oil filter investigation

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

Update on 1/24/06:

It appears WIX Filter Company has now "seen the light" as far as their "new" design 51101 (also NAPA 1101) filter goes for use on bypass oil lubricating systems as used on vintage Allis Chalmers B tractors (and others). The WIX website now lists the 51101 part number as "not found". The current part number is WIX 57011, which lists this filter as a bypass type and applicable to Allis Chalmers and others. The equivalent NAPA part number is 7011. I asked about that filter at my local NAPA store and was advised that the NAPA 1101 has been "superceded" by the new 7011 and that the Allis Chalmers Model B was the correct application. The NAPA website has not yet been updated to reflect this change. The 7011 filter looks like this:

While the old wooden dowel has been left out of the central hole, the filter media is indeed a packed media and not the pleated media that was on the revised 1101 design some three years ago. I have not yet performed a pressure test for verification of no reduced oil pressure with the 7011 filter. I will do that when I replace the oil and filter again in my '42B this spring. However, I am quite confident that this replacement design will work fine. For those where NAPA is a convenient local source for their Allis filters, I am glad to see that WIX/NAPA has apparently finally corrected the prior oil pressure problem they created. Perhaps a significant reduction in sales volume helped spur them along?

Rod(NH) - 1/24/06

Update on 5/4/06:

I decided to also get a Fram C-159 filter to compare with the new NAPA 7011 one, as well as the old WIX 51101.

Here's what the Fram C-159 looks like:

I am surprised that the Fram does not have the Fram name imprinted anywhere on the filter. It came in a Fram box and has the correct number but no Fram ID on the filter itself. It also looks surprisingly like the new NAPA 7011 filter shown above, except for the imprinted labelling. There is no wooden dowel as there used to be with the old NAPA 1101. It does have the packed media however. I have always thought that WIX (DANA Corp) was the manufacturer who supplied NAPA with oil filters and that Fram (Allied Signal Corp) was a separate and old-time distinct filter manufacturer. I'm not sure anymore.

I used the same oil in the tractor as was used in the original test on 7/10/03. It had not been changed yet. The ambient air temperature was about the same in the mid 70'sF. I ran the tractor at idle speed, cold, for only as long as it took to take the pictures - about 30 sec per filter at most. Here's the results:

Old design WIX 51101 - same filter as used on 7/10/03:

About 15 psig, the same as the standard relief valve setting in the oil pump. It seems slightly higher than the first test with this filter (12-13 psig), possibly because it is now used, not new, and may have some crud buildup in it, thereby causing a little higher back pressure.

Here's the Fram C-159:

About 11 psig.

And here's the NAPA 7011:

About 10 possibly 11 psig.

My conclusion:
There is essentially no difference between the new NAPA 7011 and the Fram C-159 filters. They both have slightly less back pressure (about a couple of psig) than the old design NAPA (WIX) ones. I found on both the 7011 and the C-159 filters that they installed on the filter tube noticeably easier than the old WIX did. There was essentially no resistance from the packed media to the insertion of the tube, despite the fact there was never that old wooden dowel installed to keep a path open. I am thinking that the media is not packed as tightly in either filter as it used to be in the old NAPA 1101. The approximate 2 psig difference between the old and new is not that great and both the new NAPA 7011 and the Fram C-159 are completely usable in my AC-B. However I think they represent a little cheaper quality product than used to be available. At least NAPA improved their new 1101 design by replacing it with the 7011.

Rod (NH)
May 2006

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