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Keeping it Cool

Everything You’ve Ever wanted To Know About Cooling Systems—and a few more things

Fielding Melish
Mar 1, 2002
Photographer: Miles Monroe

It’s Friday afternoon.and you and your jobless friends are bored. Such mindless activity arouses the proposal of a road trip and, three pairs of eyes fixate on you for silent nomination as the driver. After all, you’ve spent the past year squeezing your parents’ credit cards for those mods to your Integra. The turbocharged B18 and an engine-management system were completely justifiable expenses in your quest to be the toast of the show-car scene.

Two hours later, the tiny coupe crawls along a clogged freeway, enjoying the inti-macy of Friday-night traffic. As the car inches up a congested interchange ramp, the engine begins to run a little rough—then a thin plume of steam rises from under the corner of the hood. No one noticed the temperature needle peg itself deep into the “doom” zone. Nice work, sport—this completely unnecessary trip may have just cost you your motor.

The modified engine under your faux–carbon-fiber hood is an air pump that uses a spark to release the potential energy of gasoline. At peak efficiency, only about 30 percent of your fuel’s energy is used to create reciprocating power. The remaining 70 percent of the fuel’s potential energy escapes through the tailpipe or is converted into heat that’s absorbed by the cooling system. This absorption of heat is vital to proper engine performance. Yet important as they are, cooling systems have a tendency to get overlooked. Sure, they rate low on the scale of sexiness, especially when compared to the likes of turbocharging but remember: Engines tweaked to rev higher or handle forced induction to create more power also create additional heat as a byproduct. In these instances, a stock cooling system may not be able to handle the extra load.

Knowledge, in this instance, is your best defense. Understanding how the cooling system works, how to improve it, and how to identify possible problems can save you from making potentially wallet-devastating mistakes as well as help you to maximize performance. So learn well, grasshopper, and don’t forget to flush.

Coolant and Pressure
If an engine ran without coolant, even for a brief period, the temperatures could soar high enough to melt a piston and fuse it to the cylinder wall. Metal surface temperatures in the cylinder head and combustion chamber can rise higher than 500 degrees F, so the cooling of these surfaces becomes a vital engine-design element for power and longevity.

Honda blocks use aluminum, open-deck construction with iron sleeve inserts as friction surfaces. Most of the areas inside the cylinder head that are not of structural significance are filled with a coolant passage. The coolant—in most cases a mixture of antifreeze (ethylene glycol) and water—absorbs heat from such hot spots as combustion chambers and the backside of cylinder walls. While water absorbs heat effectively, it also freezes at a high temperature (32 degrees F) and boils at too low a temperature (212 degrees F) for use in cars in certain climates. Mixing water with antifreeze yields a solution that benefits from a lower freezing point of -35 degrees F (for a 50/50 mixture of water to antifreeze) and a higher boiling point. It also adds anticorrosion properties.

The effectiveness of the coolant—its ability to resist boiling and transfer heat—can be helped with Redline’s Water Wetter, which reduces the surface tension of the coolant. Another option would be Evans’ NPG coolant, which is designed for use without water, allowing you to run a zero-pressure cooling system. At zero pressure, this waterless coolant has a boiling point of 360 degrees F.

A major component of coolant-system maintenance is the periodic flushing of the system. Old antifreeze/water mixtures can actually become corrosive to metals after extended use, but they must be disposed of responsibly, as the mixture is toxic. Sealing the radiator’s filler neck on modern cooling systems is a rubber-gasketed cap with a spring-loaded valve that pressurizes the system and increases the coolant’s boiling point. Factory radiator caps typically increase the cooling-system pressure by 14 or 15 psi and raise the boiling point about 43 degrees F. As the engine warms up, the coolant heats up and expands, causing pressure to build up; the cap’s valve is the only place where this pressure can escape. When the system pressure reaches the cap’s pressure rating, the cap’s spring is compressed, forcing the valve open and allowing coolant to escape through the overflow tube to the expansion tank. This also permits air to escape the cooling system; as the radiator cools down, the vacuum created by the cooling system contracting pulls down another spring- loaded valve, returning coolant to the radiator. Due to the pressure contained by the radiator cap, and the fact that boiling liquid can lead to Darkman-like disfiguring burns, it’s never a good idea to open a radiator cap while the engine is still hot—and certainly never when you’re in the nude.

