Mark Houlahan
Tech Editor, Mustang Monthly
June 14, 2012

There's nothing that will ruin a day quicker than seeing your temperature gauge head north and smelling that heart-wrenching scent of coolant coming from under your hood. Cooling system issues are the bane of our world of customization and modification we call restomodding. From engine swaps and high-horsepower powertrains to custom front fascias/grilles and hoods, there are a lot of things that can affect the efficiency of your cooling system and its ability to shed the heat that your engine puts out.

Combustion chamber temperatures can reach 5,000 degrees. Aluminum will melt at 1,200 degrees, and iron at roughly 2,100 degrees. Therefore, the cooling system's main job is to prevent damage to your engine by getting rid of that extreme heat through thermal transfer from the surrounding metals to the water/coolant in the cooling system passages. Of course, there are the basic cooling system components to worry about too. Do you have a radiator of sufficient size to transfer the heat your engine generates? What about your choice of cooling fan, or even the water pump and cooling hoses? In order to effectively cool your restomod, these important questions should be answered with the entire system in mind.

Before we get into choosing the right parts, an overview of how the typical cooling system works is in order. We'll begin with the heart of any cooling system--the water pump. Driven by a typical V-style belt or serpentine belt, the water pump on Ford engines pressurizes the coolant passages of the engine block where heat from the combustion process transfers from the block to the water/coolant mixture. At the rear of the block, the coolant passes up into the cylinder heads and then flows up into the intake manifold and forward to the thermostat. Once the thermostat has reached its specified temperature, it opens to allow the coolant to flow through the upper hose to the radiator core. There, the hot water/coolant is cooled by ambient air forced through the fins of the radiator, either by ram air at speed or via a fan that pulls the air through the core.

After travelling through the radiator, the water/coolant is returned to the engine via the lower radiator hose and then to the water pump to repeat the cycle. The importance of a quality water pump can't be underestimated. As the heart of the cooling system, you shouldn't skimp on your water pump for your project. If your water pump is of questionable history/age, it's a worthwhile investment to install a new high-flow unit. Look for quality hardware like a large-bearing shaft, a billet steel hub, a cast/machined impeller (and not just a universal stamped steel one), and a quality casting. Don't rely on a $30 remanufactured parts store water pump to cool your high-compression, aluminum-headed stroker! Look to a high-performance piece to provide adequate flow. Nowadays, the aftermarket offers electric water pumps in addition to the traditional beltdriven versions. These offer great flow and great looks, but most of them were designed for race cars to cool the engine between rounds while the engine is not running. There generally is no pulley on these, so belt routing will have to be changed unless you buy an electric pump that utilizes an idler pulley.

The cooling system's thermostat is included for a reason, and we highly recommend that all street-driven cars use one. The thermostat controls the cooling system's flow through the block. When cold, the thermostat is closed and restricts water/coolant flow, allowing the block to warm up quickly (hot water is needed for cabin heater use for example). Once the water/coolant in the engine has reached the appropriate temperature the thermostat opens and allows the water/coolant to transfer to the radiator where the heat is dissipated. Using the proper thermostat rating is important. Typical thermostat ranges are from 160 degrees to almost 200 degrees. Modern EFI engines use higher temperatures, typically in the 192- to 195-degree range, while traditional carbureted engines normally will run just fine with 180-degree thermostats. It's rare these days a 160-degree thermostat is used, and using a lower-than-required thermostat in an attempt to fix a cooling system problem is nothing more than a Band-Aid fix. Find the real root of the cooling system issue instead.

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Choosing a Radiator

When it comes to radiators, there have been huge advancements over the years. From down-flow copper brass radiators to aluminum crossflow to hybrid radiators made from aluminum and plastic, the radiator is arguably the second most critical cooling system component behind the water pump. The most important factor when considering a radiator for your classic Ford is the surface area of the radiator core. Simply put, maximize your surface area for the best cooling efficiency. If your core size is smaller than your radiator support opening, you're not taking advantage of the available airflow opening/surface area. Once you've maximized the surface area of the core size, you can then go to a thicker core to further increase cooling. Keep in mind that adding core thickness does not increase the radiator's efficiency as much as maximizing the core's surface area, however. A thicker radiator core does have more resistance to airflow than thinner radiator cores, but the difference is typically minimal considering a 4-inch thick radiator core has roughly 10 percent more resistance to airflow than a 2-inch radiator core.

Core thickness has been blamed for decreased cooling many times, with the thought being that the airflow through the thicker core was decreased, and became fully heat saturated before exiting the core. Sounds great in theory, but usually what is happening is a decrease in coolant flow, not airflow. An older down-flow radiator design usually has a very narrow cooling tube design, which is fine with a stock water pump. When you move to a modern aluminum radiator with the wider coolant tubes, there isn't sufficient coolant flow to create turbulence in the tubes with a stock-type water pump. You need this turbulence to force the coolant against the outside walls of the coolant tubes for thermal transfer to the tube wall, and subsequently to the air passing over these coolant tubes. With the combination of a modern, large-tube radiator and a stock-flow water pump, the velocity decreases and you don't get the turbulence you need for proper heat transfer. If the core is doubled in thickness the coolant velocity is halved. Again, this is why we recommend the modern crossflow design, as it uses wide tubes with less cross section (thinner cores), which requires less velocity to achieve the needed heat transfer.

One thing many people don't consider when choosing a radiator is fin count. Yes, we want the largest core possible, but the fins between the cooling tubes of the core are what give the radiator surface area and help transfer the heat to the air passing through the core. The higher the fin count the better the radiator will cool. The main concern with higher fin counts is keeping the core clean of dirt and debris, as the higher fin count traps these insulators more easily, decreasing the efficiency of the radiator. When building a custom radiator to your specs, you will have to determine the best fin count for your application, although many custom radiator builders only have a few options to choose from.