expecting to dump the program almost yearly.
AM General-hmmwv were never designed to drive beyond 55 mph. The airflow from aerodynamics of the verticle windshield and angled radiator kills it. Until your above 85% throttle for long hiway hills there isn't much of a problem in hmmwvs overheating. Once head wind or foreward speed combines to 70 mph, over half the incoming airflow is lost. Hence the hood scoop I helped design down in the Cali NASA wind tunnel that was turned over to the USMC. My dads envolvement in gubmint fly fly world got me into contact with a couple NASA engineers, and a good friend in charge of a USMC base wheeled vehicles fleet. So it all worked out good, but the best improvment of getting a bigger, more efficient, verticle radiator was the final answer, along with a 100% lockup fan clutch system.
