Global warming? While the tree- huggers clamor over ex-Vice President Gore's global temperature scare, we'll lay odds that when summer arrives, car crafters will be more concerned about engine warming or, more accurately, engine overheating. Cooling systems are often the last item on the "let's get it running" checklist. Unfortunately, radiators and cooling-system components qualify for afterthought status until there's a problem. Most of the time, modified cars tend to have problems with low-speed cooling. But we've also seen many scenarios where 70-mph cruising under a midday sun that can really turn up the heat. We dove radiator-neck-deep into this subject and came up with a few cool solutions. Check 'em out.
Airflow through a radiator can be maximized only if the entire core is covered by a shroud. This Flex-a-lite Black Magic fan uses an integral shroud and rubber seal to cover all but the corners of this Flex-a-fit aluminum radiator. If the fan generates sufficient airflow through the radiator core, it will enhance cooling even when stuck in slow-moving traffic.
How Cool Do You Have To Be?
There are three basic parameters that determine cooling efficiency: radiator surface area, coolant speed through the system, and the amount of airflow through the radiator. These three functions determine the efficiency of the system as expressed in Btu of heat rejection per minute. Even a small problem with any of these variables will cause difficulties. Since this is a rather complex situation with dozens of variables, we're going to just hit the high spots on how to make your cooling system more efficient.
The biggest limitation in terms of radiator surface area is the original vehicle itself. The radiator core support generally dictates the size of the radiator, along with the displacement of the engine and the horsepower it makes. Core support and engine size are obvious, but there is some voodoo science related to the horsepower number. Let's say your car makes 1,000 hp at 6,000 rpm. It'll create a certain amount of heat at that power peak, but you are going to spend most of your idling around the pits or on the way to the grocery store. At idle, it's likely that you are making less than 20 hp, which doesn't really create a lot of heat. Manufacturers therefore must create a balance where the radiator can be large enough to handle the engine's heat pontential yet still be small enough to fit in the car and be relatively inexpensive. Be Cool, for example, offers systems for most popular musclecars in 400, 700, and 1,000hp applications.
For an extreme example, we asked Be Cool's Mitch Drouillard what he would recommend for a '70 Mustang with an 800hp big-block. Be Cool actually offers a custom modular system that includes a monster 27-inch core-width radiator that is much wider than the stock radiator opening. This requires moving the battery to the trunk and includes a pair of 13-inch electric fans to maximize airflow especially in low-speed applications, and is efficient enough to handle this combination. The key is to buy a radiator with the largest core area that fits in the car with a capacity that will handle the Btu potential of your car's engine size and horsepower.
If this small-block pulley combo is used on the street, it's almost guaranteed to cause overheating issues because of the under-driven water pump. A far better pulley ratio will use a crank pulley larger than the water-pump pulley to create a ratio of at least 1:1.1.
Coolant speed through a radiator is the second essential component. Production small-block Chevy engines from the '60s and '70s, for example, generally used a 1:1 pulley ratio that was designed for low-density radiators with 31/48-inch tubes. When a car is modified with an aluminum radiator that uses larger 1-inch-diameter tubes, it may be necessary to increase water-pump pulley speed for sufficient coolant velocity through the radiator. This increased speed is necessary to create turbulence in the tubes and expose as much coolant to the walls of the tubes as possible. This is where you may have to experiment with your vehicle to come up with the right drive ratio, but a ratio of at least 1:1 to as much as an overdriven 1:1.3 is a good place to start. Worst of all would be the combination of a 1-inch-tube aluminum radiator with a slightly slower-than-stock pulley ratio.
