Question about full flow filters

[moose]

Here's some some Truck /Industrial engine oiling factory bulletins info dated 1947......

 

.

Bob

 

 

So, I made the change to full flow by adding the plug around two months ago.  The oil stays cleaner longer now, but I still have a large fluctuation in the oil pressure.  Right after adding the plug, I kept the same relief valve(from the partial flow setup) and the pressure stayed about the same if not a little lower.  Running it this way the pressure would max around 50 cold, then drop to around 35 at temp, and speed.  Then I decided to look into the stock block(now spare) and trade the relief plungers and springs.  They were different plungers and different length springs.  Now, after the trade, upon initial startup the pressure stays at around 65 for a few minutes(no matter the rpm) then it will approach 80 at higher rpms, and drop down to 40 at idle.  After the engine heats up fully(180-185 with an IR thermometer) the pressure gets low again.  10-15 at idle and 40 above 50mph.

 

It makes me think it has something to do with the relief valve because it seems like the relief is doing its job at first(when it stays around 65), but then it doesn't seem to be doing anything after that.  It moves like a tachometer with the rpm.  Last weekend I drove my other flathead for four hours total on the highway, and the pressure never went below 40, even idling.

 

I've added the page that Bob put up last time, because it shows the by-pass plug, which on my engine installs from under the oil pan flange.  The one I installed in mine looked like the one in the picture.  In my manual it says one of the reasons for low oil pressure could be improper seating at by-pass plug, but show no picture.  So I pulled the by-pass plug from my spare(stock c48 with full flow from the factory) and it was not the same.  It wasn't the long one as shown in the attached picture, but just a short threaded plug.  Is this my problem?  Do I need to pull the pan, again??  this would be the fifth time I think.  I sure can do it quickly now, but I still don't want to.

 

Anybody have any ideas?  Is it an oil pump problem?  I could switch out the oil pumps, I suppose.  I can't find any pictures that show the stock pressure relief or by-pass plugs in my manual, or other than the attached picture from Bob.  That one is the setup for partial filtration though.

Edited June 17, 2014 by moose

[garbagestate 44]

Before you do anything else, You might want to make sure that your oil pressure gauge is displaying accurately by teeing in a second gauge to see if they agree with each other especially if the one your using is the original gauge.

[moose]

I tried that out right after I rebuilt it last summer.  I was worried about the stock gauge, but it was pretty much right on.  Thanks for the thought!