The flow rate changes from radiator to radiator. Such as back when I first got into watercooling, I used a tranny cooler, it has a low resistance since its an open tube design.
But heatercores are more restrictive and require more pressure behind them to get the water through and keep it at a decent GPH.
Back in the tranny cooler days...everyone thought that 100GPH was enough. I wouldn't have even considered anything over 150GPH(my pump was 145 in fact). But since someone discovered cars have a few other radiators, they pulled the heater core, hooked it up, and noticed the pressure drop was quite high in comparison to the tranny cooler.
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