Part II continues
Oil Cooler Thermostats
The first question is, do you really need one? In the old days, two elastic bands and a piece of cardboard were all we needed. We used elastic bands to attach the piece of cardboard in front of the oil cooler to prevent the wind from robbing the oil of too much of its heat when the weather turned cold. People with some loose change bought or made nice leather covers that did the same thing.
Lockhart, which had been making oil coolers forever, was the first company (that I can remember) to supply automatic thermostats as an option on an oil cooler for a Harley-Davidson. However, much as I tried, I could not find Lockhart’s web address or a contact to interview. I asked Marv Beals from Jagg for help. “I am sorry to report the demise of Lockhart,” Marv replied. “It is always a sad day when we lose historically strong companies in our small market. I have always been of the opinion that competition helps to define us and helps to scratch out a market for all who are competing. I consider myself an amicable competitor and look forward to the shared market cultivation from all our efforts.” There are still other American-made oil coolers on the market such as Hayden (
www.HaydenAuto.com) and Derale (
www.Derale.com).
The early Lockhart thermostats were external, technically good-looking little gizmos that rerouted the oil away from the cooler and back to the oil tank from the engine when the oil temperature dropped below a predetermined setting, like 180 degrees. This temperature is a safe one, as we do not want to lower oil viscosity at operating temperatures. Many later thermostats became internal units incorporated into the oil cooler design as with the now defunct Lockhart 600 series. Jagg incorporates a thermostat into its #4700 oil filter adaptor for mounting a cooler on a Twin Cam.
There are also manual bypass systems like the optional #4000 Jagg Bypass valve. The rider must remember to turn off this valve by hand whenever the ambient air temperature drops below 55 degrees. Do not confuse this temperature with the other noted figure of 180 degrees, which relates to the oil and not the air temperature. If you do not ride in cold weather, a thermostat is not required. If you do, then one of the aforementioned techniques is necessary, whether it is a simple cover, a manual bypass valve, or an automatic thermostat.
As stated earlier, turbulated coolers like Jagg and the current Harley-Davidson coolers have impediments built into their internal channels (or rows) that prevent the oil from flowing through the finned tubes as a fast-moving laminar stream. Regular oil coolers just have finned, straight-through tubing. Oil on the outside of the stream is able to dissipate its heat to the surrounding fins, but the oil in the center of the stream largely retains its heat, defeating the purpose of a cooler. Causing turbulence in the oil stream brings the maximum amount of oil, and therefore heat, to the cooler’s heat-dissipating fins. I think it’s obvious that a turbulated oil cooler is the one to choose over a straight, flow-through one, as long as the pressure drop (I’ll explain this in a moment) is not prohibitive. Oil coolers are in the business of dissipating heat. Therefore, the turbulated oil cooler with the most rows or passages will be the most efficient. Next, we would choose the turbulated oil cooler with the most rows that has the greatest area of finning. Finally, we would go with a black one over a beautiful polished billet or lustrous chrome one because function and not form is what we care most about with performance components.
Pressure Drop And Heat Rejection
Pressure drop refers to restriction in the oil cooler that causes the oil pressure to drop below its normal flow characteristics. Pressure drop is the measure of pressure at the oil cooler inlet less the measure of pressure at the outlet of the oil cooler. Pressure drop will usually be quantified for our purposes in pounds per square inch (psi).
Heat rejection is defined as a quantifiable measure of the ejection of waste heat from the engine into the surrounding atmosphere. Heat rejection is very similar, but not totally synonymous with, heat dissipation or dispersion away from the engine into the surrounding air. Heat exchange is another popular term. There are nuances to each not readily apparent to the novice.
The term heat rejection conveys a better description of the goal of a motorcycle oil cooler. For example, heat rejection is often the goal of a heat exchanger but not always. A car’s heater core’s primary function is to move heat from the heater core to the car’s interior seating area where its benefits can offset the cold. On a Harley-Davidson, heat ejects from the cooler to the atmosphere. Heat dissipation is subtly different from rejection, and describes more of an inactive dynamic, such as passive radiation from a heat sink or house radiator.
Article concludes in the next thread. Check back issue for pix and more information. 
Techline-Nov.08-Oil coolers-Pt II: Heat Rejection and Pressure Drop (cont.) 
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