Franklin Oil Bypass

The topic of oil cooling on the Franklin engine comes up quite often. Here are some tips shared on the Reflector.

Oil Temp

To all Franklin drivers,

The misinformation is getting way too deep for me to remain silent. Lycoming engines have "Vernatherms", Franklins do NOT. A vernatherm works somewhat like a thermostat in an auto only backwards. In an auto the thermostat is closed when cold and opens on temperature rise. The vernatherm is open when cold allowing oil to bypass the oil cooler and go directly to the oil galley. When the oil gets hot it closes the bypass forcing the oil to flow through the oil cooler. The Franklin works on a different principal. The external oil hoses are connected to an oil control housing on the copilot side of the engine. This housing contains two spring loaded valves. The one that faces away from the prop is the oil pressure regulator valve and comes set at 80 psi. The second valve facing toward the prop is the oil cooler bypass valve that is set to open at 15 psi. When the oil is cold it is viscous (thick) which causes a large pressure drop in the oil cooler loop forcing the bypass valve to open and allowing oil to flow directly into the oil galley. Hot oil flows more easily allowing the bypass to close. The Franklin in Velocity problem is the length of the oil lines to the nose cooler causing an excessive pressure drop that in turn limits flow through the cooler. Mike did the two things that solve this problem. Bigger lines and a stouter spring on the bypass valve. Oil pressure and temperature are both measured where the oil enters the galley, after the oil cooler. That applies to Lycoming too. The oil pressure will always read less than 80 psi because of the pressure drop in the external loop so 60 psi is a good number.

Larry Coen

Oil Bypass

The franklin engine I have in my 173LW is running high oil temp, 245+. Having read recent posts, I am planning to install a larger oil cooler with a vernatherm. I would appreciate comments from those who have done this. Is there a modification necessary for the factory oil bypass valve? Grover McNair N832AB

Grover,
You still haven't said where you are measuring the oil temp. As PZL memo states, you should be measuring it at point 7.1, just before the oil return to the engine. I modified my plenum by splitting it in two and sealing the base of the cylinders. Now, in Florida's heat, I can take of and climb to 5000 ft with CHT's remaining below 390 F. and oil temps hovering between 215 and 220 F. Attached is a PZL bulletin of what Brian is saying.

Click for a larger image.

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