Got a great question via PM an decided it would be good to post.
On upsizing radiators...(and then a discussion on fans, because the answers are connected)
ALL ELSE BEING EQUAL - yes, a larger (deeper) radiator core, as long as it still has enough tubes to provide an increase in surface area over stock, would improve the cooling system capacity and bring down your peak ECTs (once the thermostat is fully open of course).
However - all things aren't equal. If you double the core thickness and keep the fin spacing the same, you'll double the airside pressure drop required to pull the same volume of air through it. The fan however has a performance curve and will provide less airflow at increasing pressure. You end up with a line that slopes down to the right for the fan and down to the left for the radiator, and they operate at the point where the lines cross.
This image illustrates it where the "system" curves are the radiator. You could think of SC1 as the stock radiator and SC2 as the hypothetical thicker core, and think of one of the N curves as the stock fan at a given speed. Intersection point C is where the stock setup operates, and with the thicker core you move to intersection point D.
Note the pitch of that fan curve and how it flattens out to the left. That means as you approach the maximum pressure capabilities of the fan, very small changes in pressure can have huge changes to air flow. Since I didn't select the cooling fan for the TJ, I have no idea where we even are to start with on that fan curve....if we're out farther to the right we might not be too sensitive and not lose a whole lot, but if we're already in the flatter section, we could lose a LOT. The impact of reducing the airflow means that you're trying to distribute the heat into a smaller mass of air, which means it heats up more as it passes through the radiator and that affects the dTlm I described in the first post. You lose some of that while you gain surface area, and the result may be that you gain nothing, you gain a small amount (but less than proportional to the gain in area), or you could actually lose some cooling system performance.
Highway driving takes the fan out of the equation but the same physics are at play...your vehicle speed and the air resistance through the radiator generate a pressure differential across the radiator that will push the air through it, but there will be a balance point and driving faster isn't always an option (not just for safety/legality, but drag force, and therefore engine power required, increases with the square of speed so the heat you have to reject will outrun the capacity of the radiator).
The N1 vs N2 curves as shown above would be analogous to increasing the speed of the same fan or scaling it up in a way that maintains the geometry. It will generate more flow and more pressure as you speed it up, or less flow and less pressure as you slow it down. Different fan designs change the shape of the curve entirely, which could make it flow more or less than the stock fan at different pressures.
This begins to go off on a tangent but this plot actually compares the different curve shapes for different types of fans. You can see an axial fan, which is what an automotive cooling fan always is because of it's form factor, is the worst at dealing with higher pressures. This is why you see the hamster wheel looking fans (mostly "forward curve centrifugal") in pretty much any application that involves more than just a heat exchanger...if there's any ductwork at all, that drives the system curve steeper and you need a better fan to push/pull through it.
Further fan notes:
The wonky looking lopsided blade spacing is likely for noise. Evenly spaced blades tend to create a "blade pass tone" as they pass over nearby objects. Spacing them out unevenly "smears" that focused tone frequency into a dull whoosh, because you're hearing multiple frequencies superimposed on one another like mashing a bunch of keys on a piano instead of a single, much higher key, more loudly. Fans like the commonly used Explorer, as well as most electric fans, have swept blades, which also help with blade pass tones since there's not an entire edge passing the same place all at once.
The tip spacing (space between the blade tips and the fan shroud) is positively atrocious but it's a reality when you have a fixed shroud around a fan mounted to a torquey engine mounted in rubber. This is not doing us any favors from a flow vs pressure standpoint, but the only practical way to help that would be an electric fan.
Electric fans:
I think someone mentioned in Jezza's bolt-on dyno thread that Engine Masters did some dyno runs with and without a mechanical fan and found something crazy sounding like 14 horsepower. Scale that down to normal driving and it still could easily be pulling 3 hp, which even with a 100% efficient motor (doesn't exist) would draw
188 AMPS from a 12V electrical system. My advice for electric fans:
1. Don't do it thinking it's going to improve your cooling system. Best you can hope for is that it doesn't make things noticeably worse, so have a good reason to do it, such as packaging for an engine swap, noise reduction, etc.
2. get something similar in size and from a high-hp OEM application - they typically provide a lot more air than the junk off the shelf, even from the same manufacturer.
3. be aware that cars with electric fans were designed that way and the radiators are likewise designed to be applied with an electric fan with considerations for the different flow vs pressure situation therein as well as having the ability to run full speed at idle, etc.
