a few thoughts on oil filters

A question that we often receive is “Do I really need an oil filter on my radial engine?”  The short answer is that I am a big proponent of oil filtration.  In the olden days the design philosophy was, for the most part, to install a reasonably coarse oil screen (#20 mesh or so, which will catch errant pistons and link rods but not a lot else) and then change the oil often.  During WWII thousands of the Cessna “Bamboo Bomber” remote firewall mounted oil filters were manufactured, which used a larger version of the same relatively coarse screen. This unit also gave a location from which to drain the oil, as well as a couple of 5/8” thermometer plugs which could be used as a location for temperature probes.  These Cessna filters were later used as standard equipment on the C-195 and Waco Classic YMF-5, and many have found their way onto various other radial-powered aircraft.

While the WWII Cessna filter is a whole lot better than nothing, its filtration does not compare to that of a modern cartridge type filter.  Probably the two most popular of these filters are the Airwolf and one or other of the ADC designs.  Both of these companies produce a spin-on style adapter, while ADC also makes a “pancake” filter with a reusable element.  That one is my personal favorite.  It comes with a bypass light that lights up anytime the filter goes into bypass, and has as an option a chip detector.  While there may be some debate as to whether you really want to know every time your filter finds a chip (especially on a newly overhauled engine which is often a chip producer), knowing when the filter goes into bypass is a valuable piece of information (if the filter becomes clogged enough to bypass five hours after an oil change it is a good idea to find a place to land).  The ADC pancake also has a ribbed cast aluminum housing that functions as a mini oil cooler.

One of the big differences between the spin-on filters and the pancake style is the installation location.  The spin-ons are designed to go between the engine oil outlet and the oil tank inlet, while the pancake filter goes between the oil tank outlet and the engine inlet.  Installing a filter on the engine inlet may disturb some folks who might be concerned about restricting oil to the engine, but the ADC pancake filter flows so much more oil than a spin-on (1.5 times as much) that this becomes a non-issue.  These pancake filters were certified at the engine inlet and years of experience with this installation on the Cessna 195 have shown it to be a successful design.

The question might become, “If the filter works well on the engine inlet, why not put it on the outlet just to be extra safe?”  There are a couple of reasons why not to do that:  1.  Putting the pancake ADC filter on the engine inlet side makes the filter the first line of defense in filtering everything that comes into the engine.  2.  Anything that you put on the outlet side of the engine (be it a pancake filter, spin-on filter, or oil cooler) will create some back pressure in the return line to the tank.  With some engines and aircraft this may not be a problem, but with the Jacobs engine in certain installations it is a problem.  Creating back pressure in the return line to the oil tank does two things:  it keeps the oil in the engine longer, thus causing the oil to pick up more engine heat; and because of the Jacobs oil pressure relief valve design the back pressure assists the relief valve spring in raising the engine oil pressure.

In installations where the engine oil temperature tends to run too low this additional oil temperature may be a good thing, and adding a spin-on filter to your engine oil outlet may bring the temperatures up into a better operating range.  However, in a tightly cowled engine where oil cooling is a challenge (think Cessna 195 and some Wacos), the few extra degrees of temperature created by the oil return backpressure is enough to push the oil temps to the red line.  We have seen several people install Airwolf spin-ons in their C-195s, only to remove them a short time later after picking up an additional 10 degrees F. oil temperature.  

When we were first testing the fuel injected R755A2 engine, we utilized a water-brake dynamometer.  Since the water-brake uses no propeller and produces no wind for an oil cooler, we decided to immerse the oil cooler in the dyno’s water tank to obtain our oil cooling.  This required running about seven feet of one-inch Mil 6000 hose from the engine oil outlet to the submerged oil cooler, then seven feet of hose back to the inlet of the oil tank.  When the engine was started we immediately had 225 psi of oil pressure!  The back pressure of the oil in that oil cooler and fourteen feet of one-inch hose assisted the oil pressure relief spring in the oil pump and basically locked the valve closed, causing the pump to produce all the pressure that it could.  Attempting to adjust the pressure relief valve did nothing, but shortening the hoses to the cooler reduced the back pressure and brought the oil pressure back to normal levels.

Summing all this up, oil filters are a good thing for your engine, just be sure to pick one that does what you want it to do it terms of heating/cooling oil.  And change your oil often; it is still the cheapest way to add to the longevity of your engine.