Keeping Your Cool

Let’s talk temperature.

The basics: heat is generated by the combustion chamber, coolant surrounds the combustion chamber in the water jacket. The thermostat is a temperature actuated valve which holds the coolant inside the water jacket until it reaches a certain temperature. When the thermostat opens, hot coolant is ejected from the water jacket through the upper rad hose and into the radiator. Depending on whether the radiator is a cross-flow or a down-flow variety, the coolant flows across or down the core of the radiator and is cooled by the air flowing through the core. After being cooled in the core, the coolant is sucked out of the radiator through the bottom rad hose and into the water pump. The coolant is then pushed back into the water jacket by the water pump and the cycle begins again. At some point in this circuit, the coolant might make a detour through a small loop leading to the heater core of the vehicle, which provides heat to the passenger compartment. And finally, the whole time, the coolant is kept under pressure by the rad cap ranging from about 5-15 psi. This pressure raises the boiling point of the coolant and enables the coolant system to function at a higher temperature than if the system were open to the atmosphere.

Many hot rods have high horsepower engines which generate more heat than a regular stock motor. Combined with the increased heat, many hot rods have limited space where the small stock radiator used to fit. In order to properly cool the high power v-8s that everyone wants to install in their hot rod, it is often necessary to purchase or custom build a radiator that can disperse an adequate amount of heat. And these radiators can get expensive in a hurry.

To spend or not to spend?

That is the question. And to be entirely honest, you may not have much of a choice. Depending on how much juice your motor has, and how much space you have for your radiator, this might be an expenditure that is absolutely necessary.

Let’s start at the beginning, can I use my stock radiator?

The first rule of thumb is to compare what engine the car came with stock to what you are putting in. Did the car come with a v-8 or a straight 6? Most of you will be installing a small-block v-8 at least. If you are planning to install anything from a stock to a mildly enhanced v-8, most stock radiators will probably handle the load. You will be able to have the radiator flushed, pressuretested and painted and you are good to go. If you are installing anything from a high horsepower small block to a supercharged big block, you will need to invest in an upgraded radiator. Luckily enough, there are aftermarket radiators available for almost all common hot rod bodies. They are usually range in price up to $500. And as they are necessary, it is an expense that you usually cannot avoid.

I am planning to put a whole herd of ponies under that hood, what can I expect?

Okay, so you want the big horsepower. Whether you go small block or big block, you will have a great deal of heat to get rid of. Not only are you going to need to invest in the largest radiator that you can find, but you may also need to invest in some peripheral cooling devices to ensure your ride doesn’t boil over. Radiator shrouds and electric rad fans may be necessary to keep your air flow up. Rad shrouds are a factory device on most vehicles dating all the way back to the 1930s. But often times when installing a different engine, the fan no longer lines up to the opening in the shroud. There are a variety of solutions to this problem, but other than buying a universal shroud and then cutting it to fit, fabricating or altering the original to fit might be another option.

Radiator shrouds and electric rad fans help keep the engine cool when the car is idling or moving slowly.

Like in a parade or at a rally. The shroud closes the gap between the fan and the radiator so that the suction of the fan is concentrated on the rad surface. An electric fan effectively removes the need to have a fan blade attached to the engine. This has several benefits. The first is that the electric fan can run at full speed even when the engine is at idle. This is a major advantage. When a motor is idling or running at low rpm, the fan is also running at a low rpm; this in turn means that there is not much air flow. An electric fan can run at full speed any time; even when the car is idling. The second advantage with electric fans is space and location. The conventional fan on an engine is in a fixed spot. If you are putting a big block in an older (1920s-1930s) hot rod, there is not a lot of room in the engine compartment. Being able to remove the fan and install an electric replacement might give you those few extra inches necessary to get everything to fit.

Aftermarket electric radiator fans usually come with all the necessary wiring and components.

However, that is not to say that you cannot find a good used unit to retrofit into your hot rod. The tricky part is to find the necessary sensors to make the fan run properly. There is usually at least one sensor for the coolant temperature and at least one relay to take the signal from the sensor and turn the fan on and off. Make sure that you have all the necessary components before you leave the junkyard.

Last but not least, a quick warning about transmission coolers.

If you are installing an automatic transmission in your hot rod, it is highly recommended that you install a transmission cooler. You can burn up a brand new transmission in no time flat if the Automatic Transmission Fluid (ATF) gets overheated. In vehicles originally equipped with automatic transmissions, the radiators often have transmission coolers inside one of the tanks. Beware these arrangements. In the case where a leak occurs between the antifreeze and the ATF, the increased pressure in the radiator will push antifreeze into the ATF. This will destroy your transmission. Although this does not occur often, I am mentioning it because it has happened to me once. To avoid this problem, install one of the external transmission coolers and hang it in front of the radiator.

Note: antifreeze mixed with any lubricant will eliminate the lubricating properties of that fluid and will cause high friction overheating. (seizing of engines, transmissions…)

Here is a simple list to help you figure out your cooling system:

  1. If you plan on using a fairly stock motor in your hot rod, you may very well be able to use the original radiator that came with your hot rod. Maybe get it cleaned and pressure tested, but there is no harm in trying it out. If you have overheating problems, you can always try a couple of things.
  2. If you are going high horsepower, whether small block or big block, you are most likely going to need a beefed up radiator, aftermarket 3 or 4 core radiators are readily available for most common hot rod bodies. Expect to spend around $500.
  3. For any hot rod, high horsepower or not, rad shrouds and electric fans are an option. Radiator shrouds make sure that the suction created by the fan is concentrated on the radiator surface. Electric fans are compact, and are able to run at full speed even when the vehicle is idling or moving very slowly.
  4. Consider installing an external transmission cooler if your hot rod has an automatic transmission. These coolers are cheap and easy to install and might save you having to replace your transmission.
  5. Never underestimate the importance of a properly functioning rad cap. The increased pressure in a cooling system substantially raises the boiling point of the coolant. As well, the rad cap controls flow to and from your radiator overflow tank. (if your hot rod is so equipped, and I definitely recommend installing an overflow system.)


So that is your cooling system in a nutshell. Let us know if there is anything that we missed. And remember to tell your friends about