There is one inescapable fact about any high-performance machine: If it makes power, it also makes heat. We’re talkin’ BTU’s to the max. The combustion process and high-stall-speed torque converters are the top two on the heat-generator hit parade. If that heat isn’t managed properly, the oil in those components overheats and begins to break down. Oil quickly fails its primary missions of lubrication and cooling when it overheats. Oil temperatures of over 220 degrees Fahrenheit are considered excessive and will quickly break down petroleum-based lubricants. How do you protect your engine or transmission from a high-oil-temperature failure? Simple—bolt on an external cooler.
The Automatic Answer
Automatic transmissions are the most susceptible to oil-cooling problems. While engines also heat the oil, the engine’s radiator helps maintain oil temperature. All factory automatic-transmission-equipped cars come with some type of trans cooler, but they are usually routed through the engine radiator. With engine-coolant temperatures averaging over 200 degrees Fahrenheit in everyday driving, factory coolers are rarely sufficient help in a high-performance application.
The biggest potential to increase automatic-transmission oil temperatures comes from high-stall-speed torque converters. Increased stall speeds come from intentionally creating more slippage in a torque converter. The slippage “beats” the oil, which raises oil temperature. That’s why all high-performance automatic-transmission and torque-converter companies insist that a high-performance oil cooler be installed with any high-stall-speed converter, especially in street applications.
If the additional heat generated by a loose torque converter isn’t adequately controlled with a more efficient cooler, it will dramatically shorten the life span of the automatic, or even worse, kill it altogether. That’s an expensive lesson no one wants to learn. Compared to the cost of a new converter and automatic, a $100 cooler is a wise investment.
Now that you know why you need a cooler, which ones are the best? As you can imagine, there are literally dozens of different coolers on the market. To some extent, they will all do the job. As in most things in life, though, you get what you pay for. Generally, the higher-quality coolers from Earl’s, Russell and Hayden’s Rapid-Cool line are good choices.
These coolers combine small size, light weight and excellent durability with efficient heat-rejection characteristics. That means a relatively small cooler can do the job of a larger cooler that is less efficient. The more efficient coolers are typically a plate-and-fin design or a design similar to engine radiators such as Earl’s new Topless coolers or the new Setrab-built Russell cooler. These coolers increase the total surface area of the tube to increase the thermal transfer process. In addition, these coolers have denser fin counts that help radiate heat. Many of the cooler companies claim up to a 30-percent increase in heat rejection over the round tube-and-fin design.
The round tube-and-fin-style coolers have been around for decades and tend to be fragile and not as efficient in moving the oil through the cooler, creating flow losses. As you might guess, the round tube-and-fin coolers are also less expensive than the more efficient plate-and-fin design.
A Cooler Alternative
Typically, street-driven engines do not create excessive engine oil temperatures under normal conditions. However, with many of the new street-car competitions, such as the Silver State road-race challenge, street-car road races and events such as the Automotive Triathlon, engine oil temperatures can quickly skyrocket. As with automatic-transmission oil, engine oil temperatures above 220 degrees Fahrenheit will quickly break down a petroleum-based oil.
Synthetic oils, such as Mobil 1 and others, can withstand these temperatures for a longer period of time, but lower temperatures are still preferred. Large, external oil-to-air coolers such as the ones we’ve covered can be used. The problem is that these coolers require large -8 or -10 braided-steel oil lines and high-flow AN fittings to prevent pressure losses and flow restrictions. Unfortunately, these fittings and lines are expensive and usually locate the cooler in front of the radiator. But there is another alternative. Chevrolet offered an oil-to-water engine oil cooler on both late-model Corvettes and Camaros that used engine coolant to reduce oil temperatures. This space-efficient cooler fits between the oil filter and the engine, routing engine coolant to a sandwich cooler. These coolers warm engine oil with faster-heating coolant when the engine is cold, and then they cool engine oil when oil temperatures climb under load. While not as efficient as the larger oil-to-air coolers, they will tend to limit oil temperatures.
Automatic-transmission and engine oil coolers are by far the most popular street-cooler applications. While power steering can also be cooled, it is rarely necessary. Chevrolet did do this on the late third-generation Z/28 Camaros by routing the power-steering fluid through a long steel pipe across the bottom of the radiator. That works as an inexpensive cooler, since in aggressive turning situations, like a road course, the power-steering pump can generate excessive fluid temperatures. The ’93 Mustang Cobra R also comes with a power-steering cooler. All-out NASCAR cars and off-road trucks install oil coolers on both the manual transmission and rearends in order to maintain fluid temperatures, but that’s not necessary for street cars.
If you are considering using a more-efficient cooler for your automatic or installing an engine oil cooler, first consider installing an oil-temperature gauge. If your oil temp never exceeds 200 degrees Fahrenheit, a cooler probably isn’t necessary. But if you find that your ATF consistently spikes higher than 210 degrees Fahrenheit, the heat’s on to install a better cooler.
