426 Hemi variable oil pressure

[Dragon Slayer]

That would be a drastic leak and I doubt he could get 40psi. Did you pull valve covers and prime and look at rocker arm plugs see if they are leaking? You do have very low-pressure cold in my opinion, and inability to get high pressure or hold it. Looking at heads, with covers off, and priming manually, drop pan and inspect especially pickup. Was this block windowed before or in good shape. I had a hemi block with a crack in the lifter galley to cylinder wall. Ran pretty well at first on run stand, than crack developed and oil leaked into water. After more running it would leak water into oil also. Primmed fine, ran high pressure cold 80psi, but did start to loose pressure after run time and fluctuate toward 20psi. Hemi does move a lot of oil to the heads and drainage is slow so you can also see effects with low oil level.

Oil pressure issue are hard problems to track down.

 

[elocho]

Some more info
the original engine documentation confirms 8-10psi at 1k rpm idle is ok. I suspected to have not enough oil or an issue with a wrong dip stick… but it is a standard hemi pan 6qrt and 6qrt of oil came out. I checked one rod bearing and it is brand new. Need to check clearance on all

 

[Geoff 2]

This ^ is for wedge engine. Hemi had a 75 lb spring & larger pump. Both rod & main brg clearances were larger.

 

[Dragon Slayer]

This ^ is for wedge engine. Hemi had a 75 lb spring & larger pump. Both rod & main brg clearances were larger.

Pump was not larger it had the same pump as a 383, 440. Spring is a relief spring and only would affect max pressure not idle pressure where your no were near a relief pressure. I do think that is the spec sheet for his motor vice the Service manual for OEM motor.

Finding out why it falls off at rpm after a minute is what is going to be crucial here. It should rise and stay steady at a constant higher rpm.

 

[69 hemibee]

I suspect the front oil gallery pipe plug has been left out.

In my experience if that's the case the oil pressure will not vary as much as OP described. Usually its low to zero regardless of RPM.

 

[RemCharger]

Some more info
the original engine documentation confirms 8-10psi at 1k rpm idle is ok. I suspected to have not enough oil or an issue with a wrong dip stick… but it is a standard hemi pan 6qrt and 6qrt of oil came out. I checked one rod bearing and it is brand new. Need to check clearance on all

View attachment 1290232

I don't know where that's from, but this is a 67 service manual and they call for min 45 at 1000.
Hope it's legible

 

[dvw]

The cracked block theory is interesting. I dont believe it's an internal leak on the pressure side. Unless it is a leak that changes size, such as a crack.
Doug

 

[Dragon Slayer]

This is an interesting topic to me because I lived through it. It would be interesting to see engineering notes on how significant the clearance on mains, rods, lifters, rockers effect oil pressure, along with pan level.

I can't find it now, but I do think that the race spec (might have been the 67 RO cars) had a lower idle rpm oil pressure. I do think 8 to 10 psi at idle was acceptable.

I do think the orig poster should add a quart of oil and see if that stabilize pressure at the mid rpm.

I know my engine had normal to tight bearing clearances, but loose lifter clearances. Standard pump. I could prime by had easy and generate 60-80 psi with break in oil. Once hot pressure would be much lower. When I had fluctuation in oil pressure running higher rpm, adding oil would resolve it. I did have a leak in lifter galley into the water chamber around cylinder. Later became a two-way path. When after shutdown, hot water could spray into the lifter galley and contaminate the oil.

It would be interesting to put a site glass on a stock pan and see how oil level changes on a hemi running on a stand and how that level effects pressure. The drainage back to the pan is much slower then feed to the heads.

 

[Runcharger]

I experienced this once, that's why I think it is sucking air. I had a Chevy guy grind my 383 crank and he crossdrilled it on me. I also was running full groove mains so it was able to pump lots of oil. Driving on the highway the oil gauge would flicker at highway speeds. I overfilled it one quart and that stopped the oil gauge from flickering. Obviously it was pumping the oil up top and the pickup was sucking air at highway speeds causing the gauge to flicker.

 

[Geoff 2]

Post #24. My bad, thought the Hemi came std with a larger pump. It does come with a larger pick up. 2500 rpm is well above idle rpm, where the OP took a reading, so you would well & truly expect the relief valve to be bypassing.
I mentioned a missing front pipe plug earlier in this thread & I meant the one behind the cam spkt. Because the spkt is close to the opening, it could act as a baffle & might account for odd/changing pressure.
The amount of oil these pumps can generate at even a fast idle needs to be seen to be believed...
In the 1970s I had a 440 in the engine bay, car nosed into my garage. I had just rebuilt the engine, carb idle speed screwed in for fast idle/cam break in. Had someone start the engine, & a jet of oil shot up into the rafters before I could get to the key & switch off the engine. I would estimate at least a pint of oil lost in those few seconds. Silly me forgot to install the pipe plug at the rear of the engine...