Moto Guzzi National Owners Club
Moto Guzzi National Owners Club
Moto Guzzi National Owners Club
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Oil Filters Revisited

The Short of It

Good News for Moto Guzzi owners. Fram (a trademark of Allied-Signal Company) has recently authorized its PH6022 as a replacement filter for the UFI part number 2328700 (the filter used on recent big block Guzzis). Fram is updating its parts catalogue to include the cross reference as an authorized replacement.

The Long of It

There has been considerable discussion in the recent past regarding allowable oil filters for Moto Guzzis. This piqued my curiosity: oil filters are something we tend to take for granted. As with any subject, the deeper in you get, the more there is to learn. This is a summary of what I've learned about the fundamentals of filters and engine lubrication systems. A list of worthwhile references appears below, should the reader wish to get to the source material. University reference libraries are a good place to go to for this type of info.

(Topic for Sitting-Around-the-Campfire: Did you realize that the only high speed engine part to receive unfiltered oil is the oil pump? Think about it.)

While getting into the design of engine lubrication systems, I also contacted Allied-Signal's Engineering Applications Department to see if they had recommendations for the Guzzi motorcycle.

As a result of this inquiry Allied-Signal undertook a standard test procedure to determine the use of a replacement filter for the UFI 2328700 (used in recent big block Guzzis). This involved purchasing a dozen or so UFIs, measuring their physical dimensions, then running them through a battery of tests to determine their dynamic characteristics such as burst pressure, relief valve blow-off pressure, and filter element efficiency. The standard for oil filter evaluation is the SAE HS-806, Oil Filter Test Procedure. This is a series of standardized baseline test procedures for engine oil filter evaluations. Each manufacturer also has some proprietary tests, but SAE HS-806 is the industry standard.

The lubrication system found in Guzzi's current engine is typical standard automotive practice. Oil is drawn from a reservoir, motivated through a positive displacement pump, forced through a line with a parallel pressure-relief path available, then through a filter with internal relief valve, and then on to the engine proper. Return oil drains by gravity back to the reservoir where the process starts all over again. The Guzzi engine's oil pump has a coarse screen in front of its inlet as a further protective measure. Say, for example a piece of piston skirt should break off, it won't end up jamming the oil pump.

A positive displacement pump does not generate pressure, per se: it generates flow. Oil pressure is generated only by attempting to restrict that flow through either the oil's inherent resistance to flow (viscosity), or by mechanical restriction. The pump used in the Guzzi is a gear pump of the "external gear" or "two-shaft" design (versus an "internal-gear" pump). This type of pump has been around for over a hundred years and its characteristics are well suited for engine lubrication. It is efficient, simple, has good wear characteristics, and ideally, will force a fixed volume of oil for each rotation of its shaft.

For example, a given gear pump will flow 1.0 gallon of oil per minute at 3000 rpm. At 6000 rpm, that same pump's flow rate will be approximately 2.0 gpm, and at 1500 rpm the flow rate will be approximately 0.5 gpm. This type of performance is nearly ideal for automotive type engines, which can benefit from a lubrication flow rate proportional to engine speed. The pump will cause these amounts to flow, regardless of discharge pressure.

This is "ideal" performance. A real pump operating at high pressures experiences a slight reduction of flow compared to ideal as a result of oil being forced back through the clearances in the pump's side casing and gear teeth. Still, a typical automotive oil pump in good operating condition is a reasonably good approximation of an ideal positive displacement pump.

As mentioned, positive displacement pumps do not primarily generate pressure. For such a pump to register pressure on its outlet side, it is necessary to attempt to restrict the oil flow which has passed through the pump. Take, for example, the case in which hot, thin oil is pumped through an engine which is running at a fairly slow speed (idling). Imagine having an oil pressure gauge sensing the oil's pressure between the pump's outlet and the oil filter. Since the thin oil flows readily through the filter, and the oil's flow rate is pretty low, there will not be much "attempted restriction" to the oil's flow, and the pressure registering on the gauge will be low. Now imagine spinning off the oil filter. The restriction to flow will be even less. The pressure gauge will measure an even lower pressure, even though the flow rate passing through the pump has not changed at all.

Now that we've removed the oil filter, imagine poking a finger into the port from which the oil is coming. (Wear an imaginary glove so you don't get burned.) The harder you push your finger into the hole, the higher the oil pressure gauge registers. The volume flow rate through the oil pump will not change significantly at all. It'll just squirt faster through the gap it has forced between your finger and the port. In fact, a whole range of pressures is available depending on how much you "attempt" to restrict the flow, even though you have not measurably affected the oil flow rate.

