I am putting a water cooled engine in a Porsche and want to design a custom radiator. How do I calculate the area necessary, air flow across this area, and temperature under different conditions assuming the power rating of the motor is 250 HP and is probably 35% efficient at best ? I have a mechanical engineer who works for me but the size he came up with does not seem to match what I see in production vehicles.

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 Recognitions: Science Advisor The basic heat exchanger equation is $$Q = UA \Delta T_{lm}$$ where Q = heat transfer rate U = overall heat transfer coefficient A = Area of heat transfer $$\Delta T_{lm}$$ = log mean temperature difference between the two media exchanging heat. The difficult part is coming up with U. It is usually found by coupling this equation with the energy balance for the system and iterating a result. It's not a straight forward process. Chances are the reason why the estimated area your ME came up with is because a radiator has an incridible amount of perforations and bends in it to increase the overall heat transfer area while not increasing the overall envelope of the radiator.
 Recognitions: Gold Member Science Advisor More things to consider: Your engine might be 35% efficient "at best". However, have you considered "at worst"?! I'm not being facetious, but you won't have much problem when the engine is really working (140mph down the autobahn). You're going to come unstuck when you hit the traffic, you have no ram-air effect, and the poor thing is trying to dump an insane amount of heat out through an inadequately sized rad. Have you had an opportunity to do a heat balance on the engine, or are you still at the drawing board stage? When we specify a new radiator, it's pretty much a full-time job for the thermo engineer to do the calculations and predictive work. I wouldn't set about trying to calculate the air flow across the radiator. This is going to be something decided for you, essentially by the package constraints and the heat rejection at varying speeds, based on the possibility of (say) a 45 deg C ambient.

Try the Lytron website for radiator sizing.

 Lol I guess this is the day everyone doing a watercooled swap into an aircooled car is coming to the forum for help! Check out my post "airflow thru radiator". If you go to kennedy engineering's website they have a lot of useful information in their catalog that is avaliable for download regarding radiator sizing and also have weight info of engines and such.
 If you can, I would just use the radiator that was designed for whatever engine you are running, that way you know that it is up to the task without going larger than you need, and if anything should happen to your radiator, like a rock damaging it or something, you know you can get a replacement at your local autoparts store, all without spending big bucks on a custom radiator
 Why are you doing this?
 Recognitions: Gold Member Science Advisor Note that the OP was made 2 and a half years ago, and he's only made 3 posts....