[dekolds]

FWIW, i have a 982 in the Atlanta heat. I get home and pull into unconditioned garage w 194f engine and 238f oil temps. The purge fans run about 20 mins in garage. I then checked temps and they were 194f engine and 206f oil. The heat is still emanating from rear tire wells but fans no longer running. 3 1/2 hrs after turning off engine at 10:30 pm and w garage doors still open the temps are 155f engine and 142f oil. I question what the purpose of the fans really are since the temps dont really drop by the time the fans turn off. Engine is still hot and obviously cooks for a while as it very slowly cools down. Also, The temps in morning after 12 hrs are typically 100f for engine and oil.

 

[dekolds]

The engine bay of a 981 is an upside down bathtub that traps heat in the "bottom of the tub" which is the intake area. The purge fans are there just to vent some of that hot air out as needed. They have nothing to do with reducing engine or oil temps. That is where the heat being vented out is radiating upward from, and has already occured at that point

I totally get the upside down bathtub and heat trap effect . The point about the temps and particularly the water temp is that the engine is essentially still just as hot as it was when it was turned off 20 mins prior and yet the fans turned off. Yes, the headers and exhaust have cooled some but the engine mass is still hot at 194f (yes, i realize the water temp is a bit of a mystery and is not accurate but it did not droo) and emanating heat which I can feel from the wheel wells. Although i have not measured temp of the bay, just from the feel of heat from wells it does not feel different over course of 20 mins. Maybe the fans are intended to run long enough for the super hot headers & exhaust to drop to a reasonable temp? I assume headers get up to 700-800f for normal driving? My 2-stroke kart runs at about a 1,000f/14,000rpm so i assume our cars run a bit cooler.

 

[dekolds]

Thanks for the reply but I am doing a horrible job of explaining. I have no concerns about how long the fans run or how long it takes to cool down or thinking that anything is abnormal. Nor was I implying that part of their function was to cool the exhaust. Been wrenching vehicles for 40+ years so I understand hot engines but am always trying to learn in all that I do. To simplify the question/comment - is the engine bay really much cooler say 10 mins after the little fans turn off versus the engine bay temp just after shutting off the engine? The engine/transmission is essentially a 500 pound relatively large and dense mass of metals within a bathtub/heat trap that per the water temp and oil temp have not appreciably cooled over the course of 30 minutes which is why it takes 12 hours just to cool the engine from approx 194F to 100F in my garage. The garage BTW is about 90-95F in the morning/after 12 hours. The reference to exhaust temps was simply recognizing that although the headers are significantly hotter (100's of degrees) than the engine, they are also substantially lighter and have less density than the engine. As a result they shed heat more quickly and cool down much faster than the engine. As a result the heat being shed by the headers are what is creating the majority of the heat within the bay after the engine is turned off. Therefore, I was leading to the possibility/question is that maybe the little fans stay on long enough so the heat being shed by the headers have reduced to the point that they are not contributing much more heat to the engine bay than the engine is contributing (ie, somewhat of an equilibrium state) and therefore that is the point the fans turn off. Hope this explanation makes more sense. Again, no concerns, just seeking to learn.