Okay, now spin the oil filter back on our imaginary engine and, instead of hot thin oil in it, feed it with cold thick oil. As soon as that cold oil hits the oil filter, the pressure gauge's reading will jump up to a higher level. The reason, of course, is that the cold oil has a greater resistance to flow (viscosity), and attempting to force a thicker oil through the small pores of the oil filter at the same flow rate of the thin oil requires more pressure. The oil pump doesn't care, it just keeps pumping out the same amount of oil, regardless. (Of course the power required to turn the pump shaft increases as a function of both flow and pressure, and we're assuming the power is there.)

Over the "normal" operating range of the engine, you can repeat this experiment and get the same results. The whole point being made here is that the oil pressure downstream of the pump is a function of the "attempted restriction" of the flow, not the pump itself.

If you try and pinch off the flow at the pump's discharge and have enough power applied to turn the gear pump's shaft, it will develop some pretty high pressures. Pressures in excess of 2000 psi are common in hydraulic systems, and the same kind of gear pump is used in both automotive and hydraulic applications. Automotive oil gear pumps routinely deliver over 300 psi and can go higher; no need to test higher. I was unable to find a high pressure performance curve for an automotive gear type oil pump, but I have no reason to believe that achieving 1000 psi would be out of the question. The important thing to note here is that just about any oil pump in good operating condition can blow up just about any oil filter if it were allowed to. The typical spin-on oil filter is designed with a burst pressure of approximately 270 psi. Designing an oil filter with a case and seal burst pressure of 2000 psi or more would boost the per unit cost into the stratosphere. Besides, that is what the bypass pressure relief valve is for: limiting the oil pressure at the pump's discharge to an acceptable level. On the downside, if the bypass relief valve is plugged or stuck, immediately there is serious trouble. A burst oil filter case is a real possibility.

The bypass relief valve is typically a spring loaded valve preset to begin lifting off at a certain pressure. The valve on my Jeep Cherokee is set at about 60 psi. When the oil pressure downstream of the pump exceeds 60 psi, the relief valve begins to lift and pass oil back to the reservoir, creating a shunt flow path and maintaining resistance to flow at close to 60 psi. Oil pressure will exceed 60 psi if the oil is cold and the flow rate is high; the relief valve's "regulation" is not perfect. It has its own flow/pressure curve, and increasingly high flows through the relief valve will increase the pressure drop through the valve. I've seen a max of about 70 psi registered on the pressure gauge of my Jeep. This occurs at about 2300 rpm with cold oil; I don't exceed this engine speed until the engine warms.

The positive displacement pump/bypass relief valve system has an interesting synergy when the engine's lifetime is considered. As the engine ages, clearances loosen up and more oil flows through bearing surfaces at a given engine speed. This translates into a reduced resistance to oil flow in the engine. As an engine normally ages, the extra oil flow capacity it needs is already built in; the bypass relief valve just does not open up as frequently. When things get really loose, well, it's time for new parts.

The spin-on oil filter has its own bypass relief valve: an "internal" relief valve. The filter's internal relief valve allows oil to flow past the filter element without passing through it when the filter element's resistance to flow is above the filter's relief valve setting. This performs the dual function of protecting the filter element from damaging high differential pressures, as well as protecting the engine from potential oil starvation caused by a plugged filter. SAE HS-806 lists five standardized internal relief valve settings. Relief valve settings of 9-12 psi tend to predominate for automotive use. Higher bypass pressures are also available (20 psi, 29 psi, 36 psi) depending on the engine manufacturer's design philosophy.

I tried to find out why there is variance in this parameter. What value is having a 27 psi internal relief valve setting compared to 12 psi? Consider the problem from a comparison of probable situations. For example, if a vehicle is being driven with cold oil in it, the filter with the higher internal lift-off pressure will more strongly resist bypassing the filter element (a good thing), but the increased pressure drop will mean less pressure at the filter's outlet (a bad thing). If the engine has a long distance to the farthest pressure-fed bearing, the reduction in pressure at the filter outlet may end up unbalancing the oil flows within the engine. Worse, the increase in filter pressure drop may force more oil through the oil system's pressure relief valve, reducing total oil flow to the engine.

The best I have been able to find out is that lubrication systems with a high system bypass relief valve setting (above 60 psi) will probably have a higher filter internal bypass specification. Perhaps enduro type racing applications also fit this need.

Regardless, automotive engine designers are apparently leaning toward the theory that bypassing the filter at a lower "differential" pressure (more often), is preferred over fewer incidences of oil filter bypass.

And small wonder. Remember the observation: "The oil pump is the only high speed engine part which receives unfiltered oil." Just because the oil filter momentarily bypasses unfiltered oil doesn't necessarily mean there is anything harmful in it. The oil passing through the oil pump clearly is not loaded with harmful particles. If it was loaded with harmful particles, the life of the typical oil pump would be short, and it is not.

