You know what’s going to happen. You’ve been here before. You’re stuck in traffic, and the temperature gauge is doing that inevitable slow climb—210, 220, 230 degrees. Hot rodders seem to put up with cooling-system problems as if they were some kind of divine curse that must be endured. Forget that! Even in the mix ’n’ match world of Hot rodding, applying a couple of the simple ideas presented in this story can help you build a cooling system that will perform best when you need it most.
How Hot Is Too Hot?
First we should examine overheating. Some hot rodders think coolant temperatures over 200 degrees Fahrenheit are approaching nuclear meltdown. That’s overreacting. Most new cars operate normally in the 210- to 220-degree Fahrenheit range, and for good reason. That elevated temperature helps burn off exhaust hydrocarbons for cleaner tailpipe emissions. Of course, those higher temperatures aren’t necessarily ideal for ultimate performance, which is why drag racers prefer to run their engines with hot oil and a cool intake manifold. Elevated engine temperatures also increase the risk of detonation, which is why it’s not advisable to mash the throttle at low rpm with a hot engine. A general rule of thumb is an engine’s spark sensitivity to detonation increases 1 degree of timing for every 10-degree increase in coolant temperature. In other words, a hotter engine will rattle easier than a cooler one.
In everyday driving, engine temperatures of over 210 to 220 degrees Fahrenheit are not bad, especially if the cooling system doesn’t boil over. However, if you’ve experienced the steam genie dancing around your engine compartment, there are a number of solutions for keeping your cool.
There are really two areas that deserve attention, and both have to do with fluid flow. The first and perhaps most important fluid is air. A number of O.E.M. and aftermarket cooling designers and engineers we spoke with believe managing airflow through your radiator is the best place to control cooling-system temperatures. The second fluid is the engine coolant itself and how efficiently it travels through the engine.
Airflow
Let’s start with the most basic system, an engine-driven mechanical fan and a radiator. The fan is designed to create negative pressure behind the radiator to pull air through. But the farther away the fan is from the radiator, the less effective it becomes. Flex-A-lite recommends that the fan blades be within 1 inch of the radiator and no more than 2 inches away. But that may not be enough.
The accompanying illustrations show numerous ways to manage the air and force it to travel through the radiator in order to maximize the radiator’s effectiveness. Most street-car overheating problems occur at low vehicle speeds because of reduced airflow through the radiator. The best way to combat that is with a fan shroud. A properly designed fan shroud acts like a wind tunnel in pulling air through the radiator. The best shrouds are designed to pull air from the entire radiator rather than just in the center around the fan.
Flex-A-lite recommends that the fan blades be positioned so that approximately ½ of their depth projects into the shroud and fan-tip-to-shroud clearance be ¾ inch. Reducing that clearance will pull the maximum amount of air through the radiator, especially if you are using a fan with a significant pitch-angle blade. A tighter tip clearance is better, but it must allow for engine/chassis movement. Generally, the larger the pitch angle of the blade, the more air the fan will tend to pull through the radiator, especially at low speeds.
If you are experiencing high-speed cooling problems, a shroud might not be the best solution. The shroud can act as a restriction since the air must funnel between the engine-driven fan and the shroud. If you need a shroud to prevent low-speed overheating, you could create spring-loaded pressure-relief doors in the shroud, which would open as pressure increases inside the shroud at high speed.
Ford engineer Wayne Lawson gave us one especially interesting trick. His experimentation has shown that an electric fan located inside a shroud behind the radiator that rotates in the opposite direction of the engine-driven fan dramatically improves low-speed cooling. Lawson describes it as creating the equivalent of a near-zero tip clearance for the engine-driven fan. Keep in mind that the electric fan must counter-rotate while pulling air through the radiator. That will require using the correct fan blade to create that situation.
