Wow... a ton of suggestions and more. OK, how about I post some pix of the setup. That might help. But first a simple explanation of the PWM setup. It is not infinitely adjustable. The manufacturer offers 160°, 180°, and 195° sensors that are attached to the radiator tank next to the outlet hose neck. My current thermostat is a 160° EMP/Stewart and I just switched last week from a 180° EMP/Stewart. Haven't noticed much of a change in the situation, it just takes a couple of minutes more to get to that 195° level where the fans kick in and don't stop.
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The shot above shows the engine side of the custom aluminum shroud which measures 23" wide and 16" high, same as the radiator core. The two fans are 12" and the fan assembly is out of a Ford Contour SVT. Supposedly with zero static pressure, they can pull around 3500CFM total. That means maybe 3000 CFM behind a radiator and AC condenser. There's no official rating, that's a guesstimate made by numerous people who have used these fans on other rides compared to supposed known CFM dual 12" fans.
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This shot shows the radiator side of the shroud. I have a dozen BeCool rubber pass thru flaps mounted so that at highway speeds, I can get more airflow through the system. At slow speeds the draw created by the fans keeps the flaps tight against the shroud so its not pulling engine heated air in. The shroud is sealed around the perimeter with a dense cell foam (I used a corvette kit for that).
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Between the big block and the radiator, there's not much space when you run a serpentine belt system. I don't think this affects the heat but I'm showing the clearance to point out that there isn't much room for conventional fans and factory shrouds. Likewise, I choose this fan setup because others have used them on similar big blocks with good results. So if I switch fans, I have to be very concerned about depth.
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This shot shows the shroud without the fans in place. Theres about 15/16" of air space between the inside of the shroud and the radiator core. The way its set up now, I can lift the entire shroud/fan assembly out with minimal wiggling.
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This overhead shot shows the entire layout of the engine bay and the radiator closeout panel that supposedly helps the road air on a path through the condenser and radiator.
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Not the best shot to show the closeout panel removed, but you can see the plumbing for the condenser before the condenser was actually fitted in there. Its almost the same size as the radiator core, maybe 3/4" thick? One option is a "pusher" fan mounted in front of the condenser switched on at 195° to increase airflow.
The way the PWM is setup, I can't switch the fans off at 200° while riding on the highway to see if airflow alone would do the job. In theory, a belt driven clutch fan would not be normally engaged at highway speeds and you would rely on a 45-65 mph breeze to flow the radiator properly. It sounds like changing the coolant mix would have little to no effect so that probably points to more airflow via different or an additional 3rd pusher fan. But keep the comments coming, its interesting to see all your thoughts.-Mike
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p.s. here's my beast.