Now, don't go out and remove your bike's oil filter after reading the above paragraph. The reason the oil remains so pure is because the oil filter keeps it that way. Oil passing through the engine usually returns back to the reservoir just about as clean as it left the filter. Usually. That's the problem.

Modern lubrication systems are based on recirculation. (The days of once through lubrication are very long gone.) Left unfiltered, harmful suspended particles will stay suspended, build up in concentration through recirculation, and turn the oil into an abrasive fluid.

Years ago, oil filters were more typically used in a bypass mode, in which the filter was parallel to the main flow of oil. About forty five years ago, full flow oil systems began to gain popularity and have become the norm for automotive engine design. The full flow oil filter is named for being able to process the entire flow of the oil pump's discharge and it is placed between the oil pump's discharge and the engine's pressure fed lubrication surfaces. The system pressure relief valve is on a line common to the oil filter's inlet.

The full flow oil filter has the advantage of insuring that potentially damaging particulate matter can be removed from the oil with near certainty, prior to its contact with lubrication surfaces. With a parallel flow filter system, the engine was typically assured of getting a constant flow of oil, but one never knew for certain if there was that one (or a million) particle which just happened to not pass through the oil filter and repeatedly scoured its way through the engine. One disadvantage of the full flow system, however, was that under conditions of high restriction (such as cold oil) through the oil filter, the oil filter could actually restrict the oil flow to the engine and cause much of the oil to bypass through the oil system's pressure relief valve.

The solution to this potential problem was installing a pressure relief valve inside the oil filter itself. What happens is that during conditions of high oil filter restriction (typically during cold oil operating conditions, but this could also occur from filter plugging), the relief valve opens up to allow unfiltered oil to pass to the engine's lubrication surfaces. This insures the engine gets its necessary amount of oil, albeit unfiltered. However, as should be clear by now, if the filter bypass doesn't operate very frequently, it should not make any significant difference.

I tried to find out how often the filter bypass can be expected to operate during a "normal" filter lifetime, i.e. fifty to one hundred hours of operation. I hit a brick wall on this one. There must be some SAE paper out there where somebody instrumented his car and found the answer to this question. Regardless, it would be a fun experiment to hook up a data recorder to the inlet and outlet pressures of a Guzzi's oil filter and drive it around for awhile to monitor the filter bypass function.

I sat down with a calculator and punched some numbers in and discussed the result with Allied-Signal. My guess is that something between one ten thousandth and one hundred thousandth of the total oil flow in sixty hours of operation bypasses the oil filter. As a result, the advantages of using a full flow/filter bypass system are many compared to its disadvantages, assuming "normal" operating procedures are followed.

Of course it is the "abnormal" operating conditions that cause most of the problems. If an engine is operated in a manner which chronically operates the oil filter bypass, increased engine wear can be expected. Some filter bypass is nearly unavoidable, is harmless and assures necessary oil flow to the engine. Too much filter bypass, or "unnecessary" filter bypass, simply increases the wear rate of the engine.

Most problems associated with contemporary automotive lubrication systems (of which the Guzzi is one) can be traced to operating outside intended conditions: too cold, too thick, too old, too fast, or all of the above.

In summary, if we love our Guzzis (And if you've read this far, you must.), what this all boils down to from an oil filter perspective is the following:

First, use only authorized filters. (There are now at least two.) An unauthorized filter may have too high an internal relief valve setting. Under cold oil conditions, your engine may not get the oil flow it needs. Also, an unauthorized oil filter has no warranty of performance. (You're on your own.)

Second, follow recommended practices regarding oil viscosity, oil and filter change frequency. Oil that is too thick/cold means increased "unnecessary" filter bypass. Same goes when filters are too old. Minimizing unnecessary filter bypass is desirable.

Third, don't whack the throttle open on a cold engine. Everything that you don't want to happen does. The engine needs a large flow of oil and doesn't get it. The oil pump tries to comply and tries to force a large volume flow of oil, which then tries to act like taffy. The system bypass valve acts like it's partially plugged; the filter bypass valve does the same thing; and the filter element tries to implode. The oil pump discharge pressure goes through the roof and the filter tries to blow off its seat. If you've got a good pump, stiff oil and a small bypass valve, you just might be able to accomplish all these, as well as score a bearing or piston.

List of References

  1. SAE Handbook (thread spec., engine lubrication systems)
  2. SAE HS-806 Oil Filter Test Procedure
  3. Marks Mechanical Engineering Handbook
  4. Hydraulic Pumps and Motors (Vickers)
  5. Oil Pump Basics from Melling (Melling Automotive)
  6. Engine Oiling Systems: Pressure vs. Flow (Melling Automotive)
  7. Mr. Gary Cross, Melling Automotive (personal communication)

Mr. Gary Bilsky, Allied-Signal (several personal communication)

The Internal Combustion Engine - Theory and Practice (C.F. Taylor)