Operating temperature difference - what causes it?

I've got an '88 XJ 4.0, and I've had two different motors in it, one original and the other is an '89 motor, so both non-HO, stock head, intake, the whole deal, both also had two different 195* t-stats. Both motors run under 190* according to my Auto Meter temp gauge, the newer lower mile '89 motor barely breaks the 170* barrier once its fully warmed up (about two miles runs up to 180*ish then comes back down), and actually goes down more when I kick the heater on. The older original motor would hover around the 190* mark, which was tolerable. A little more background info; replaced the factory single row radiator with a two row for a '94 XJ and at that time I removed the pressurized bottle and added an overflow canister, 13 pound cap, so essentially the whole cooling system is setup like a post '90 XJ, minus the water control valve.

My question is: What makes the HO motors I've all seen run at 210*? I'd find it very hard to believe that something changed in the block or head to make them run 30 degrees higher than mine. I know my t-stat isn't stuck open, but perhaps its opening too soon? I would suspect the temperature gauge to be screwing with me, but heater performance suffers in conjunction with temperature readings (IE if it i let it sit idle with the electric fan off until temp hits the 200* mark the heat is very warm, otherwise its fairly lame).

What do you guys think; Swap t-stats and see what happens?
Any insight is greatly appreciated.

End rant.

Well, your fan clutch could have failed so it is always engaged and pulling too much air, it should just be coasting at about 20-30 percent of shaft speed until the air flow hitting the bi-metal spring is 170 degrees (coolant in the engine would be about 30 degrees hotter). If the fan is locked up it will make things too cool.

The thermostat could be funky. I recommend the Stant Premium 195 degree only.

Are you running a 50/50 mix of coolant with distilled water? Any cooling system additives like water wetter?

If you have an automatic/Air conditioning/Power steering cooler, those would ADD to heat.

I would check the fan clutch. Drive around the block from cold start. That will allow the silicone fluid in the fan clutch to spin back into the reservoir (it leaks out overnight). Shut off the engine and see if the fan spins loose.

Same Autometer temp sender used in both engines? Have you tested the sender? Did you use too much thread sealer on the sender--single wire senders ground through the engine and too much sealer can effect them.

Congrats on having mostly the opposite cooling system than most others! There have been some recent posts on overcooling, but seems that they all had 3-core radiators.

You can always put some cardboard in front of part of the radiator to boost temperatures.