The Pump and the Thermostat
Coolant must flow through the block and head in such a way that it can absorb and transport heat without boiling. When coolant boils, its capability to absorb heat is diminished, causing temperatures to rise dramatically.

Coolant makes its way from the bottom of the block and out of the cylinder head to the radiator by a mechanically driven centrifugal pump. The water pump also draws coolant from the radiator and forces it through the engine at a higher pressure.

A thermostat regulates the flow of coolant from the block to the radiator. By varying the size of its aperture, the thermostat slows coolant flow to ensure that the coolant will spend enough time in the block and cylinder head to absorb heat. On a cold engine, the thermostat completely restricts flow to the radiator block outlet. The thermostat uses a wax-filled cylinder to open at the prescribed temperature. A rod connected to the spring-loaded valve in the cylinder presses against the wax. As the wax heats up, melts, and expands, the rod is pushed out of the cylinder and opens the valve. The thermostat is located at the top of the engine at the coolant outlet, where coolant temperatures are highest.

Typically, a car runs most efficiently when the coolant temperature is kept around 200 degrees F. At this temperature, the combustion chamber is warm enough to vaporize the fuel mixture for improved combustion, and the oil’s viscosity has lowered sufficiently to reduce parasitic drag.

The Radiator: Heat Transfer and Airflow
The modern radiator is constructed of densely finned, aluminum cores usually with plastic tanks. Aluminum has a very efficient rate of heat transfer and the structural strength to withstand higher system pressures.

From the factory, most radiators are designed to match the heat output of a stock motor. The radiator sits behind the grille opening in the path of air that rushes in when the vehicle is in motion. When super-heated coolant is pumped from the top of the engine into the radiator, it flows through a structure of tubes. Folded aluminum fins connect the tubes, and the metal absorbs the heat of the coolant. Air entering through the grille moves across the tubes and fins, and cools them by transferring the heat to the ambient air.

The fins-per-inch measurement gives an indication as to how effectively the unit will transfer heat. More fins of folded metal result in greater surface area for air to flow over and increased heat transfer. After- market aluminum radiators, such as those offered by Fluidyne, not only have a high density of fins, but they also have wider aluminum cores to provide greater cooling surface area and coolant capacity.

The rugged, all-metal, welded or epoxied construction of such radiators can also handle higher coolant-system pressure, and it maximizes the temperature differential between the coolant entering the engine and the super-heated fluid entering the radiator. A high-pressure radiator cap (such as a 24-psi unit) is available from many aftermarket-radiator suppliers, but such pressure puts greater stress on the rest of the system (clamps, hoses, gasket surfaces, and so on). When a vehicle is at rest, or moving slowly, the airflow must be maintained through the radiator so it can continue dissipating heat. A fan provides constant airflow through the radiator; the fan is electric in most front-wheel-drive cars, since the engine’s power output is oriented toward the side of the car. Thermostatically controlled pusher or puller electric fans (aka “blow” or “suck” fans) draw air through the radiator core once the coolant reaches a predetermined temperature.

Lost Coolant
One of the most common causes of overheating is a low coolant level, which reduces the system pressure and the coolant boiling point. Pressurized systems with full coolant levels and functional expansion tanks are more effective at maintaining temperatures.

Cracked hoses, faulty hose clamps, bad thermostat housing gaskets, radiator pinhole leaks, leaky expansion tanks, and tired radiator caps are common culprits of pressure loss. Leaks can often make slight hissing sounds and can be identified visually after the car has been running. If a head gasket is compromised, however, coolant can escape through a combustion chamber, mixing oil with the coolant. Traces of exhaust gas and oil can be seen floating in the coolant, and this means that the top of the engine must be removed and resurfaced to replace the gasket.