The most common musclecar cooling-system problems concern low-speed cooling, most often attributed to low airflow through the radiator. Assuming the radiator is sized properly and there is decent coolant speed, then increasing airflow at low vehicle speeds should remove sufficient heat from the radiator to keep the engine at a manageable temperature. Engine-driven fans can move a tremendous amount of air but are also compromised by slow engine speed at idle while delivering sufficient airflow at higher engine speeds. Electric fans have become popular with the OEMs because they can move enough air at low speeds to keep the engine cool, relying on vehicle speed to push air through the radiator at highway speed. This reduces parasitic horsepower losses at highway speeds by eliminating the engine-driven fan. While this may seem trivial, dyno testing in the May '00 issue of Car Craft resulted in losses of 35 hp at peak horsepower from a simple one-piece, plastic, engine-driven fan. Clutch fans lost between 8 and 19 hp depending upon the clutch model, while a Flex-a-lite Black Magic electric fan driven by the alternator cost 1 hp. All these numbers were generated using a 496hp small-block Chevy at 6,300 rpm.
Radiator Basics
Let's start with radiator materials. Our automotive forefathers were pretty sharp guys and used copper/brass radiators for a reason. Copper has an excellent thermal-conductivity rating. A copper-fin's thermal-conductivity rating is more than 50 percent higher than an aluminum fin. Brass, which is an alloy of copper, is not as good a conductor as aluminum but is used for the tubes because of its strength. One difficulty with copper is that the lead solder used in older copper/brass radiators has a terrible thermal-conductivity rating, which limits the efficiency of lead-soldered radiators. So companies such as U.S. Radiator have instituted a newer process that improves efficiency by changing the flux and solder and its contact with the fins.
If you've ever wondered why some copper/brass radiators are cheaper than others, it's all in the construction. The original radiators built in the musclecar era used 11/42-inch tubes 91/416-inch apart that are generally the least expensive. More modern radiator construction moved those centers closer together, with the 11/42-inch tubes 31/48-inch apart. This creates room for more tubes in the same-size radiator core. There are even copper-brass radiators now with 11/42-inch tubes on 51/416-inch centers. Each of these versions can be obtained in two-, three-, or four-row applications. As the radiators become denser, they become more expensive.
Tubes in all radiators are flattened to increase surface area that contacts the fins. Aluminum radiators use 1-inch-diameter tubes like this cutaway from Afco. American-made radiators such as this one also use thicker wall tubes that are less likely to fail under high pressure.
Then why have aluminum radiators become so popular? One big reason is that the OEMs saw the potential for a significant weight reduction and lower material costs. Racers are also big on aluminum radiators for that reason, with a weight difference of around 10 to 15 pounds. Plus, aluminum radiators start with 1-inch cooling tubes roughly 31/48 inch apart. Fin counts are also a critical radiator-design component, but a higher fin density (measured in fins per inch) may make airflow more difficult and not necessarily work well for street applications.
The two major designs for radiators are vertical flow and horizontal flow. As far as efficiency is concerned, there is no advantage to horizontal-flow radiators other than that they tend to allow a larger core to fit into a given engine compartment. Virtually all production-based radiators are built with a single-pass design, where coolant enters from the engine into the top of the radiator and travels across the core to the outlet on the opposite side. While dual-pass radiators have been around for a long time, they are now beginning to show up in high-performance and racing applications. A dual-pass horizontal-flow radiator moves coolant across the top half of the radiator on the first pass, then directs the coolant across the lower portion of the radiator face for a second pass. One reason this works is because the velocity of the coolant roughly doubles when the coolant is forced to travel across half as many tubes per pass. This creates turbulence in the tubes, exposing more coolant to the radiator tube walls and improving heat transfer. This also presents an increased load to the water pump, which means using a dual-pass radiator demands a better water pump if the system is to take advantage of the dual-pass concept.
Coolant Chronicles
What you pour into a radiator is also an important decision if you want to protect all those expensive aluminum engine components. Straight water is the most thermally efficient coolant, but anticorrosion issues and cold weather demand antifreeze. According to Jay Ross at Applied Chemical Specialties, the best water to use is soft water. Distilled water is not a good idea because distillation strips ions from the water. When it is introduced into the cooling system, the natural chemical-balance process will pull the ions from light metals such as aluminum or magnesium that are exposed to the water. This ion transfer greatly enhances the corrosion process called electrolysis. Soft water is treated with sodium chloride that replaces the lost ions and minimizes the electrolysis process. If soft water is not available, then bottled water or tap water is the next best solution. If you insist on distilled water, Ross says mixing it 50/50 with antifreeze will pull ions from the antifreeze rather than from your cooling system itself.