Another tip worth considering is that a large engine-driven fan with a large pitch angle could become a restriction at high rpm and act like a large, flat disc that reduces airflow. Hot Rod performed a series of engine-driven fan dyno tests a few years ago and found that a Flex-A-lite thermostatically controlled, clutch-operated fan with six blades offered as little restriction as no fan at all on the dyno. That was because at temperatures below 180 degrees, the clutch disengaged the fan from the engine. The disadvantage to the clutch fan is that it is large, and the fan size creates difficulties in fitting the fan into a tight engine compartment.
Electric cooling fans have become popular with both the factories and the aftermarket, but there are certain guidelines you should follow to optimize their cooling efficiency. Everyone we spoke to, including Flex-A-lite engineer Chris Horyn, emphasized that electric fans should always be mounted behind the radiator for optimal cooling. These fans do a better job of creating a low-pressure area behind the radiator than they do of pushing air through a radiator. According to Horyn, switching a 12-inch electric fan from in front of the radiator to behind it will increase airflow by a solid 10 percent, based on a 70-percent coverage of the radiator with the fan. A 16-inch fan would increase airflow by 15 percent.
Generally, engine-driven fans are more efficient than electric fans, unless the engine-driven fan is slowed down with underdrive pulleys. These pulleys not only slow down the fan so it pulls less air, but they also slow down coolant flow through the radiator and engine, which can contribute to overheating as well.
One other point worth mentioning is that the air must be able to escape the engine compartment once it has passed through the radiator. Extremely tight engine compartments (like an early Chevy II) might contribute to overheating problems because the air cannot easily escape the compartment. That could also contribute to a high-speed overheating problem, creating high pressure under the hood, which would block airflow through the radiator. That was the original reason for hood louvers in street rods.
Radiators
The task of optimizing airflow must also take into account the radiator. According to Ford’s Lawson, radiator design has changed significantly in the last 10 years, with many new cars moving to high-fin-density-count radiators. Fin count is determined by counting the number of fins in one row in 1 inch. Mid-’60s vintage radiators made of brass and copper typically fall in the 10- to 14-fin/inch category. Late-model radiators now reach a fin density of over 20 fins per inch. As you can imagine, it creates a more difficult path for the air to pass through. That generates a greater pressure drop, which occurs as the air passes through the radiator.
Lawson says that engine-driven fans with deep pitch-angle blades work best with low-fin-density radiators, while electric fans with shallow pitch-angle blades tend to work best with high-fin-density-count radiators. While you can use an electric fan with a low-fin-density radiator, its efficiency will not be as great. That relationship is based on the pitch angle of the fan blades. Because high-pitch angles require more power to operate, most electric fans utilize low-pitch-angle blades.
While it is true that brass and copper radiators offer thermal conductivity advantages over aluminum radiators, they are limited to a maximum coolant tube diameter of 5/8 to ¾ inch. Since brass and copper are very soft, larger tubes cannot handle the pressure. Aluminum radiators, such as those available from Modine and Griffin, can be built with tubes up to 1-½ inches in diameter. The larger tubes allow the radiator manufacturer to place more fins-per-inch, which improves the radiator’s thermal efficiency. That reduces the thickness (and weight) of the radiator and also improves airflow through the radiator. For example, an aluminum radiator with two rows of 1-½-inch coolant tubes is probably more efficient than a four-row brass/copper radiator. Not only would the brass/copper radiator be heavier, but its added thickness would present a more restrictive path for the air to travel, especially at low vehicle and engine speeds.
To put that another way, it is possible that replacing a three-core brass/copper radiator with a four-core brass/copper unit might actually reduce cooling efficiency at low speeds, especially if airflow through the three-core unit was already marginal based on identical fin densities. That’s because the increased thickness created a greater pressure drop across the radiator, especially with no shroud. You could make the four-core radiator work, but it would require improvement of airflow management with a shroud, a bigger fan or both. In other words, a thicker radiator is not always the answer to an overheating problem.
There are far more details to optimizing cooling systems than we have room for here, including concepts such as air/fuel ratio and timing, which can have a dramatic effect on low-speed cooling. We suggest experimenting with some of the suggestions presented here and investing some time into making your cooling system work more efficiently. You’ll find that you can have a hot rod that’s also a cool ride, even in the worst heat and traffic.
