Every engine has an optimal temperature range where horsepower is best generated. The cooling system’s job is to make sure the engine stays in its happy place, temperature-wise, to make power and not melt down. In this article, we’re going to dive into the pool of cooling system principles and talk with Don Meziere of Meziere Enterprises to learn how to properly design a cooling system for a race car.
Cooling System Principals Explained
There are multiple factors that play a role in how well your cooling system will function. Besides the actual cooling system parts, the engine’s tune can actually affect the operating temperature. If your engine is running on the lean side, or it has a lot of ignition retard, it’s going to heat up quicker — this is because the leaner fuel mixture burns hotter, and an excessive amount of retard will make the engine work harder to compress the air and fuel mixture. So, it’s important to remember to watch your tune-up to help fight excessive engine heat.
Now, provided you have your engine’s tune under control, there are five factors to understand when it comes to cooling systems: heat production (BTUs/HP), radiator capacity (heat dissipation), air flow, water flow, and pump and system pressure.
A British Thermal Unit, or BTU, is a unit used to express a specific amount of heat (specifically, the amount of heat needed to raise one pound of water at maximum density by one degree Fahrenheit). You can actually calculate how many BTUs your engine generates based on how much horsepower it makes. Each horsepower an engine produces is equivalent to 42.42 BTU. When the engine is running and the cooling system is being used, the BTUs generated by the engine are absorbed by the coolant and must be dissipated by the radiator.
Meziere walks us through an example of how you can use your engine’s horsepower, and the 42.42 BTU measurement, to calculate the adequate size of a radiator.
“A 500 horsepower stock car will need much more cooling capacity than an 850 horsepower dragster. The stock car’s engine RPM will cycle above and below peak horsepower twice a lap, heat-soaking the cooling system with 180,000 BTU in a 10-minute event. The dragster, in one round, might idle less than 10 minutes and make an 8-second run at a 750 horsepower average. Running 10 seconds at full throttle, the dragster would release about 318,150 BTU. In the case of the dragster, the system must be adequate enough to prevent detonation under power and maintain temperature at idle.”
A radiator’s capacity refers to how much heat it can dissipate while the engine is running – it has nothing to do with how much coolant it can hold. Most high-performance radiators are filled with tubes that are 1 to 1-1/2 inches wide. Since these tubes are wider than the 1/2 to 3/4-inch tubes that radiators used in the past, there’s more surface area to assist with heat dissipation.
The dual pass design of newer radiators also helps with how well they dissipate heat.
“Dual pass designs force the water to travel the length of the radiator twice, increasing the amount of temperature drop capable for a given size; unfortunately, the restriction is much more than doubled. Surface area is king when it comes to radiators. Doubling the square inch of your radiator will double the heat dissipation, whereas doubling the thickness is less effective and restricts airflow,” Meziere states.
An entire article could be dedicated to the theory of cooling system airflow, but we’re going to just cover it at a high level here.
Airflow has a massive impact on the efficiency of a radiator. You want to have the correct size opening in the front of your vehicle to ensure the radiator is being exposed to the proper amount of fresh airflow. Fans that are properly sized and sealed to the radiator with a shroud will help in the airflow process, too. The fan and shroud need to be matched to your radiator’s size and the engine’s cooling needs.
“The radiator transfers heat to air as it passes through the core. For proper function, the air stream must be under high pressure at the front side of the radiator, and lower pressure behind. This pressure differential drives the fresh air past the fins. If air pressure builds up in the fan shroud or the engine compartment and the difference in pressure is decreased, airflow across the radiator can stall. Therefore, thoughtful planning should be done to consider both ‘at rest’ and ‘at speed’ conditions, and how fresh air can be presented to the radiator effectively in both situations,” Meziere says.
Surface area is king when it comes to radiators. – Don Meziere, Meziere Enterprises
You can have the perfect radiator that has optimal airflow around it, but it’s worthless if you’re not moving enough fluid through it. The right water pump for your application is what will ensure the correct volume of water is flowing through the cooling system.
“Where rules and conditions permit, electric water pumps can be a solution with multiple benefits,” Meziere explains. “The constant speed of an electric pump eliminates high- and low-RPM problems. The bonus is that you can run the pump when the engine is shut off. You should never run your engine without the water pump on because hot spots can form in the cylinder head before your temperature gauge begins to register. When an electric water pump is mated with a good electric fan, you can easily regulate water temperature for consistency and rapidly cool the engine between rounds after shutdown.”
A cooling system needs to maintain the proper amount of pressure to function correctly. Each pound of pressure that’s introduced into a closed cooling system will increase the boiling point of the coolant by 3 degrees Fahrenheit. So, if you’re using a 16-pound radiator cap, you can expect the boil-over point of the cooling system to be 260 degrees Fahrenheit. You need to keep this in mind when you’re designing a cooling system so you can select the right parts.
