You have built a great engine with lots of power. Now you want to be keeping it cool in your car. All too often we see guys build fantastic cars, but neglect to pay attention to a critical component that keeps everything running smoothly: the cooling system. Tough to believe it gets overlooked but think about how many times you have been at a show, cruise, or autocross event and seen a really nice car on the side of the road with overheating issues.
Assuming you do not have any tuning issues causing your car to overheat (too much ignition advance, excessive lean condition, plugged exhaust), there are basic factors that affect cooling system function and efficiency. Let us give you a little background first on keeping it cool.
Keeping it Cool With Radiator History
While all radiator cores might look the same, they perform vastly different in keeping it cool based on tube spacing and fins per inch. The heat transfer points of a radiator are where temperature is allowed to leave the radiator and that occurs where the fins are bonded to the tubes. The more transfer points a radiator has, the greater the temperature drop will be between the inlet and outlet.
For comparison, a ’60s-style core typically had tubes spaced -inch apart; that is -inch of fin between the tubes. By going from a two-row radiator to a four-row core design we were able to double the heat transfer points which resulted in a 15-20-percent increase in temperature drop without changing the other variables such as air or coolant flow.
The move to aluminum radiator construction was purely financial. The raw materials to build a radiator are purchased by the pound and a finished aluminum radiator weighs about 25-percent of a copper/brass unit (dollars per pound being almost equal at that time). The result was a huge financial savings for car companies.
The Differences Between Copper/Brass Vs. Aluminum Radiators
When it comes to the difference in performance between copper/brass and aluminum radiators you may find the tests surprising. We found that temperature drops at all operating ranges were virtually the same, with a slight advantage going to the copper/brass unit. But consider this: The thermal conductivity or heat-transfer rate of copper is 92 percent versus aluminum at 49 percent.
However, the copper fin is bonded to the tubes or water passages using lead solder, which is very inefficient and slows the heat transfer rate to just slightly better than that of aluminum. This can be a disadvantage if the bonding process does not allow the copper fin to touch the brass tube and why not all copper/brass cores of similar design, but different manufactures, transfer heat equally.
Copper/brass radiators, because of their weight and durability, have been around a long time and are easily disassembled and reassembled for cleaning purposes. Not the case with aluminum unless speaking of the O.E. version that comes with crimp-mounted plastic tanks. As a result, the life expectancy of the aftermarket aluminum radiators will be far less than that of copper/brass.
Heat Production (BTU/HP)
BTU (British thermal units) measure how much heat the engine produces. One horsepower is equal to about 42.44 BTU. About one third of the heat generated by the engine goes into the coolant/water mixture and must be dissipated by the radiator. When you are trying to calculate the amount of BTU your engine produces, you only need to consider the engine’s horsepower that is continuously being used, not its peak power output. A car that cruises a lot and runs in the meat of its power band continuously for extended periods of time will need more cooling capacity than a trailered show car or one that sees light driving duty.
How much heat an engine displaces though the water system will determine how much radiator is needed to cool it. The horsepower is just one of many factors. You can cool a 650hp LS motor with the same-sized radiator as you would use with a 65hp flathead motor. The flathead motors are usually extremely hard to cool because so much heat is transferred to the water jackets, whereas the new LS motors are very well designed.
Recommended Engine-Operating Temperatures
Most hobbyists are not concerned with fuel efficiency, so our recommendation would be 175-195 degrees. Higher operating temperatures will burn fuel more efficiently, but the increase in operating pressure and metal distortion can easily create problems over time.
Determining The Size of a Radiator
There are formulas to determine appropriate radiator size based on engine heat output (operating BTU’s) and radiator heat-transfer rates (also stated in BTU’s). They can be found in any engineering handbook, but my recommendation to a hobbyist is to put in the most efficient radiator that fits the hole or intended application, up to a four-row copper/brass or two-row aluminum core. Everyone knows by now that copper/brass units use -inch tubes while aluminum uses 1-1-inch tubes. That way the thermostat or lower limit control can maintain the lowest temperature you have determined best for all driving conditions.
Radiator Capacity (Heat Dissipation)
The radiator’s capacity is the amount of heat it can dissipate, not the amount of coolant it holds. Radiators cannot be judged on physical size alone these days because of the varied materials they are being made from. In the past, most radiators were made from copper because of its superior heat dissipation properties. The drawback was the solder used to assemble radiators would inhibit the copper’s ability to dissipate heat. The advent of aluminum radiators has allowed the switch from 1⁄2- to 3⁄4-inch-wide tubes to 1- to 1.5-inch-wide tubes and the use of double-pass tanks. Wider tubes have more surface area, which allows for increased heat dissipation.
Dual-pass radiators force the water to travel the length of the radiator twice, increasing the amount of temperature drop capable for a given-size radiator. The downside to dual-pass designs is the coolant flow restriction is more than doubled. Surface area is the most crucial factor with radiators. Doubling the square-inch surface of your radiator will double the heat dissipation capacity, whereas doubling the thickness is less effective and restricts airflow.
Another factor is whether your car is running air conditioning and/or an automatic transmission or engine oil cooler. A typical A/C condenser is right in front of the radiator and exchanges heat with the air just like what it sits in front of. If you do not have enough radiator capacity, then every time you hit the A/C button, your car’s bound to overheat.
Other factors playing a role in radiator design and function are fin count per inch and configuration, i.e., downflow (top-tank) or crossflow (side-tank) radiator designs. Inlet and outlet size also play a key role.
