Bypass Setting Question

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The OE filter for my Audi has a bypass spec of 2.5 bar (36 psi). I know that means that if the pressure differential across the media is greater than 36 psi the bypass spring will start to open.

Does this mean that it takes a greater loss of oil flow/pressure to trigger the bypass to let unfiltered oil to the engine? If I'm thinking about this correctly, would that potentially starve the engine of oil whereas a filter with a bypass of 10 psi would at least send some oil through to the engine at a lower pressure diff?

I don't know if it really matters, but I envision oil flow volume decreasing when the oil is cold and viscous as it creates a greater PSID until it heats up and flows better through the filter element. I would think they would want the opposite so the turbos would always have an adequate oil supply, even though it may be unfiltered.

Am I on the right path here? Why would Audi spec a filter with such a high bypass setting?
 
They way I've always seen it, oil pumps are generally positive displacement pumps, so what pumps out of them has to go through the filter and to the engine, or gasp! To the ground.

The only purpose of a filter's bypass setting is to protect the media from rupturing.
 
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Originally Posted By: JTK
They way I've always seen it, oil pumps are generally positive displacement pumps, so what pumps out of them has to go through the filter and to the engine, or gasp! To the ground.


When the positive displacement oil pump hits its pressure relief valve setting (say 80~90 PSI), then the excess oil is sent back to the sump or pump inlet. When the pump is at max pressure relief point, that is the point where the volume of oil going through the filter & engine is maximum based on the resistance of the system and the outlet pressure of the pump (80 ~ 90 PSI).

Originally Posted By: JTK
The only purpose of a filter's bypass setting is to protect the media from rupturing.


True
 
Originally Posted By: threeputtpar
I don't know if it really matters, but I envision oil flow volume decreasing when the oil is cold and viscous as it creates a greater PSID until it heats up and flows better through the filter element.

Generally flow, not pressure, is not temperature related at the pump.

The bypass is set with both the engine design and the use in mind. If it's a vehicle which the manufacturer expects to be driven hard, the bypass setting is likely to be set higher than if it's something your grandmother might drive to the store.

An oil filter constantly in bypass doesn't do much filtering.
 
Originally Posted By: threeputtpar
I don't know if it really matters, but I envision oil flow volume decreasing when the oil is cold and viscous as it creates a greater PSID until it heats up and flows better through the filter element.

Generally flow, not pressure, is not temperature related at the pump. [/quote]

As long as the oil pump is not in pressure relief mode, then the flow volume of the oil is the same if it's cold or hot as long as the positive displacement oil pump is not in pressure relief.

But when the oil is cold, the pump's outlet pressure is higher in order to pump the same volume through the engine & filter than when the oil is hot. That's why the oil pressure might be 65 PSI at 1500 RPM when cold and only 35 PSI at 1500 RPM when hot. Both cases would have the same GPM of oil flow if the pump wasn't in pressure relief. When the oil is cold, the oil pump may hit pressure relief at pretty a low RPM, whereas with hot oil the pump may not even hit pressure relief even at redline.
 
Originally Posted By: ZeeOSix
When the oil is cold, the oil pump may hit pressure relief at pretty a low RPM, whereas with hot oil the pump may not even hit pressure relief even at redline.

I was involved in a study almost a quarter century ago involving VW Golfs, Golf GTs, and BMW in which a low bypass pressure filter for the VW Golf was interchanged with the higher pressure filter for the GT, and even higher pressure filter for the BMW among the three applications. An aftermarket filter was used with identical case, media, and they differed only in the bypass pressure.

At moderate speed and acceleration the lowest pressure filter worked fine in all three.

As the driving became more vigorous with frequent hard acceleration the lowest bypass pressure filter began staying in bypass mode a greater percentage of the time. And it did so with all three vehicles. The conclusion was that hard drivers could benefit from using a higher bypass pressure filter than stock.

This is the reason weekend racers often use a high bypass pressure filter, or modify the filter to limit bypass mode.
 
Originally Posted By: Wilhelm_D
Originally Posted By: ZeeOSix
When the oil is cold, the oil pump may hit pressure relief at a pretty low RPM, whereas with hot oil the pump may not even hit pressure relief even at redline.

I was involved in a study almost a quarter century ago involving VW Golfs, Golf GTs, and BMW in which a low bypass pressure filter for the VW Golf was interchanged with the higher pressure filter for the GT, and even higher pressure filter for the BMW among the three applications. An aftermarket filter was used with identical case, media, and they differed only in the bypass pressure.

At moderate speed and acceleration the lowest pressure filter worked fine in all three.

As the driving became more vigorous with frequent hard acceleration the lowest bypass pressure filter began staying in bypass mode a greater percentage of the time. And it did so with all three vehicles. The conclusion was that hard drivers could benefit from using a higher bypass pressure filter than stock.

This is the reason weekend racers often use a high bypass pressure filter, or modify the filter to limit bypass mode.


Interesting test, and makes sense on how the filter's bypass valve would behave in those situations you described.

Just to be clear, the part you quoted from me was talking about the oil pump's bypass/pressure regulator valve, not the filter's bypass valve.
 
Originally Posted By: Wilhelm_D
I was involved in a study almost a quarter century ago involving VW Golfs, Golf GTs, and BMW in which a low bypass pressure filter for the VW Golf was interchanged with the higher pressure filter for the GT, and even higher pressure filter for the BMW among the three applications. An aftermarket filter was used with identical case, media, and they differed only in the bypass pressure.

At moderate speed and acceleration the lowest pressure filter worked fine in all three.

As the driving became more vigorous with frequent hard acceleration the lowest bypass pressure filter began staying in bypass mode a greater percentage of the time. And it did so with all three vehicles. The conclusion was that hard drivers could benefit from using a higher bypass pressure filter than stock.

This is the reason weekend racers often use a high bypass pressure filter, or modify the filter to limit bypass mode.


Thanks for the answers, guys. So if I take my thought process to the next step, are you saying that if the car sees some spirited runs the filter with the higher bypass setting will be in bypass less often (or even not at all) than a filter with a lower setting? If yes, then is this a tradeoff between keeping the filter out of bypass at higher revs and engine loads versus protecting the filter media from greater PSID and possible media damage?
 
Originally Posted By: threeputtpar
Thanks for the answers, guys. So if I take my thought process to the next step, are you saying that if the car sees some spirited runs the filter with the higher bypass setting will be in bypass less often (or even not at all) than a filter with a lower setting? If yes, then is this a tradeoff between keeping the filter out of bypass at higher revs and engine loads versus protecting the filter media from greater PSID and possible media damage?


Keep in mind that if an oil filter has a bypass setting of say 36 PSI, then the filter designer has done so knowing that the filter could see 36 PSI of delta-p, and therefore has designed the filter to take that much delta-p (and more) before damage occurs. The bypass setting on an oil filter has to match what the filter can take in terms of delta-p.

So yes, in general an oil filter with a higher bypass setting would be better to use on an engine that sees a lot of high RPM use and maybe even use of a heavier oil then originally specified by the engined manufacturer.
 
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