Filtration vs. Flow vs. Bypass Valve

[sky jumper]

maybe a bit of a noob question, but if a filter with better filtration and lower flow (pure one) results in a higher pressure across the filter, wouldn't the bypass valve open easier and spew dirty oil into your engine? if so, then wouldn't you want a filter with as much flow as possible, coupled with a higher BPV spec, to prevent the BPV from opening in the first place? even if it provided less filtration, wouldn't that be better than good filtration some times and no filtration at other times?

 

[kanling]

All else being equal, Yes, but I also want the bypass valve to open if it's really necessary because I would rather have dirty oil than no oil at all.

Anyway, the oil in the pan shouldn't be all that dirty- after all it was filtered before. A few passes around the engine without filtering isn't going to dirty it very much more.

The only issue is with filters with closed-end bypasses where the oil washes over the dirty media to bypass. In that case, you might really have dirty oil coming out in bypass mode.

 

[sky jumper]

quote:

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Originally posted by kanling:


The only issue is with filters with closed-end bypasses where the oil washes over the dirty media to bypass. In that case, you might really have dirty oil coming out in bypass mode.

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how do you know which filter have this close-end bypass??

thanks.

 

[kanling]

Most do, unfortunately. If you know what you're looking at, you can usually tell by looking in the center hole. Search around to find pictures of cut open filters and you'll soon figure it out. If there is a gadget at the end of the filter when looking through into the hole, it most likely is the bypass valve.

Wix filters typically use a threaded end bypass as do the new Champ Labs E-Cores. Those are the widely available exceptions to the closed-end bypass.

 

[kanling]

For Auto RX, I would want to use the largest filter I could fit. That way, even if lots of "globs" got caught in the filter, hopefully the pressure drop would still stay lower than the bypass opening pressure. I've never done Auto RX, but I suppose I would want to do it in the summer when the bypass valve is least likely to be needed, as well.

Yes, as long as the filter physically fits in the space, and the gaskets are same dimensions, bypass pressures are similar, you can use an oversize filter. There are several threads here about oversize filters. Watch out that an extra long filter, for example, doesn't hang down dangerously close to the road though.

Also an oversized filter may not be the best choice for short trip driving since it will take just a little longer for the oil to get up to temp.

 

[sky jumper]

quote:

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Originally posted by kanling:

Yes, as long as the filter physically fits in the space, and the gaskets are same dimensions, bypass pressures are similar, you can use an oversize filter. There are several threads here about oversize filters. ..

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thanks kanling. I've got some searching and learning to do.

 

[Ugly3]

Check out Gary Allen's testing of filter bypass operation. Oversimplified summary, once the oil is warm (maybe 1 minute after start up) he can't get the filter to go into bypass no matter what he does. And the oil viscosity was not a factor (he ran 20w50 and 5w20). The testing was done with a Purolator Pure One filter!

 

[sky jumper]

thanks for the info, I'll search around. this seems like a really important feature, especially if you are doing an Auto-RX cycle where all that grime gets caught in the filter.

I'm wondering if anyone has done a study to determine just how often the bypass valve opens in normal driving, and how much unfiltered oil gets through with different filter designs. would be good to know.

 

[Bill Kapaun]

I think as a general rule, expect the BPV to be open on cold starts and closed once the oil is hot.

If you use a "thicker" oil, it will be open more often/longer.

As mentioned above, the oil in the pan only has one trip through the engine since it was last filtered. Considering how much "junk" is deposited in the oil on just one pass, that oil can't be considered to be THAT DIRTY.
Lets face it, most of us on this forum probably keep far cleaner oil than the average motorist and their cars don't seem to die that much sooner.

 

[sky jumper]

quote:

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Originally posted by Bill Kapaun:

As mentioned above, the oil in the pan only has one trip through the engine since it was last filtered. Considering how much "junk" is deposited in the oil on just one pass, that oil can't be considered to be THAT DIRTY.

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right - the oil in the pan might not be that bad, but from what kanling says, most filters in bypass mode push the oil right over the dirty filter media before pumping it back into the engine - now that is something to worry about. as I prepare for my first Auto-RX cycle, I am very concerned about it as ARX supposedly clogs your filter with a lot of grime. I will certainly get a filter with the threaded end bypass (once I figure out exactly what that is and how to spot it).

 

[sky jumper]

on a related topic - larger diameter filters with larger filter area should flow better, right? or at least keep the pressure across the filter lower for the same flow rate.

so - is it possible to use a larger diameter filter that may not be spec'd for your car as long as the gasket and threads are compatible?

it seems to me that many filters appear to be exactly the same, yet they are not spec'd for the same cars - why? what is different about them?

the SuperTech filter for the Accord is huge (large diam), but the OEM is a smaller size, and looks identical to the Camry and Maxima filters. Can I use the ST Accord filter on my Maxima???