Car radiator gives poor heat exchange

[Mr Sippy]

I used to run water through my condenser from tap to waste. Used 25-30 gal water per run. The water came out at a fast trickle around 130-140F

Recently I was given fan/radiator from '05 Chevy Malibu. Coupled with a pump and reservoir I thought it would make for good recirculating cooling not dissimilar from what I see others have put together.

First I set it up like it was in the car; fan pulling air through the radiator. Coolant output temps got hot pretty quick and I shut down. Then reversed the fan to push air and put a little separation between fan and radiator. Not much difference.

Some particulars: 5500 watt element at 100% in a well insulated boiler. The pump is returning approx 6 qts/min. The 2 speed fan (2 motors) is powerful. In the car it was on a 25A circuit. Putting my hand in front I'd guess over 300 cfm. That's using a 150 cfm fart fan as a reference. The coolant out temps rose to about 150F in 13 minutes before I shut down. I didn't get the sense it would level off. The temp differential between in and out was steady. The temps were monitored with rtd sensors.

So I guess I am wondering: For those using similar setups; do yours run that hot? Do they plateau eventually? Water and air flow seem to move plenty good. Unless I'm missing something a car radiator just doesn't seem adequate for 5000+ watts of power. Thanks