Casting imperfections, such as hairline cracks or weeping freeze plugs, relieve system pressure just as easily as pinhole cracks in the radiator do. Temporary solutions for leaks include JB Weld (a “stronger than steel” epoxy welding agent) applied to the crack or fissure, or such radiator sealing agents as Alumnaseal or Bars Leak. These agents use a ceramic or metallic medium that mixes with coolant or water to help seal leaks. Most sealing agents are only temporary repairs at best, and ultimately, parts will need to be replaced or welded.

While some radiators can be repaired, most shops will just replace the whole unit. This plastic-tank/aluminum-core construction has proven to be the weakness of the Honda radiator. A radiator passage can become clogged with debris, reducing coolant flow through the core and fins and reducing heat transfer. While debris can be removed with rods that clear the radiator core, most shops will just opt to replace the radiator.

How It Can All Go So Wrong
If a cooling system maintains the correct pressure and fluid level, and yet the engine still runs hot, then the problem has more to do with the fluid flow rate or airflow/heat exchange rate. Coolant flow is managed by the churning of the water pump and the variable restriction provided by the thermostat, and the operation of these two components can be altered to suit the engine’s cooling needs. Typically, water-pump failure is accompanied by the squealing noise of cashed-out impeller-shaft bearings, or the pump just leaks at the seals.

Water-pump impellers are designed to be most efficient at pushing water through the cooling system at the lower speeds at which street engines tend to live. At very high rpm, however, temperatures can rise because some water pumps pump more air than coolant. In such instances, many racers increase water-pump pulley size to reduce the water-pump impeller speed at higher rpm, therefore getting more effective cooling during races.

A slipping belt is also an overlooked cause of high-rpm overheating. It results in a pump impeller that spins more slowly relative to crankshaft speed and thus can’t move coolant quickly enough.

Thermostats
If overheating persists after the operation of the thermostat has been tested (using a pot of boiling water and a thermometer), consider that switching to a lower-temperature thermostat will allow more coolant into the engine sooner.

Thermostats that open sooner can help increase coolant flow, but the coolant spends less time in the engine absorbing heat. Switching to a lower-temperature thermostat is just a Band-Aid remedy, however; it’s also necessary to increase the pressure of the cooling system with a stronger radiator cap.

With an adequately pressurized system and an efficient radiator, a higher-temperature thermostat (190 degree F instead of 180) will improve engine cooling—it will slow the coolant and do a more thorough job of absorbing heat. The larger temperature differential between the ambient air and coolant temperature will need peak radiator efficiency to control temperatures.

Better, Stronger, Faster
Unless you’re a sorcerer, not a whole lot can be done to control the temperature of ambient air. You can, however, control the temperature differential between the coolant and incoming ambient air. Ideally, you want the coolant to absorb as much heat as possible before it turns into vapor pockets, and then send the heat to the radiator for the most dramatic temperature reduction possible. Typically, a 100-degree differential is the desired target.

Quite a bit can be done to improve the flow of air through the radiator, thus maximizing its heat-exchanging capabilities. The electric fans that provide airflow for most front-wheel-drive cars are designed to draw air through the radiator of a motor with a near-stock power level. The heat generated by higher horsepower requires more aggressive airflow. Aftermarket companies, such as Flex-a-Lite and SPAL, offer a range of electrical fans that can move larger quantities of air than OE cooling fans; these fans also provide different mounting possibilities. Whether it be pusher or puller fans, such devices are crucial to help maintain airflow when the vehicle is in traffic or just moving slowly.

A fan shroud fitted around the perimeter of the radiator also helps direct airflow through the aluminum heat exchanger instead of around it. Radiator airflow can also be improved on a moving vehicle by an air dam mounted under the car’s front bumper, which forces air that would otherwise go under the car up into the grille opening and through the radiator.

The majority of enthusiasts spend so much time combing ads for the next great power-adding device or scheming of ways to make the larger engine swap fit, that they never consider the extra heat that these modifications will generate. Addressing a performance engine’s cooling needs as part of a buildup will save you time and headaches on your dream car project.

By Fielding Melish
2 Articles

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