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Discussion Starter #1
I like using the Mobil-1 oil filter because (a) it has the anti-drainback valve in it and (b) I have more money than brains.

But perhaps not, because it occurs to me that our oil filters screw in from the bottom up, and they naturally sit upside down, with the opening at the top.

So, smart guys, since gravity is going to leave the oil filter full of oil when I switch off anyway, what's the big deal about the anti-drainback valve on the Mobil oil filter?
 

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Just my guess on this. If the valve is left off you are correct that the oil will all flow back into the filter. The problem lies in that there is a limited volume of oil to be returned, and if all of it goes into the filter and oil pan, the oil galleys in the pump housing, etc. can be emptied (oil flows to lowest points) causing lack of lubrication until they are filled again after pump replenishes. By having a check valve more of that oil is kept in the upper parts of the motor, reducing start up oil starvation.
 

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Lovbyts
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Like steve said. That is the reason. Where would you rather have the oil? In your filter and pan or on top where it can do some good? Good explanation.
 

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Discussion Starter #4
So, then, a drainback valve is of value only in that it prevents oil from remaining in the filter? If true, why would oil so displaced not just default down to the oil pan anyway, completely bypassing all the advantages you guys listed?

I'd always assumed that the opposite trait was the more desirable, that by retaining oil in the filter, oil was that much closer to, and got to, the concerned engine parts that much more quickly when next the engine was started.

If your theory is true, then the only filters that need a drainback valve are those which screw into the engine at or below the horizontal, such that they would hold oil by gravity. For example, my truck's filter screws in by pointing down about 45 degrees, so gravity will suck the oil out of it. And you're saying that this is a good thing?

Please don't infer that I'm calling you guys liars, it's just that this is such a volte-face concept for me to be accepting it too readily.
 

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Perhaps my explanation was incoherent as they are known to be. The idea of the valve is to keep oil in the upper areas of the engine. This can be by trapping oil in the filter if it is mounted opening facing down and thereby keeping oil in a higher area (filter)than just the oil pan. Or, if the filter is mounted with the opening facing up it can help keep oil higher in the engine than the filter or oil pan would since the filter and the oil pan are at around the same level and you want to keep some lubrication in higher areas.
As I stated, I am not an engineer but I believe this is the reason for the valve.
 

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Discussion Starter #6
Sorry Steve, I must be too full of tea this morning (among other things).

As I re-read your first post, it makes sense. I incorrectly inferred you guys to be implying that an empty oil filter was a great thing. I realize now that you meant that (1) not only does my oil filter stay full (because of gravity), but (2) the drainback valve acts like a finger plugging the bottom of a drinking straw, and any oil that is in the engine above the filter, and that would otherwise naturally fall back into the filter absent the pressure from the pump, will instead be blocked by the valve and will stay where it is, in the engine.

Thus, rather than all my oil being in the pan and being miles away from anything, not only do I have oil in the filter that is closer to the engine, but I also have oil sitting in the passages immediately after the filter, which is even closer to the engine (if not already right where it's needed).

I suppose in the case of a filter that points down when mounted (like my girlfriend's Camry, where it's on the top of the engine), while not necessarily having the additional advantage of stopping any runback out of the passges and into the filter, the valve does still at least keep oil in the filter, bringing it that much closer to the engine parts requiring lubrication at start up time.

Seems like mounting the filter at the bottom of the engine, and facing up, is the most sensible thing to do, then. Which makes me wonder why the Stealth is the only vehicle I've seen like that. In the experience of others, is our filter mounting common or uncommon?
 

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Lovbyts
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This is very comon proctice. I know even my 67 Camaro and 85 Trans Am both had it mounted this way.
 
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