I've been searching a lot on this just to see what people here have to say about the placement of the aux cooler. mostly I was looking for info on thermostats. I dealt a lot with tranny coolers when I had my Impreza, since I did rallycross a few times. Temps tend to run fairly hot when you rallycross regardless of wether it's a WRX or a lowly EJ25 powered Impreza.
I'm all for placing the aux cooler before the radiator cooler. It's been mentioned how well this works in cooler weather for keeping the temps up by reheating the fluid to the proper temperature. There were two points I didn't see directly brought up so I thought I'd add my input for the record.
1) Some people advocate completely removing the water (radiator) cooler alltogether. I would strongly reccommend against this. The main reason being that water has several times the heat capacity of air. around 8 times the heat capacity IIRC. I might be wrong on the exact number, but it is several times that of air. This means it will absorb a LOT more heat than air. it will cool the fluid much more quickly even if it's diffusing the heat into 160 degree water instead of 80 degree air.
2) more importantly, heat transfer is directly proportional to the difference in temperature between the hot and the cold side of a system. so the hotter the fluid is in the aux cooler, the more heat it will transfer out of the system. basically, it will work more efficiently if it's before the radiator than if it's after. given a fluid temp of 220*, more heat will leave the system if it flows through the aux cooler first then the water cooler. The heat transfer process is already very fast in the water cooler since the heat capacity is so much greater than air, so considering differences in temperature for its placement in the system is less important.
basically, the air cooler depends more on temperature difference for a fast reaction, and a water cooler relies more on it's heat capacity for absorbing heat. So, leaving the water cooler in is strongly advisable, and placing the aux cooler in front of the water cooler is optimal.
that's all I got. hopefully some of it is new info, or at least explicitly stating what's only been implied before, and not just me neglecting to search hard enough
. I got this stupid degree in physics, and I've used it more on modding/maintaining my cars than I have in the job world