Purple Ice and No-Rosion are excellent anticorrosion additives that can be used with either straight tap water or mixed with antifreeze to reduce the formulation of corrosion and scaling in the cooling system, which can reduce the performance of your cooling system.
If you are a drag racer who is required to use straight water, a high-quality anticorrosion additive is essential. We've found the No-Rosion additive from Applied Chemical works very well. A pint of this additive applies a thin anticorrosion layer to the cooling system to fight deposits and limit the effect of electrolysis, yet it does not hurt heat transfer. Royal Purple's Purple Ice is another anticorrosion product that uses additional additives called dispersants to help reduce the formation of steam pockets in the cooling system, which can reduce heat transfer from the combustion chamber, causing detonation and boilover. Additives such as Red Line's Water Wetter and Purple Ice address this by reducing the size of these steam pockets. When steam pockets form, they act as insulators, preventing heat transfer out of the combustion chamber. While it may seem obvious, it's worth noting that these additives will not help a car with problems such as an undersized radiator or insufficient water or airflow. These additives are not a mechanic in a can.
The Electric Side of Cool
What most car crafters rarely consider is that the early '60s and '70s vintage alternators rated at 60 to 70 amps were not designed to crank out maximum amperage at idle. Late-model alternators or high-performance alternators rewired by companies such as Powermaster are designed to generate greater amperage at idle. These more efficient alternators are capable of delivering the 40 amps or more required of dual fans running at full boogie along with a big electric fuel pump, lights, and maybe a thumpin' stereo. Add the draw from a pair of headlights and perhaps a defroster or A/C fan, and a load of 50 to 60 amps from the alternator at idle is not unusual. This will also require large 8- or 10-gauge wiring from the alternator to the underhood power source for your fans and multiple solid-ground circuits between the engine and the chassis. A good ground also means the ground wires should be of equal size as the power leads. The biggest electric fan won't run at anywhere near peak efficiency if the ground circuit suffers from resistance. A simple voltage drop test will tell you if the wiring circuit is the culprit.
Electric Fans
The toughest question when choosing an electric is how to pick the right one. There are dozens of electric fans out there and unfortunately no accurate backyard test for fan efficiency, but we've uncovered some handy shortcuts that can help you choose the best fan for your application. As a general rule, straight-blade fans move more air than curved-blade fans, but you'll pay the price in terms of increased noise.
There is no common industry standard for rating electric fans. Most companies use a cfm rating, often expressed in free-flow and not when placed behind a radiator. This makes comparisons of electric fans difficult. Spal publishes its test data on its Web site for each electric fan. Any fan's highest cfm rating occurs with zero static pressure, or with no airflow restriction in front of the fan. Spal expresses this restriction in terms of inches of water. As the restriction increases (with a thicker radiator core, for example), flow volume drops while current flow increases slightly. According to Jason Schmidt at Spal, one rule of thumb is 10 amps of current flow per 1,000 cfm of air. This is not accurate in all cases, but if you find a fan rated at 3,000 cfm that only requires 10 amps, the cfm rating may be optimistic. Spal rates all its fans, and the three we investigated revealed 17 amps for 2,000 cfm, 21 amps for a 2,360-cfm fan, and a third pulling 26 amps at 3,000 cfm, all rated at zero static pressure.
Twin fans can be a plus in tight-clearance situations, since staggering the two fans moves them away from the water-pump pulley. This is a twin Spal-fan setup that incorporates small rubber flaps that open at higher vehicle speeds to improve airflow.
