I frequently hear the younger guys say that they want to run a 160F t-stat because they hear that cooler air is denser and holds a higher oxygen content, and while thats true the fuel air mix in an intake port can very easily be traveling 100-300 feet per minute, that means that in a typical intake manifold at lets assume a low cruise speed the time it takes an individual cubic inch of air to be blended with fuel mist and too move from the carburetor or throttle body to combustion chamber could easily be less than 1/20 of a second, how much heat transfer do you thinks accomplished in that time span?
I just came across this thread again and after watching Engine Masters Season 4 Episode 43 they did some very relevant testing on engine coolant temps vs. manifold temps and whether and why cold coolant makes power.
The test:
598 cubic inch BigBlock Chevy
Test #1: Cold coolant temps (135F coolant): 790 ft lbs @ 5,100 RPM; 854.8 hp @ 6,200 rpm
Test #2: Hot coolant temps (200F coolant) 744.2@5,100; 836.1 hp @ 6,300 RPM
+15.8 ft lbs and +18.7 hp with the colder coolant.
The interesting part of the test is that they were testing the intake air temps in the runners and the 65 deg F increase in coolant temps only resulted in a +3 deg F increase in the air temps, largely for the reasons that Grumpy detailed in the test. (There's not much time to change temps; and the evap cooling capacity of the fuel helps reduce the air temps, too.)
Their working theory is that the colder chamber temperature creates a bigger pressure differential across the intake valve so they are getting greater airflow into the cylinder / across the intake valve when running cooler coolant temps vs. hotter coolant temps.
In Season 7 they go back and test the impact of cold vs. hot fuel, which makes a much smaller difference on the 464hp SBC 383 they did that test on. (+9 hp and +3.3 ft lbs improvement with cold gasoline vs. warm gasoline).
-Not related to oil viscosity, but very cool none-the-less (pun intended).
I tried to take screenshots of the dyno charts to share but my Engine Masters subscription is through Amazon Prime Video and they have DRMed the crap out of all their shows and you can't take screenshots in any browser any more... ;-(
Adam