Given the significant amount of concern and negative experiences that I have recently read about concerning loss of oil pressure with the new style NAPA 1101 oil filter for AC tractors, I decided to do my own investigation. This page is the result.


Here's what the new style filter looks like:
And here's what the old style looks like:
I had an old style 1101 filter on my '42 AC-B that has not been changed for about 3 years. Probably no more than 200 or so hours on it.
I started the tractor to check the oil pressure. The oil was SAE 30W with about 100 or so hours on it. The ambient temperature was about 75 deg F. The oil pressure was taken at idle speed immediately after starting. In other words, the engine was not up to operating temperature. I got about 14 psig as shown here:
I then put on a new, unused, new style 1101 filter and checked the oil pressure again. NO OTHER CHANGES WERE MADE. Same oil. Tractor at idle speed. Engine at ambient temperature. I got about 8 psig as shown here:
I had a new, unused, old style, WIX 51101 on hand that I installed next. The WIX 51101 is identical to the NAPA 1101 except for the branding. As before, NO OTHER CHANGES WERE MADE. I got about 12-13 psig as shown here:
There is a significant drop in oil pressure in going from a unused old style filter to a unused new style filter with no other changes being made. From about 12 psig to about 8 psig represents a 33% decrease in oil pressure! Such a drop in oil pressure results in a larger quantity of oil being bypassed through the filter and a reduced quantity being delivered to the engine bearings and other lubrication points.
Curious as to why this drop in pressure occurs, I cut both styles of the 1101 open.
Here's the old style:
The oil flow is down from the top (left) after having been supplied to that area by the riser tube in the oil filter housing. You can see that oil flow through the media is not perfectly uniform but can "channel" itself. The media is a tightly packed stranded material that fills an area about 3.25 inches in diameter and 3.5 inches deep. There is a perforated plate that retains the media in place. This plate has about 212 holes in it that are about 0.045 inches square. Typically the purpose of such a perforated plate would be to cause a pressure drop across itself and result in fairly even flow across the entire area taken up by the plate. That is in addition to the purpose of retaining the media. You can observe that oil channeling is more prevalent in the upper portions of the media, indicating that the perf plate is doing that job of "speading" the flow out. The total vertical flow area of the media is therefore a little more than 8 square inches for a length of 3.5 inches. The total flow area of the perforated plate holes is about 0.34 square inches. After the oil passes through the holes in the perforated plate, it flows by gravity to the oil pan.
The new style 1101 filter looks like this:
The oil flow here is up the center towards the right via the riser tube in the filter housing. Oil exits the center core area through petal shaped openings at the top (right) of the core (black rubber shown here). There are six of these petal shaped openings that are each about 0.6 inches long and an average of 0.2 inches wide. This gives a total flow area of about 0.7 square inches.
After passing through the petal shaped openings, the oil flows down around the outside of the cylindrical, pleated media. The clearance between the OD of the filter pack and the ID of the can is about 0.040 inches. This is maintained by four bumps on the OD at each end of the media pack. These "bumps" are not shown in this photo. They are out of sight in the back. The OD of the filter pack is about 3.15 inches. The flow area here then is 0.045 x 3.15 x 3.14 = 0.45 square inches.
The media is pleated as shown here:
You can now see the "bumps" mentioned previously. There are about 50 pleats that are about 0.5 inches high by 3 inches long. That gives a flow area of about 150 square inches. That's a lot. Pleats really do increase the surface area . The filter media is only about 0.036 inches thick. So there's a LOT of surface area and only a little distance for the oil to travel through the media.
After the oil passes through the filter media, it has to pass through a perforated plate that makes up the ID of the filter pack. That plate looks like this:
This perforated plate performs the same purposes that the perforated plate does in the old style filter. It's just cylindrical instead of flat. There are about 200 holes in this plate. They are about 0.035 inches wide by 0.150 inches long. That's a total of about 1.1 square inches for flow through this plate. After passing through this plate, the oil flows by gravity to the oil pan.
The following summarizes the flow areas available after the orifice restrictions in the riser tube on the tractor:
Old style: maximum 8 sq in through 3.5 in of media, then through 0.34 sq in of holes to oil pan.
New style: 0.7 sq in petal holes, then 0.45 sq in filter pack clearance, then 150 sq in through 0.036 in of media, then through 1.1 sq in of holes to the oil pan.
I was unable to perform any specific pressure drop test through either style of filter. No time and no test equipment. That would be a good laboratory test with the right equipment though. However, my own opinion is, given the flow areas involved in the two styles, that the new style represents a significantly lower pressure drop for any particular oil flow rate.
I would have expected that the orifice restrictions in the top of the riser tube would be the controlling factor that determines bypass flow through the filter. In other words, the pressure drop through the filter itself would be several orders of magnitude LESS than the drop through the orifice restrictions. That is apparently not the case with the AC-B and others like it. The restriction presented by the filter itself seems to be a significant portion of the total drop from the pressure gage through the riser tube, the riser tube orifice restrictions and the filter itself to the oil pan. The filter resistance can therefore significantly affect both the filter flow and the engine oil pressure. The restrictions of each portion of the path are additive just like resistors in a series electrical circuit. It is the TOTAL resistance that dictates the bypass filter flow rate and the engine oil pressure.
My personal conclusion:
The new style NAPA 1101 oil filter has less of a flow restriction (for the same flow) than the old style. This lower restriction results in an easier path from the oil pump through the filter to the oil pan. This easier path results in a lower oil pressure (by over 30%) and a lower oil flow to the engine bearings, etc.
I don't intend to use any NAPA 1101 (or WIX 51101) filters in the future until the design is changed to get the engine oil pressure back to where it is supposed to be. It's bad enough to have low oil pressure due to worn bearings and excessive clearances. You should not have to deal with new filters that make the situation worse. NAPA/WIX/DANA can rationalize their filter all they want but they have lost a customer in me until they fix the problem.
Rod (NH) - 7/10/03
Addendum: 9/5/03
Hastings LF405 Filters
Further testing on Hastings LF405 Filters show that they maintain pressure.

These tests were performed on a ’39 Allis Chalmers B
Jim Danforth
We also get several calls about low pressure after oil changes with good filters.  This standpipe is  common issue.  It serves to regulate the flow to the filter and frequently is thrown out with the old filter since it can get stuck inside and not noticed.