Originally Posted By: ZeeOSix
Originally Posted By: xtell
ZeeOSix
Let me see if I got this right, because I do not want to pass along inaccurate information. When I mentioned "very high pressure" causing an oil filter to go into bypass, what I was referring to was pressure on the inlet side of the filter media, like if it was clogged up with debris.
I give you some examples to help explain what's going on with a delta-p situation in an oil filter.
As a filter gets clogged the delta-p across the media increases. The "pressure on the inlet side" of the filter is relative. In other words, you could have 150 PSI on the inlet side of the filter, and 145 PSI on the outlet side, so the delta-p would only be 5 PSI. If the bypass valve was set to 12 PSI it would never open in this case.
Or you could have 60 PSI on the inlet side and 45 PSI on the outlet side, which would be a delta-p across the filter of 15 PSI, in which case a bypass valve set to 12 PSI would be opened up.
The filter's bypass valve operation doesn't care what the inlet pressure is on the filter, just what the delta-p is across the media.
The main factors that cause delta-p across the oil filter are:
1) The flow resistance of the media (The resistance goes up as the filter gets dirty. Also media area is a factor to flow resistance).
2) The viscosity of the oil (Get more delta-p with thicker oil).
3) The flow rate of the oil through the media (Get more delta-p with increased flow. More flow with increased RPM).
It's a good idea to not rev up engines too much in really cold weather after a cold start is because of 2) and 3).
Originally Posted By: xtell
The second thing is if you have a small filter and a real big filter, would not the oil flow through both be the same pressure wise and volume wise? I'd appreciate any input to set me straight.
xtell
If all things stay the same except for the size of the filter, the delta-p across the media in the bigger filter will be less because the increase in total surface area helps the filter flow better. The inlet pressure will be a hair lower in the larger filter because the delta-p across the larger filter will be slightly less by just a few PSI.
Yes, the volume going through them both will be the same at the same engine RPM because of the positive displacement oil pump. If you put the same volume of oil through a media area that is twice as large then the resulting delta-p across the media will be less. That's also a reason that good flowing oil filters typically have a bypass valve with a lower opening pressure.
If the oil pressure sensor is located after the oil filter, it will read the same with both filters installed (as long as the pump is not in pressure relief) because the flow rate at the sensor is the same at the same PRM (positive displacement oil pump again). The oil pressure is a product of X GPM of oil flow going through a fixed flow resistance - ie, the whole oiling system of the engine in this case. If the oil pressure sensor was located before the filter (rare case), then you would see the pressure at that location go up slightly if the filter was more flow restrictive, or go down slightly if the filter was not as flow restrictive.
Completely and utterly correct. Your explanation and examples are spot on.
Too bad you have to explain this over and over and over and over and .....
I will add this ... There are only two ways to view this topic
1) filters often go into bypass
2) filters rarely go into bypass
As for condition 1, what of it? If this is frequent, show me that it matters, please. Anyone? Anyone got any data to prove that frequent BP events cause harm? We see bazillions of UOAs that have great wear data. If BP events are frequent, then apparently we can conclude nothing ill comes of it.
Or, condition 2; so what? If BP events are rare (which is my personal belief based on Allens data), why worry? What does it matter? If this is infrequent, the damage must be minimal.
UOAs tell us that wear rates are typically low on most equipment that is reasonably maintained and in good shape. The brand/grade of oil you choose, the type filter you select, and the frequency of BP events, are all moot. Macro data proves that these topics are NOT the controlling factors in wear rates.