Usually, the size of the radiator is defined by the size of available area. If you build the “biggest” radiator you can get into the area, it`s hard to go wrong. The available space will determine whether a radiator will be a downflow or a crossflow. Water doesn`t care if it flows up and down or side to side. You just must be careful to keep the tubes covered with water. Getting air pockets in the water system can do a lot of damage. You need a recovery system with a crossflow radiator.
Cooling Air Flow & Fans
Air flow is the most critical factor in the cooling system and affects a radiator’s cooling efficiency the most. A vehicle’s speed, be it a streetcar or racecar, is the point most considered when figuring out airflow necessary for proper cooling. Maintaining adequate airflow at a car’s various operating speeds is crucial and complex. First, the radiator must be supplied with fresh air. The grille opening or air inlet can make all the difference here. Ideally it should be facing squarely into the wind. With older cars, frontal/grille area usually is not an issue, except for Corvettes. The size of the grille opening should always be proportional to the vehicle’s operating speed(s). Big-block-powered C2 and C3 Corvettes are notorious for cooling issues because of the smaller front surface area they have, along with their tighter engine compartments.
A radiator transfers the heat in the coolant to the cooler air passing through the fins and over the coolant tubes, or more simply put, the radiator’s core. For the radiator to work properly, the flow of air must be under high pressure at the front side of the radiator and lower pressure behind. This pressure differential pushes the air past the fins. If air pressure builds up in the fan shroud or the engine compartment and the pressure differential is decreased, the airflow across the radiator can slow down and “stall” much like the airflow over the wing of an airplane. When planning out your car’s cooling system, you must consider both idle and cruise conditions and how fresh air can be presented to the radiator effectively in both situations.
Electric fans versus mechanical/clutch-type fans are really an easy decision. There are normally two types of overheating situations. If you are running hot at highway speed, you do not have enough radiator capacity. If you are overheating at idle/slow speed, you do not have enough airflow. This is where the electric fan works, and the “engine” fan does not. The type and quality of electric fan is especially important. Accurate airflow cfm numbers are critical. The more air you can move through the radiator, the more heat you can dissipate.
Water Pump & Flow to Keep it Cool
Coolant flow is usually the last aspect of the cooling system to be addressed. Ironically, it is also the usual cause for overheating problems. A typical stock water pump has excessive clearance and straight impeller blades, usually open front and back. With the engine running at low rpm, this produces little coolant flow and is typically responsible for cars overheating in traffic at idle speed. At high rpm, this design will cause cavitation and aeration, which can also cause the coolant flow to be reduced to the point of engine overheat. A common Band-Aid fix for this problem is to run underdrive pulleys, which slows down the revolutions of the water pump/impeller. While the high-rpm cavitation problem is solved, this solution usually contributes to a low-rpm overheat problem because the water pump is not turning fast enough. With an engine-driven water pump, the only remedy is an aftermarket race-style pump with tight clearances and a swept-blade, closed-impeller design
Electric water pumps are a highly effective solution to these problems with multiple benefits. The constant speed of an electric pump eliminates high-rpm cavitation problems and low-rpm insufficient flow issues. A bonus is being able to run the pump when the engine is shut off, especially useful racing applications.
The third benefit is the elimination of parasitic horsepower loss from the engine having to turn the water pump off the crankshaft.
Cooling System Pressure
Cooling system operating pressures are determined by water pump operating pressures, and we prefer to keep it under ten pounds. We all know by increasing the system pressure by one pound we increase the boiling point by 3 degrees, so by running a 12-pound cap our water will not boil until it gets to 248 degrees. Trust me, an engine that wants to run at 248 degrees will open that cap up long before it gets that hot. To deliberately increase the operating pressure to increase cooling is redundant in my opinion and again only points out the need for more efficient heat transfer.
Pressures will increase in the system just after turning off the engine as the coolant absorbs existing engine heat but cannot move through the radiator to dissipate it. The resulting increase in pressure pushes coolant past the cap and hence the need for a coolant recovery system.
Once the coolant in the idle engine starts to cool, a vacuum is created and another valve in the cap opens and prevents the radiator from collapsing a top tank, but more importantly, returns the coolant to the radiator so no outside atmosphere or air (contamination) enters the sealed system. Unfortunately, most aftermarket recovery tanks are smaller than the needed capacity and that varies with cubic inches and size of the engine.
Even though your temperature gauge might never go beyond 192 degrees, you can have hot spots around the combustion chamber that will be more than the coolant’s boiling point. A lack of pressure in the cooling system allows boiling to start prematurely. Gasses produced by the coolant boiling push water out and simultaneously aerate the coolant, making the cooling inefficiency worse.
The more pressure the water pump produces, the less chance there is of steam pockets forming. The same boiling point law mentioned earlier works here too. Racing-style water pumps can generate pressure in the water jacket more than 30 psi to minimize hot spots and reduce detonation/pre-ignition.
Finally
We all know classic cars and trucks are very cool. On the other hand, vintage cars that run hot are not keeping it cool. There is nothing worse than steam coming out from under the hood of your prized classic car while boiling coolant pours onto the ground. Overheating can be an inconvenience or a catastrophe depending on when and where, and to what degree (no pun intended) overheating takes place. But the fact is, with a properly designed cooling system, overheating should not be a problem.
Hello classicautoadvisors.com owner, Thanks for the well-organized post!