edit: It was a clogged oil filter. The oil wasn't old at all but the air filter was shot.:

It's a Melling 155 High Volume Pump and a stock pan, but the pan being drained isn't the problem. Running 10W-30. Engine only has about 10k on it. It acts the same whether it's in neutral revving, or driving. All of the bearings were fine, and I took the pump apart and it's fine. Was using a Fram stock type oil filter. I checked the pressure with an external gauge through the oil supply line for the supercharger, and the gauge in the dash is telling the truth. (The supply for the SC is taken by drilling into the block above the filter, so that's the point the external gauge was reading at.) I checked a couple clearances with plastigauge and they were reasonable, so that shouldn't be it.
Any thoughts? I can think of two possibilities:
1) the pump is exceeding the capacity of the pickup screen piece at the end of the tube. I'm looking at it now and the pickup screen is about 30% smaller than the one off a stock pump, although the tube itself is thicker. The build sheet has the pump and screen listed separately, with the screen being "155S Oil pump screen/LT1".
2) The spring is too weak. (But I compared it to the one from the stock pump that was working fine in another engine and couldn't tell any difference.)
The engine is out of the car right now so I don't want to put it back in and still have the problem. I plan to put the stock pump on it anyway and see what happens, but I'd appreciate any thoughts.

 

Do not use Fram oil filters.

40psi on a HV sounds quite low. Is it 40 psi even at cold start?
I thought maybe the oil pressure sending unit was wrong, but if you checked that than I do not know.

 

I was under the impression you don't want a hv oil pump with a stock pan because it will suck it dry?

 

whats wrong with a fram TG oil filter? I have one on my car right now as well as previous cars and havent had any issues yet.

 

I think they have one filter that is ok, but every test I have ever seen on these boards shows that fram suck.... bad.

I'm sure you have been told a million times, but HV and stock pan is dangerous.

If the pickup opening is smaller that could be an issue. My canton pickup has a much larger opening than the stock one.

Check the sender.

Rock solid as in the gauge never moves at all? Could the gauge be stuck? Or does it move when you start/stop the car?

Trying the stiffer spring couldn't hurt, but it is a pain to get to. (well assuming you are not sucking the pan dry it couldn't hurt) But even with the 60psi spring I can still see more than that on a normal Melling pump with thick oil and a cold day.

 

It's a little higher when it's cold, like 43. And if I rev it to 3000 and let it sit there for about 5 seconds it might climb from 40 to 41, but that's it; it doesn't budge basically. I'm putting new bearings in now, and a new stock-type oil pump, so I'm afraid I might never know if it was loose clearances, or if it was the pump, and it's too much work to be taking the pan on and off to find the cause for sure. Thanks for the thoughts.

 

It's fixed. I put a stock oil pump in when I rebuilt it (again) and the pressure does what it should do.
The problem was one of three things:
-the spring in the pump was weak (unlikely since I couldn't tell any difference by hand with the OEM spring)
-the pickup was too close to the bottom of the pan (unlikely since the angle was the same on another pump I used which had no issue)
-the pickup was too small (my money would be on this one)