Oil pressure differential due to filter

[tubman]

I don't want to hijack someone else's thread, so I'll start a new one. In my adventures with the Edmunds heads on my '51 Merc I've had them off a couple of times. To make it easier, I ran it with out the oil filter for the interim runs. I just connected the mechanical gauge to the filter port. This is an engine with a stock bypass filter. I checked and the filter housing and fittings and it does have the proper restrictor fitting installed. I couldn't help but notice there was a difference in oil pressure with and without the filter. Plumbed direct, it registered 45 lbs cold at 2000 RPM. With the filter hooked up, it was right about 40 lbs at the same speed. I haven't really thought about why this would be, but I thought it was kinda interesting.

[51 MERC-CT]

That is the same thing that happens when the bearings wear.
To check the phenomena make up a pressure side feed line with a valve in it (preferably a ball valve) and watch the pressure change as it's switched.

[Ronnie]

Normal

R

[G.M.]

Surprises me that you would see a drop with the small restriction
hole in the return line. Not enough drop to worry about. G.M.

[tubman]

I'm no expert on fluid dynamics, so I'll ask a simple question. My tests show about a 10% drop in oil pressure with the filter out of the system. Does this mean that about 10% of the oil flow from the pump is passing through the filter?

[JSeery]

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From a Street Rod article:

When Ford did introduce an oil filter it was the bypass style, which meant only a portion of the oil was filtered. Oil was delivered to the filter from a boss at the back of the block where the oil pressure gauge sender was located. From the filter oil was returned to the pan. A simple arrangement, it’s always been said that 10 percent of the engine’s oil passes through this style filter, which Joe Abbin’s recent testing confirmed (see his sidebar on page 83 for more information).

Sidebar Information:
How much oil does the Ford Flathead oil pump circulate and how much gets filtered?
The stock Flathead oil pump is a positive displacement gear type, capable of circulating about 15 quarts per minute under ideal conditions when the engine is spinning at 2,000 rpm (the high-volume version pumps approximately 20 percent more). A recent test at Roadrunner Engineering demonstrated that a stock ’46-53 oil filter with restrictor flows about a quart and a half of oil per minute through a clean filter under meaningful conditions of oil pressure (55 psi), temperature (180 degrees F), and viscosity (15W-40). This means that about 10 percent of the oil pumped by the stock pump goes through the filter under these conditions and the rest goes unfiltered to the engine. Therefore, under these same conditions, I estimate that all the oil in a 5- or 6-quart system is filtered in less than five minutes at highway speeds. Not bad and certainly worthwhile!

[tubman]

Everything said so far seems to jibe. I have always believed that the stock bypass filter system on these Fords is more than adequate for them, especially given the (usually pampered) lives they lead. When was the last time you went 5 miles on a dirt road?

[51 MERC-CT]

Originally Posted by JSeery

From a Street Rod article:

When Ford did introduce an oil filter it was the bypass style, which meant only a portion of the oil was filtered. Oil was delivered to the filter from a boss at the back of the block where the oil pressure gauge sender was located. From the filter oil was returned to the pan. A simple arrangement, it’s always been said that 10 percent of the engine’s oil passes through this style filter, which Joe Abbin’s recent testing confirmed (see his sidebar on page 83 for more information).

Sidebar Information:
How much oil does the Ford Flathead oil pump circulate and how much gets filtered?
The stock Flathead oil pump is a positive displacement gear type, capable of circulating about 15 quarts per minute under ideal conditions when the engine is spinning at 2,000 rpm (the high-volume version pumps approximately 20 percent more). A recent test at Roadrunner Engineering demonstrated that a stock ’46-53 oil filter with restrictor flows about a quart and a half of oil per minute through a clean filter under meaningful conditions of oil pressure (55 psi), temperature (180 degrees F), and viscosity (15W-40). This means that about 10 percent of the oil pumped by the stock pump goes through the filter under these conditions and the rest goes unfiltered to the engine. Therefore, under these same conditions, I estimate that all the oil in a 5- or 6-quart system is filtered in less than five minutes at highway speeds. Not bad and certainly worthwhile!

Did an actual test ( not an estimate ) that by measured volume of oil at idle speed, 5 qts. of oil in 11 min.
So the 5 min. estimate at highway speed sounds about right.