Thanks.
If you mount a cooler in the back there generally needs to be a fan on it. Having it in front usually provides enough airflow for the fins but if the cooler is larger it can block airflow to the radiator. If you’re sitting at idle the temp will creep up the same way the cooling system does. Probably wouldn’t matter as much with an electric fan.
I mounted it under the trunk mostly because the engine bay is cluttered and tight. My RF brake line is no longer on the firewall and runs along the frame rail and under the radiator so there is little room left for the two cooler lines. Trust me though, routing the lines to the rear is just as much of a PITA as running them to the front.
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I also have larger headers and wanted to keep any plumbing away from heat. The rear tubes can get fairly close to the frame on the driver’s side.
My car is EFI. I am using a temp gauge in the pan as an input to the ECU. When it reaches a certain temp, the ECU then grounds a relay through an output and turns the fan on.