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The mkiv Supra Owners Club

Calculating radiator size.


tbourner
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Does anyone know if there is a precise way to calculate the correct size of radiator required for an engine? Taking into consideration idle/WOT etc.

 

A mate is trying to calculate the HxWxD and core density required of a radiator for a transplanted engine in a hot rod, but all the 'professionals' keep saying just 'suck it and see' or use the most commonly used size - but we want to work it out properly and get it precisely right for the energy output of the engine.

 

eg.: 165bhp Rover V8 3500cc, idle 750, redline approx 5200, ideal temp 90 degrees, thermostat opens at 82.

Confusing factors will be flow rate of fan and whether it's belt driven or electric, wind speed to front, revs in each gear affecting air flow speed, thermo syphon effect and/or coolant pump size.

 

Is this impossible? I'm visualising overlapping graphs for each factor, where the overlap area is the correct size.

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You need to keep in mind that all radiators are under-specced, usually they can hanldle in the region of 30% of the engine's max power output.

 

This is because the 'duty rate' is expected to be below that.

 

So a diesel engine (in an earth-mover) making 300bhp will have a coolant rad *many* times larger than a street car making 300bhp.

Why?

Because that diesel engine will be specced for duty rate closer to 100%

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