• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

Struggling to find the right oil pressure balance

Most of the newer pumps have really crappy inlets where it comes out of the block into the pump and as it opens up toward the rotor separation. Very rough casting and irregular shape. I have been cleaning them up (die grinder) before I even put them on.
If you look at a original pump they are much smoother casting.
I do not put a high volume pump with stock pan and tight clearances, hard on oil pump drive.
High volume is for 6-6500k rpm or better, or if you have loose clearances. If I'm going to push up to 6k regularly I put a washer in behind the stock spring in a stock pump.
A big block normally will not suck the pan dry because it just relieves back to suction side not dumping excess back into pan and having to suck it back up, like a small block.
 
Last edited:
A high volume pump is thicker than a std pump, the rotors are wider, and it needs longer bolts to mount it.
 
First assume we have a properly working gauge. Let’s also assume the inlet side is not sucking air. Oil pressure is low at idle because the pump can’t move enough volume to build resistance to flow. Why? Thin oil, internal leaks from sloppy lifter bores, large bearing clearances. Shimming the relief does nothing for idle pressure. It’s already shut. Shimming will raise pressure as the rpm goes up. A high volume pump has thicker rotors. It moves more volume. Thus it can produce higher pressure. Pressure is no more than a measurement of resistance to flow. Granted if a standard pump produces the desired pressure, there is no need to increase volume. With larger clearances it will require more flow to maintain pressure. If that volume of flow is substantial then it would be smart to have additional pan capacity.
Doug
 
Last edited:
The high volume pump has more access to volume when needed. Ie hot idle. The relief valve does the regulating.

As per mentioned by DVW and BSB67
 
Put a washer or two in the cap behind the bypass spring.
My experience is that on a healthy engine it will nudge the pressure up across the board.

It’s easy and basically free to try.

My 383, with a used factory(140,000 miles used) oil pump, in which I installed the MP high pressure spring, had hot idle(1100rpm) oil pressure of about 40psi.
It was a rock steady 80psi all the way down the track.

Cold start oil pressure would peg the gauge(100psi) at about 2500rpm.
After 114 dyno pulls and 3 seasons of bracket racing the bearings looked mint.
 
The last 3 Melling pumps I bought (15 years ago) were very disappointing. Made one work on my 508 and still using it. I kept the rotors from the other two and bought two used factory originals at a swap meet for $5 each. I Will never buy a new oil pump again.
 
I would assume your turbos are being lubricated from the engine oil? Might that have an effect on the lower oil pressure at idle?

@dvw Your son's turbo Plymouth, did it show any difference in oil pressure at idle because of having to supply the turbo with engine oil?
 
BSB67 said:
The last 3 Melling pumps I bought (15 years ago) were very disappointing. Made one work on my 508 and still using it. I kept the rotors from the other two and bought two used factory originals at a swap meet for $5 each. I Will never buy a new oil pump again.
Click to expand...

Had a 440 on the dyno here a few years ago.
This was a freshen up of an engine I’d built in 2005.
This engine had a problem with one lifter bore, which is why it had to come apart.
As part of the freshen up, and because the oil pump had passed some iron grit thru it, the pump was replaced.
The cam got changed from a SFT to a HR as part of the freshen up.
The old oil pump was a HP Melling, and was replaced with the same thing.

On the dyno the oil pressure was quite a bit lower than previously.
The initial thought was it was possibly a result of additional leakage around the HR lifters.
The old oil pump was still around, so the spring from the old pump was transferred into the new pump.
Problem solved.
The bypass springs were visually different, like the new pump didn’t actually come with the heavy spring installed in it.
 
Last edited:
khryslerkid said:
I would assume your turbos are being lubricated from the engine oil? Might that have an effect on the lower oil pressure at idle?

@dvw Your son's turbo Plymouth, did it show any difference in oil pressure at idle because of having to supply the turbo with engine oil?
Click to expand...
Nope, on 2 engines feeding the turbos had no effect on psi.
 
I swapped the spring from the old pump to new as well and I thought that had fixed it but it just took me a couple drives to get heat soaked to see the low idle psi again.
I understand what folks said about how the valve works and I value Dwayne's expertise as well. I'll throw a couple washers at it since it's free and 15min of work
 
I think the standard rotor is 0.94” deep. Measure the rotor from the oil pump that was on the motor. Then you’ll know if it is a HV or std. pump and might explain the difference in oil pressure between the two pumps
 
I just put a new Mellings HP pump on my 440, while trying to chase a problem of my original pump not wanting to prime. It turned out my pick-up tube got bent down near where it mounts into the pickup screen. My poor pump was trying to suck a milkshake through a soda straw! I will let you know how this works out, once I get the engine running. My original pump had a blue spring in it. I replaced this with a stiffer black spring I had in an attempt to get this thing to prime. I think the new pump has a blue spring in it, because it had a blue spot of paint on the restrictor boss.
 
khryslerkid said:
I would assume your turbos are being lubricated from the engine oil? Might that have an effect on the lower oil pressure at idle?

@dvw Your son's turbo Plymouth, did it show any difference in oil pressure at idle because of having to supply the turbo with engine oil?
Click to expand...
The turbo feed didn't bother oil pressure at all.
Doug
 
I have a bunch of Fall/Winter chores to do around the house, so I went to the garage to find my stash of oil pumps to measure stuff. Apparently I forgot how many oil pumps I've accumulated. No great revelations, but a couple comments.

First, this statement that I made earlier is incorrect:

BSB67 said:
........older Mopar oil pumps have a larger relief valve, I.e will bypass more oil when open, than the newer Melling pumps.
Click to expand...

Every pump had identical relief valves, and all associated key design and machining on the valve and on the pump body related to the relief valve were nearly identical. In short, with the same relief spring, they should perform the same regarding bypass pressure relief.

Second, the four springs that I tested all went solid at about 1.38". Stacking up all of the related dimensions, it looks like shimming the relief valve much more than about 0.060" will start restricting the volume of oil that can be bypassed through the relief valve.

Finally, the standard pump rotors are 0.94" -ish, and the high volume is 1.19"-ish.
 
Last edited:
Ok, so if I'm measuring ones I have externally that's a very small difference if even manifest in the casting. I'll say in 30 yrs I've not encountered needing different bolts for the oil pump.
 
You must log in or register to reply here.

Similar threads

Road Grabber
New Engine Build - Low Oil Pressure Problem
2
Replies
24
Views
1K
Road Grabber
Road Grabber
Moparjack 489
Oil pressure /viscosity 440-512 cui Stroker roller Engine in „hot „Idle question
2
Replies
20
Views
3K
not so famous bob
not so famous bob
C
Another oil pressure discussion
2
Replies
25
Views
2K
Car #4
C
Y
Oil pressure irregular
Replies
19
Views
2K
gtxrt
gtxrt
hunt2elk
Lets talk about oil pump relief valve springs
3 4 5
Replies
87
Views
6K
Krooser
Krooser
Auto Transport Service
Back
Top