How Thick is too Thick?
An increase in thickness that allows a greater fin bond surface will yield a greater temperature drop. When going from a two-row to a four you double the fin bond or heat transfer points. The increase however isn't a one to one because the transfer efficiency of the trailing rows is adversely affected by the increase in air temperature from the previous rows and the decrease in air velocity caused by the increased thickness.
There are formulas to determine appropriate radiator size based on engine heat output (operating Btu's) and radiator heat transfer rates (also stated in Btu's). They can be found in any engineering handbook but Armstrong recommends installing the most efficient radiator that fits the hole or intended application, up to a four-row copper/brass or two-row aluminum core.
Radiator & Condenser
Nowadays any hot rod, no matter how individually built, can have air conditioning. Because companies like Vintage Air manufacturer universal as well as vehicle-specific A/C units, the installation is well within the reach of all hot rodders. We stopped by our friends from San Antonio to gather some input about A/C and how it impacts a hot rod's cooling system.
Since I am not allowed around any sharp edges I could only wave to Vintage Air head "cool" dude Jack Chisenhall who was on the manufacturing floor as I spent my time safely locked up in the Vintage Air office of Rick Love, who is equally conversant on the subject of A/C and its impact on the cooling system.
Love wanted to make sure that we stressed the importance of the proper mounting of the condenser with respect to the radiator. He told us that should the condenser be mounted more than 1/4-inch away from the radiator, it's important to consider some type of shroud or seal between them. Air will always take the path of least resistance (according to Love, "like a woman". I might have said "like electricity", but, hey, I just swing at the pitches I'm given!). The shroud or "packing" (OEMs use foam or rubber seals) will ensure that cool air will be drawn by the fan is flowing through both the condenser and the radiator, not just pulled in around the condenser.
Radiator Caps & Overflow Tanks
Other radiator components are just as important to a properly cooling system. To guarantee peak performance the proper mixture of clean coolant, radiator caps, and a working overflow system are all critical. All late-model engines are sealed systems, which require a proper cap. There are basically two types of caps: open system caps and sealed system caps. To tell the difference simply hold the cap up at eye level and if the silver valve button in the center bottom of the caps hangs loose it's an open system cap and not recommended for late-model use. If you can pull on the silver valve button and it snaps shut when released it's a pressurized cap and is working properly.
Cooling system operating pressures are largely determined by water pump operating pressures and according to Armstrong he recommends keeping it under 10 pounds. Armstrong tell us that, "We all know by increasing the system pressure by 1 pound we increase the boiling point by 3 degrees so by running a 12-pound cap our water won't boil until it gets to 248 degrees. Trust me, an engine that wants to run at 248 degrees will open that cap up long before it gets that hot. To deliberately increase the operating pressure to increase cooling is redundant in my opinion and again only points out the need for more efficient heat transfer."
Pressures will increase in the system just after turning off the engine as the coolant absorbs existing engine heat but can't move through the radiator to dissipate it. The resulting increase in pressure pushes coolant past the cap and hence the need for a coolant recovery system (canister, tank, bottle). Once the coolant in the idle engine starts to cool a vacuum is created and another valve in the cap opens and prevents the radiator from collapsing a top tank but more importantly returns the coolant to the radiator so no outside atmosphere or air (contamination) enters the sealed system. Unfortunately most aftermarket recovery tanks are smaller than the needed capacity and that varies with cubic inches and size of the engine.
Overflow bottles should be used on all pressurized systems to eliminate air in the system and maintain proper coolant levels. For our refurbishing of an aging cooling system we opted to install a Dan Fink Metalworks polished aluminum overflow canister (tank). Our tank will hold 2.2 quarts and measures 4x4x9-1/2 inches. Once the radiator cap is installed on the radiator all coolant level checks should be done at the overflow canister. Only if the overflow is empty should the radiator cap be removed to visually inspect levels. Be sure you have an adequate size canister. If there are no other visual leaks in the system and the canister continuously goes empty you either have a leak in the hose between the radiator and the overflow or the overflow is inadequate for the amount of expansion your engine produces when shut off. If necessary, replace the rubber hose between the cap and the overflow or replace overflow with a larger capacity canister to prevent air from entering the system during cool down. Keeping air out of the system is paramount when it comes to maintenance. Radiators don't rust but most engines blocks will if exposed to air so keeping the system tight and annually changing the fluids will provide years of trouble-free operation.