Two fans usually can cover more radiator surface area than one large fan, which makes the twin-fan systems generally more efficient. Twin-fan performance is also often enhanced by built-in shrouds that pull air in from the entire core surface as opposed to just the area of the radiator covered by the fan. To boil it all down, if you're experiencing overheating difficulties and the rest of the cooling system is optimized, increasing airflow with a pair of smaller fans covering the entire radiator core will generally improve airflow and efficiency.
Aluminum Radiators on a Budget
Aluminum radiators are nice, but they can be expensive, costing between $400 and $550. But cruising through the Summit catalog we ran across Summit race radiators. These are universal crossflow aluminum radiators with no mounting tabs and with either GM- or Ford-style inlet and outlet configurations. These radiators are a two-row design with 1-inch tubes, and come with a machined-aluminum filler neck welded into place. For a person who is willing to do some simple mount fabrication, these radiators can be fitted to many different applications. We decided to bolt one into our 455-urged '64 Olds F-85 that was getting by on the stock vertical-flow radiator originally intended to cool a 330ci V-8. Because the F-85 uses rubber saddle mounts on the top and bottom, it turned out to be an incredibly easy installation. The only extra work we had to do was to add an external B&M trans cooler to accommodate the automatic transmission since these universal radiators do not come with internal trans coolers. One limitation to using a completely separate trans cooler is that in heavy traffic, a loose converter may raise the trans temperature due to extra slippage. If that's the case, this may require a small electric fan attached to the trans cooler. Of course, the extra trans cooler and fittings also add to the overall cost of the radiator swap, so do your homework first before just buying the least-expensive radiator.
In our case, there wasn't enough room for an electric fan between the radiator and the water pump, so we had to stick with our engine-driven fan, which is unfortunate since it definitely eats horsepower. We'll also have to fabricate a shroud for this combination to optimize airflow through the radiator.
Cooling-System Tips
While the cooling system may seem simple, consider not only the variables of coolant flow, airflow, and radiator efficiency, but also how other engine systems affect cooling. If the charging system is lame, your electric fan won't spin as fast. If the ignition curve is slow, that will affect cooling. We've assembled a series of tips and tricks that can often make the difference between an overheating monster and a docile street machine that can handle gridlock in 110-degree weather.
*Ignition timing has a direct effect on cooling-system performance. Retarded ignition timing begins the combustion process later in the cycle and makes heat. Initial timing numbers of 12 to 16 degrees and a curve that's all in by 2,500 rpm is a good starting place.
*An electric fan placed on the engine side of the radiator (as a puller) is always more efficient than a pusher fan. However, additional airflow can be created by using a second pusher fan on the front of the radiator.
This '57 Chevy's builder went to extraordinary lengths to ensure that all the air entering the grille would travel through the radiator. Building small-block off-plates between the radiator and the core support to direct all the air through the radiator will accomplish the same thing.
*Third-generation ('82-'92) Camaros came with an airdam placed directly under the radiator, which on older, high-mileage cars might be damaged or removed. These airdams are essential to create a low-pressure area behind the radiator to move air through the radiator.
*Jason Schmidt is an engineer with Spal, and he told us about a customer who had connected the power wire for a large electric fan directly into his fusebox. When large fans start, they can pull as much as 80 to 100 amps for 0.10 second. This large current draw pulled the voltage down far enough that the engine died. Wiring the power lead for the fan through a relay that sources power nearer the alternator cured the problem.
*The ideal tip clearance for engine-driven fans is 11/42-inch with the fan blade extending roughly halfway into the end of the shroud. This will create the greatest amount of air movement past the fan.
*Most engines are thermally more efficient at a coolant temperature of 195 to 200 degrees Fahrenheit. Pressure is also a critical function of coolant efficiency. A typical street-car cooling system operates at 15 psi. This pressure also increases the boiling temperature of water. As a rough rule of thumb, for every 1 psi of cooling-system pressure, the boiling point of straight water will rise between 2 and 3 degrees. Water boils at sea level at 212 degrees Fahrenheit, but at 15-psi gauge pressure, water boils at 250 degrees Fahrenheit.
