High-performance cars and horsepower are hotespecially under the hood. Hot rodders know that an internal combustion engine heats things up by combining fuel, compression, and ignition to create horsepower. But theres a fine line between heat that generates power and a parts-damaging furnace.
To keep heat in check, todays engines use a forced liquid circulation system that consists of a water jacket, radiator, water pump, thermostat, fan, and hoses. When combined with the appropriate airflow, coolant flow, and system pressure, this system allows the engine to operate at the high temperatures needed for optimum performance and reliability.
Because performance enthusiasts thrive on adding extra power that places additional demands on cooling capabilities, we decided to take a look at cooling systems with a high-performance perspective and outline many of the systems and products available. We asked Stewart Components to share some of its cooling system experience.
The radiators job is to cool the liquid that has been heated by the engine. A variety of designs are available that enable both coolant flow and airflow to occur efficiently, but the most important temperature-reducing aspect of a radiator is the size of its surface area. Modern radiators use wider tubes with cross sections that provide more surface area per cubic inch of coolant compared to older designs that used narrow tubes with shorter cross sections.
Aluminum vs. Copper-Brass
Many new performance radiators, such as those from Griffin and Be Cool, are made from aluminum that offers weight savings over copper-brass. While aluminum does not dissipate heat as well as copper-brass, modern radiator design more than makes up for the difference. Old copper-brass radiators were soldered together, but solder is a poor thermal conductor and this reduced the ability of the fins to remove heat from the tubes.
Coolant flows through a radiator tube at a velocity sufficient to cause turbulence. This moves the liquid in the middle of the tube to the edges, allowing better thermal transfer between the coolant and the tube surface. Aluminum radiators, with wide tubes and smaller cross sections, need less velocity to achieve optimum thermal transfer and thus provide improved cooling.
A common misconception is that coolant flowing too quickly through the system will not have time to cool adequately. However, the cooling system is a closed loop, so if the coolant stays in the radiator longer to allow it to cool, it remains in the engine longer too, which increases its temperature. If this occurs, coolant in the engine can boil away from critical heat areas within the cooling system and cause hot spots, which can cause engine damage.
Crossflow vs. Upright Radiators
New cars have come equipped with crossflow radiators for more than 30 years, and today many modified cars from the 60s and earlier have them installed. Crossflow radiators are superior to upright radiators because the radiator cap is positioned on the low-pressure (suction) side of the system, which prevents the pressure created by a high-flow water pump from forcing coolant past the radiator cap at high rpm. Upright radiators typically have the cap on the inlet side and thus subject the filler cap to the pressure drop of the radiators core in addition to the system pressure. >> The disadvantage of this is that it can lower the effective pressure of a 22-psi cap to as low as 10 psi.
Higher radiator pressure allows a higher boiling point for the coolant, so its best to use the highest pressure cap that the radiator manufacturer recommends. Higher coolant pressures also transfer heat more efficiently from the cylinder heads. Performance radiators will typically accept 22-24psi caps, although coolant will generally only build to 16-18 psi due to expansion up to 200 degrees F.
Most stock water pumps are manufactured to operate at relatively low engine rpm and do not provide an adequate coolant flow to meet high-performance demands. Aftermarket water pumps, such as those from Weiand, Edelbrock, and Stewart, are designed for high-performance use yet consume minimal horsepower during operation. High-flow water pumps can often flow 35 to 40 percent more than stock pieces, have higher quality components, and are frequently made from aluminum instead of cast iron.
A cooling system relies on some type of fan to move air at slow vehicle speeds. While mechanical fans usually work fine, when cooling conditions become complicated, electric fans not only provide superior airflow over mechanical fans, but they also commandeer less horsepower (some large mechanical fans can consume over 15 hp at 6,500 rpm). Furthermore, most mechanical flex fans are not designed for high-rpm use and can present serious vibrations concerns due to air turbulence.
Electric fans operate best when installed as a puller (behind the radiator), and sometimes multiple electric fans can outperform a single large one. A high-performance V-8 typically needs about 2,800-2,900 cfm for adequate cooling. Just be sure your charging and electrical systems are up to the task.
Pulley size plays an important role in an engines cooling capability because the diameter of the pulley affects the water pumps operating rpm. Although many pulley systems have a 1:1 ratio with the crank pulley, improved cooling can often be achieved by running a water pump 35 percent faster than the crank pulley. Exceeding that percentage is not recommended. Pulley manufacturers like March offer a variety of choices to change water pump ratios.
Thermostats allow an engine to reach operating temperature quickly. Used with a high-flow water pump without internal bypasses, the thermostat may need to be modified by machining three 3/16-inch bypass holes directly in the poppet valve to allow some coolant to bypass the thermostat even when closed. This modification results in the engine taking slightly longer to reach operating temperature.