Ian,
a few weeks ago I was climbing a grade (unloaded) of 1500' in 1.8 miles and my trans temp got as high as 220F. I probably was going no faster than 20mph with the OD off. This got me thinking about additional cooling and so I ordered a Perma Cool cooler/fan combination.
As Neal mentioned, if you keep the original cooler then the Perma Cool fan is only used when the factory cooler cannot keep up. The fan on the 10"x14" unit moves 900cfm which is equivalent to driving at 18.75mph.
Mounting the Perma Cool close to the factory cooler with the fan in the front as per their instructions will result in the fan spinning constantly under normal driving conditions (20~70mph). This will cause added wear on the fan bearings, brushes etc and the increase in cooler surface area isn't needed under these conditions.
As mentioned, the fan is only needed under certain conditions i.e. low speed/high load where convective cooling from outside air is a minimum. My decision to "possibly" mount the cooler horizontally with the fan blowing upwards is based upon the fan drawing cooler air from beneath the truck. The fan will be out of the main airflow and so won't be rotated by the air under normal driving conditions.
I don't think any or at least much natural airflow will be blocked by the fan because it's free to rotate. I'm not sure about hooking up two coolers in parallel. Series is the usual way to go. I'm installing an external trans filter in addition to the cooler so I haven't figured out how I'm going to do this yet. I'll let you know once I'm done.
I'm always open to, and welcome other perspectives.
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'00 5.4L AUTO 4X4 F-150 Lariat
'98 4.0L 5spd Jeep Wrangler Sport
'95 3.8L SC Thunderbird
'02 F-350 PSD CC DRW 6SPD 4X4 Lariat (order this year)
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