Understanding Oil Coolers

When engine output rises beyond a certain threshold per liter of displacement, an oil cooler becomes more important, critical even. There is a lot to the selection and installation of an oil cooler, so to find out more, we caught up with Zac Beals, a technical sales representative with Setrab USA, a Swedish company that specializes in a full range of heat exchangers and radiators for OEM applications, and oil cooling for motorsport. There are right and wrong ways to add an oil-cooling system, based on application and a number of other factors, but there are two key tenets to follow when adding an oil-cooling system: get expert help and don’t skimp on materials.

“Oil is the only thing preventing metal-to-metal contact, and any high-performance engine is designed with its own optimal oil temperature range based on how much work the oil is doing in that system,” Beals said. “The demands on the oil in a high-revving turbocharged four-cylinder are different from the demands on the oil in a naturally-aspirated V-8, and the differences only get more specific from there.

“What we do know for sure is that most generally, temperatures in excess of a normal operating range will break down the ability of the oil to do its traditional lubrication job,” Beals added. “A rule of thumb is that every 20 degrees in excess heat will half the life of the oil. This has a related effect on every internal component the oil touches.”

An oil-cooling system consists of the fittings and hoses to get the oil out of the engine to the cooler itself and back into the engine. It seems pretty simple, right? Not exactly.

Any fluid flow is restricted by the smallest orifice it flows through. The size of the fittings, the length of the lines and bends in the plumbing system can affect oil pressure and flow. The range of common plumbing sizes range from a AN -6 to a -16, with the majority of fittings falling somewhere in between at -8, -10, -12. For an in-depth story on AN fittings, see the Understanding Performance Fittings – Straight and AN Threads vs. Tapered Thread from March 2017 Speed News Article.

“Finding the ideal plumbing size is a matter of considering the oil flow at the expected RPM in your most demanding situation, considering the distance from the oil source to the oil cooler, how many bends the oil lines might make, whether oil supply is being accessed on the pressure side or return side of an oil system, and can be further determined by the type of oil system, whether wet or dry sump,” Beals said.

There is no one-size-fits all for the plumbing, which also is true for the heat exchanger itself. The efficiency of a heat exchanger is measured in BTUs per hour. What’s really nifty is that any heat relieved by an oil cooler is relatable to some amount of horsepower that is not lost as waste heat in the engine. Beals said the rule of thumb is 1 horsepower equals 2,544 BTU per hour.

To find the right size heat exchanger, the first consideration is power output, then allotted space for mounting it and how the vehicle will be used, and this is where good components are critical. Cheap oil coolers can result in either low oil-pressure drops with poor performance, or adequate performance with extreme pressure drops. Properly designed and planned oil cooling systems can approximate or come close to zero net pressure loss.

To find a suitable place to mount a cooler, Beals advised mounting it on as many points as possible, and isolating it from vibration using dampened mounts. Further, chassis mount points should be such that they vibrate on the same plane, such as a radiator support. If you mount it on different planes of motion, like a linear frame rail and a lateral radiator support, you could end up tearing the mounts off with chassis flex.

It’s also a good idea to flow oil through the filter before routing it through the cooler. Doing it that way reduces restriction by filtering warmer, thinner oil and supplies clean oil to the cooler.

To prevent untethered oil hoses from “whipping,” secure them as close to the cooler as possible. Whipping can stress the oil cooler connection and lead to fatigue over time. For fittings, minimize the severity of bends to make the smoothest curves possible. For instance, don’t use a 90-degree fitting where a 45-degree fitting will do.

While trying to keep the heat exchanger as close to the engine as possible, you also should mount it in a position to maximize air flow. A heat exchanger should be placed most ideally to face directly into the oncoming air stream and, just as importantly, there should be a proper path of exhaust from the rear of the heat exchanger. If you need to use a cooling fan, it’s better to pull air than push it.

“One oil radiator may look similar to another, when there can actually be significant engineering and manufacturing differences between them,” Beals said. “True cost of ownership is really illustrated by the difference between paying less for a less expensive, poorly-engineered and manufactured oil radiator versus investing in a good value.”