In most of the articles I have read (including Steve Jack's in the tech section here), all reference 1.25" 2 row aluminum working great up to about 450 HP. That is about where my 383 should be. I am concerned about being at the upper end of the limits. I am about to order my radiator and am looking for suggestions on what to do. This is in a 1939 Chevy 2-door sedan.
Thanks
Mike

Mike check with Dick Z. here on SRS about the PRC radiator you can't beat the Quality or price and they'll custom make for your applcation!
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Mike,Charter DRSA Member

Actually the hp rating is a bit of a mismeasurement so to speak. If you never use the PEAK HP over a given time to cause heat generation sufficient enough to outstrip the system then the HP yardstick is a bit moot. But, you have to have enough capacity to cool under moderate loads.

The worst case scenerio is either a racing application where the motor is at full tilt utilizing all the hp all the time or in towing situations where you can pull significant loads under duress with your foot planted. That's why towing requires so much extra cooling capacity.

I
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Steve "Jack'Stands" Jack

Ooops.....pushed the wrong button with my new laptop...sorry.

Anywho.... I was considering a PRC for my 57 with it's new 383/450ish hp, plus air, plus Atlanta traffic, plus hot summers and I have to pull moderate grades going to cruise the Smokies. My calculations shown the downflow twin 1" PRC to be just barely adequate and would probably suffice, however with ALL the factors working against me including running a 2700cfm electric and that's all, I decided to upgrade to the Griffin 1.25" radiator and keep it a down flow, but with larger area available. A crossflow should have okay too with the 1" tubes, but did not want to move all the hardware to the "six" position. So, the big tubed downflow for me was the answer.

For your 39 I would recommend a PRC if you can get alot of airflow at idle...say twin fans of some sort, however if you are running air and live in a hot climate I would upgrade to a larger tubed animal even tho I hate to shove you away from the good folks here. If you are not running air then you will be fine with the PRC in all cases...that should suffice. You can make up for area with higher airflow figures to a certain extent. For example, A 3000cfm-fanned, 1" tubed alum radiator is as good as a 2100cfm-fanned, 1.25 alum radiator of the same area.
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Steve "Jack'Stands" Jack

Steve,
First of all thanks for your thorough reply. I live in Virginia Beach, hot and humid during most of the heavy cruise months. I am running a Vintage Air Gen II Compact, H/C/D. I am planning an electric pusher and soon to find out if there is enough room for a mechanical too.
As this is my first build, I am assuming to keep things in the stock position, I will use a downflow. I plan on driving this bad boy a lot and I really don't want the overheating issues that I hear about. Would reduction pulleys also be advantageous?
Thanks for the help

Mike, when selecting an aluminum radiator, find out if it can be repaired, if necessary.

Some local guys that bought Griffin's found out that not even Griffin will repair them. .....PRC says theirs can be repaired.

Although you directed your question to Steve, I suggest that you do NOT run reduction pulleys. They do NOT aid in cooling, and can actually be detrimental to cooling.

Many factory SBC applications had waterpumps OVERdriven by 26%. This means the water pump was turning FASTER than the crankshaft. This was done to promote cooling. ......Reduction pulleys, which are UNDERdrive pulleys, actually reduce the coolant flow in the system ---NOT a good thing on a street engine!

The reduction pulleys were originally designed for drag racing applications, where sustained cooling is not a big factor, and a few horsepower were freed up. ....Their popularity on street rods is more for looks than for function! ....The reduction pulleys can also adversely affect the charging system by slowing the speed of the alternator so much, that there is little or no output at low rpm's.

Yeah...what Bruce said! He is absolutely right.

Run overdrive pulleys and high flow pumps for better cooling.

The "high five" of cooling are: high heat exchanger area, high water flow, high airflow, high water content, and high turbulence content! These all make the cooling system efficiency go up!
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Steve "Jack'Stands" Jack