Meziere covers why it’s important to have a properly sealed cooling system.
“A poorly sealed system, low-pressure cap, or low water level can allow a runaway boil-over. The lack of pressure allows boiling to start prematurely. Gasses produced by this boiling push water out and aerates the coolant, compounding the situation. Water is diverted around these steam pockets, leading to more serious problems; surface distortion, metal fatigue, and cracks. Once this process begins, it will not stop while the engine is under load. Water flow, temperature, and pressure all work to manage this boiling at hot spots, which can produce steam pockets that insulate the metal from the coolant.”
Designing A Cooling System
Now that we have an understanding of cooling system basics, it’s time to talk about how to design a cooling system.
Just like anything else in the high-performance world, there’s really no “one size fits all” solution for a cooling system. You have to look at your specific application and how you intend to use it before you start buying parts, otherwise, you could have to deal with an overheating situation at an inopportune time.
“How the vehicle will be used really does matter a lot. If you’re building a street/strip car that will see some spirited driving, with the possibility of no cool-down period, you need to be mindful of how you size the cooling system’s lines. It’s best to use big lines that are comparable to the OEM size that the vehicle came with. Keep in mind that the lower hose on a stock V8 usually has a 1.75 or 2.40 square-inch inside area. The stock top hose is usually 1-5 or 1.7 square-inch inside area,” Meziere says.
What this translates to is that using larger hoses will help fight the possibility of cooling system restriction issues. These larger hoses are also going to give you more fluid capacity in the system. Be mindful of the fittings you plan on using to connect the cooling system hoses to the radiator and water pump. For example, if you use a 1-3/4-inch hose with a 3/4-inch NPT fitting, the coolant flow will be restricted to what the smaller-diameter fitting can handle.
You also need to remember that automotive cooling systems use centrifugal-style water pumps. This will impact how you design the cooling system.
“Centrifugal pumps only push fluid, they do not create suction or ‘pull’ any fluid. That’s why it is so important to get the system 100-percent full. If air gets into the lower hose and into the impeller cavity, everything stops. We get lots of tech calls involving this problem. The pump will not move coolant if the chamber is full of air. That’s why filling from the proper spot is so critical, and also why, when the lower hose is really long, like in the case where the radiator is moved to a remote location, the lower hose has to be air-free. That can be tough to accomplish if that lower hose goes through a rise and fall. It also can be a problem if you’re trying to fill the system from the top of the intake manifold,” Meziere states.
The size of the lines you use will have an effect on the flow, and the pressure will also impact the fill point of the cooling system.
“If you use large lines and have a good amount of flow while filling, it becomes more important to fill at a point that has low pressure. When you use smaller lines and are restricting flow, then you need to fill from the high point. A fill point at the top of a single or dual pass radiator will likely be the correct spot if you have good flow. This is true even if it is 2-4 inches below the top of the engine,” Meziere explains.
If you’re building a street/strip car that will see some spirited driving with the possibility of no cool-down period, you need to be mindful of how you size the cooling system’s lines. – Don Meziere, Meziere Enterprises
Most vehicles use a traditional cooling system layout where the radiator is in front of the engine. This makes routing all of the cooling lines fairly simple, provided there’s nothing in the way. If you have to relocate the radiator to another part of the vehicle, that’s going to change how you route lines, and it creates some additional challenges.
“Vehicles with a relocated radiator face issues with air getting trapped in long lines. Often, this trapped air will cause persistent problems by air-locking the pump when you least expect it. The fill point and the route of the low-pressure line are of special concern when you’re running long lengths of cooling lines, so be aware of that,” Meziere states.
We talked earlier about how the vehicle’s tune will have an impact on how hot the engine will get. This also applies to how much horsepower the vehicle is producing. You want to think about just how much horsepower you’re going to make and how long it’s going to be made- this will help you understand just how much heat your cooling system will need to exchange.
If you’re using a power adder with your vehicle, that will also need to be factored into the design of your cooling system.
“If you’re going to run a lot of boost, the combustion chambers are going to heat up quickly. Block pressure and coolant volume both become very important in a boosted application. Hoses need to accommodate both of these requirements and need to be accounted for from a safety aspect. Extra clamps or aftermarket hose solutions, like a clamshell clamp or a Wiggins fitting with crimp ends, should be considered when you’re plumbing a boosted application’s cooling system,” Meziere says.
Cooling systems may seem fairly simple, but there’s still plenty of planning that needs to go into their design. A well-thought-out cooling system will not only keep your engine running cool and making plenty of horsepower, it will also be easy to maintain and prevent any issues that could cause engine damage.