How can an oil filter affect the oil pres to the engine?

[Chris A]

This has bothered me much with all of the talk about restrictive filters vs. high flow filters.

First, I believe we all agree that the oil pump is a positive displacement pump, so it will pump the same amount of oil regardless of the restriction. This oil will go to either:
1) Oil filter
2) Oil filter bypass (let's assume SBC bypass in block)
3) Oil pump bypass and back to the sump

So, let's say the oil filter is plugged. The oil is still going to go through the filter bypass valve and the bearings will still see the same oil pressure until the pressure reaches max-pump-psi - filter-bypass-psi.

So, let's say a SBC has a pump bypass setting of 60psi and the filter bypass is 8psi.

Worst case, the plugged filter scenario is only allowing 52psi vs. 60psi of a fliter which does not bypass at all. Again, the difference is only once the oil pump begins to bypass. So wouldn't it be wise to put in a higher psi bypass spring?

Can anybody please help me understand how a filter can affect the oil pressure when not near the max pressure of the pump?

 

[greencrew]

quote:

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Originally posted by Chris A:
This has bothered me much with all of the talk about restrictive filters vs. high flow filters.

Can anybody please help me understand how a filter can affect the oil pressure when not near the max pressure of the pump?

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Chris, I agree with your point. Until the pressure exceeds 60 psi the oil is going to get pumped through the system one way or another. The only difference is a more restrictive filter will result in more oil being forced through the by pass at a slower pump rate.

The oil pressure will reduce engine rpm, but given the hp of a typical engine the impact will be very small and can be ruled out.

It's realy the by-pass that makes the difference. no matter how restrictive the filter is, the by-pass will ballance out. If the combination of the two are more restrictive the oil will flow at the same rate, with a higher pressure as a result.

Bob, I understan your point, but the pumpability of the oil does not change, just the pressure. I say that because you can't compress oil, and this is a closed system. It has to go somewhere no matter what the restricion, at lest until the pump by-pass opens. The increased pressure is a result of the filter's restricion on oil that will flow one way or another.

 

[cangreylegend]

oil pumps are not perfect. There is some "slippage" or some bypass in the pump as pressure increases. They are not a theoretically perfect positive displacement. So more resistance to flow will allow less flow given that the oil pump can only pump so much pressure at a given rpm and oil viscosity. Go to the extreme and think of a worn out pump. Well they all start wearing from day one.

 

[dickwells]

I'm with you Chris. One of my buddies bought a Dodge Polara with about 43K on it and when we changed the filter, it was factory and packed full of crud. The engine ran ok but the lifters were all tapping. Additionally this shows that the filter may be in bypass but it still keeps filtering somehow until it is packed to the gills!

 

[BOBISTHEOILGUY]

Chris,

Oil flow restriction occurs mainly when going from a low pressure condition to a high pressure condition. It would/should only be a temparary condition. Let's say at idle of 18-25lbs and you accellerate to the 60lbs you refer to. At that point, oil will first butt up against the filter media and pressure valve, at that point the resistance will create a backpressure until it meets an excess of 8-12lb difference from inside to outside of filter media, at which point then it will allow the valve to open, which is going to be gradual, and open up enough to allow equalization between the in and outer of the filter media. If this didnt happen, it would and does crush the media inward due to the over pressurization between the pump and filter.

It is those momentary times that causes bearing scuffing, ie, why engines run in town tend to wear on the oil (and bearings) than those run over the road due to the constant up and down oil pressure for intown lights.

[ September 08, 2003, 12:59 PM: Message edited by: BOBISTHEOILGUY ]

 

[jason07-1zzfe]

What cangreylegend and Ken2 said.

 

[Jim]

Why don't engine designers put the pressure gage at the input to the engine oil supply?? Also why don't they put a sensor on the filter bypass?

I guess they figure most drivers could care less. Fancy cup holders are more important I guess.

 

[cangreylegend]

Jim, a little sarcasm here but maybe if you put it at the tail end where the pressure is least, maybe you'll wonder where the pressure went on a cold winter morning with dino. It might scare some folks really. I dunno.

 

[greencrew]

Sounds easy enough to do with a remote filter. Put a pressure gage on the hose leading to the remote filter, and one on the return line.

 

[Ken2]

Let's call the valve in the oil pump the "pressure control valve" and the valve in the filter assembly the "filter bypass valve."

Isn't the oil pump pressure control valve always open except with warm oil and an idling engine? The pressure gauge on my truck is almost steady from about 1500 rpm to redline.

So, with everything constant...warm oil, constant engine rpm, the same resistance to flow in the oil system of the engine, if the pressure drop is increased in the filter assembly the pressure control valve opens farther, and a lower volume of oil is pumped to the engine. We'd see lower oil pressure above the filter if we had a pressure gauge there.

A clean filter with low-restriction media and warm oil might have a pressure drop of, say, 2 psi. If this filter gets dirty, the pressure drop will increase up to the point where the filter bypass valve is wide open, maybe 10-12 psi on a nominal 8 psi filter. This means that you have 10-12 psi less oil pressure going to the engine's oil system. What happened to the oil that made up this missing psi...the oil volume is less because the oil pressure control valve is open more to hold a near-constant pressure at the oil filter inlet. The same thing will happen with a clean filter made with high restriction media. If you have a constant supply pressure and increased resistance, you have less volume of flow. This lower volume would show as lower pressure if we install a pressure gauge at the filter outlet.


Ken

[ September 09, 2003, 12:45 PM: Message edited by: Ken2 ]

 

[Chris A]

First, all of my opinions are mine and are based on how I think logically. I tend to question a lot with oil filters. This is probably due to the large amount of unknown (unproven, untested, etc) characteristics of oil filters.

Bob, I agree that it is possible for flow to be restricted momentarily (small fraction of a second) when the pressures would increase. But, I would also beleive this would only occur if there is a dramatic increase. Since the oil pump knows nothing other than current RPM and RPM doesn't increase *that* rapidly, I don't believe that situation will occur in this application. Also, I believe "in town" driving cause more wear due to the loads being placed on the engine vs. constant cruising.

cangreylegend, this sounds like a problem with oil pumps in general and a filter is not going to change the loss much since the majority of the pressure is due to the bearings which will always be there.

Ken2, I don't believe that is the optimal setup for the oil pump bypass. If it is constantly opening, you are using a little bit of HP to pump that oil and you are also unnecessarily heating the oil that you are pumping just to dump it back into the pan. I do agree that in this situation (assuming that the filter is in bypass mode), the engine sees less pressure. But the fact your gauges does't move indicates to me that the filter is not bypassing.

My theory (which I have 0 proof of) is that hot oil rarely, if ever, causes the filter (pretty much any big name filter) to bypass. I base this on the experiences of other SBC owners who have plugged the filter bypass valve and run filters without bypass valves. These are mainly fair weather vechicles and they always warm up the cars before driving and have noticed no signifigant change in oil pressures than with the bypass spring installed. I have also never heard of someone collapsing an oil filter whild doing this.