Originally Posted By: jstutz
Alternate numbers are PH8A and PH16. It is common thought that the 51515 is just the longer version of the 51085, but per the wix look up page, they are different. The smaller one actually has better filtration numbers and the media is different.
Unless I'm missing somthing, you are incorrect.
Wix states the Beta 2/20 for these filters as the following:
51515: 13/23
51085: 15/28
Lower numbers are "better" filtration in Beta ratings. The 51515 is "better" at filration. It is larger. This is the common phenomenon, but there are examples where this is not true.
For example -
51311: 8/21
51307: 7/18
These two are by far more efficient that the two you mention, use the same thread pitch and gasket, have the same can diameter, and are SHORTER in height. Therefore, they are actually "smaller" in total volume. However, volume of the can does not tell the whole story. Other things are in play, such as the type of media, density of media, total area of media, etc etc. Also, you have to look at the other characteristics such as bypass settings; the 51311 is at 16 psi, where the other three are around 8-11; food for thought.
Further, what the second two filters may not have in contrast to the first two are as long a life cycle. As our dear departed Gary Allan used to profess; you cannot avert the filtration triangle. Efficiency, size and lifecycle are inter-related and if you hold any two as a constant, the third characteristic generally moves inversely, (typically but not always) in proprotion.
For any "normal" OCI, I suspect the 51307 will do a better job at filtration and have plenty of life cycle; it is "better", yet ironically smaller, than either the 51515 or 51085.
When selecting alternative filters, you must CAREFULLY assess all the criteria, and not just one characteristic.