Early Hemi Oil Pressure Concern

I am running a stock rebuilt 331 Chrysler hemi and I have some concern about oil pressure. Also, please note that I'm using a stock rebuilt oil pump. My concern is this, upon initial start up, the oil pressure maintains at about 45 lbs. Once the engine is warmed up to full operating temperature, the oil pressure maintains at 45 lbs until I start to slow down or come to a stop light. The oil pressure then drops to about 20 lbs a stays there until I start up again. The oil pressure then goes up gradually as I build up speed again. I have been told that this could be the function of a slow turning oil pump at slower speeds. I just simpy want to make sure there's nothing else going on. I don't want to pay for another rebuild before its time. Comments and/or explanations will be appreciated by those who understand the oil pressure principles that can explain this situation. Thank you for your time and comments.

 

Im not a Hemi expert but I dont think thats all that unusual, I'm pretty sure 20 PSI at idle isnt going to hurt anything.

 

I'm with Russco. Sounds allright to me.

 

My '54 331 is running a hi-po 340 oil pump. Thing pegs and 80 lb gauge when it's cold and maintains 65- 70 lbs when hot.

But, I'm with the guys doesn't really sound like you have a problem.

Johnnie.

 

20 psi at an idle-hot is more than enough. If you have a mechanical gauge you can see a quicker response at the gauge by going to a larger pressure line

 

I agree doesn't sound like a problem, but it would make me wonder what wasn't done properly on the rebuild. Were the rocker arms/shafts reconditioned or simply cleaned and reassembled????

 

Thanks for all of your responses. They are reassuring.

In response to HemiRambler, the rockers were not reconditioned,but rather cleaned thoroughly and reassembled.

Thanks to all again.

 

I agree that the slow rise is more a sender/gauge issue.

 

Normally oil pressure warning light switches activate at a round 8 psi so 20 psi on the idle is a good sign. There will be some variation between an engine at idle and aone with some rpm on it - the pump is working harder and the pressure rises however the drop off is a sign that there is some wear in the engine.

I would not worry about it - great to see you have seen this however it may just be the "nature of the beast".. Stock pumps do not have a high pressure normally and if its still an older one the pressure relief spring in it may be what was set from the factory years ago. The normal rule of thumb is 10 psi for every 1000 rpm or there abouts - 45 psi is a good level even at 5000 rpm on a stockish engine. The engine may have wear in the rockers or the cam bearing or what ever and it may loose some pressure when its all on idle and the engine is not "working"
I'd keep an eye on it yet go and enjoy it - it still has life in it.
A 340 pump has higher volume and pressure than a stock early hemi but don't knock the pump you have - its doing fine

 

All my early Plymouth , Dodge and Chrysler cars maintained 65 to 70 psi and about 40 at idle which is high due to typical tighter clearances Chrysler ran on brg. clearances. I suspect whoever rebuilt your engine did not run quite as tight clearances on the brgs which does not mean you need to be concerned, you're oil pressure is more than adequate for good lubrication.

 

yup 10psi. for every 1,000 rpm.

 

I put a rebuild kit in my 341 and a 1/16" shim on the oil pump spring. With a drill turning the shaft I got 80 PSI. Maybe your engine is just a little loose. A rebuild or fresh bearings should bring the pressure back up. On these old hemis the oil leaves the filter and heads for the galleries on each side of the block. From there the oil goes directly to the crank and then up to the cam. The oil actually travels through the cam journals and flows up to the rocker arm shafts only when the cam is turning and in the right position every 2 turnes of crank rotation. It actually sends oil to the rocker arm shafts in spurts. Most likely oil pressure drops when crank and rod bearing get worn or pounded out of round. If you're not hammering the moter, it should be no big concern. Just use a little heavier oil and one that contains zinc and phosphorous addititives. Castrol makes some and they are available from Auto Zone and have a yellow label attached to the containers to make them stand out. This will help prevent cam and lifter failure. Stay away from additives like STP oil treatment because that crap can cause the rings to lift away from the bore at higher RPMs. Bill

 

okay...but STP?....always liked to run some on a road trip....

 

i am re-assembling my stock oil pump and would like to know the torque value of the pump cover. i sure dont want to pull the threads out of the body. please help. thanks

 

You are fine. I have been running a 331 on and off for 50 years, pretty steadily for the past 20 years, and my pressures are in that range.
Understand that pump pressure does not directly lube the engine where the bearings need pressure in the thousands of psi to prevent metal wear. All the pump does is supply enough volume so the bearings don't starve. Lubrication is by hydrodynamic boundary layer effect. The lube oil is dragged into the space between the shaft and bearing and is squished to sometimes a couple molecules thickness. Pressure in that tiny clearance reaches several thousand psi which is enough to suspend the shaft and prevent metal to metal contact. Bearing wear in an engine comes mostly at startup when the lubrication has drained out and the shaft is lying on the bearing metal. Newer oil formulations pretty much eliminate even this wear process.

20psig at idle and 45psi at freeway speed is more than enough to flood the bearings. It is a mistake to overpressure the oiling system. All that does is chew up power and wear out the pump prematurely.

Hydrodynamic lubrication was discovered by accident, like a lot of technological advancements are, in the early days of railroading. They would lube bearing with tallow poured in holes on top of the truck bearings. No matter what plug they put in the hole, the hydrodynamic pressure would blow out the plug!

 

WOW! in 40 years of racing, restoring, building engines, etc. I've never heard that theory before.

 

Still takes an engineer to design it. Have to have bearings with enough surface for the pressurized oil to suspend the shaft over the bearing metal. Too little surface area and the boundary film is broken. Too much surface area and friction losses are too high. Higher viscosity lube allows easier film formation, too high viscosity causes extreme friction and power loss. Build necessary surface area with larger diameter journals results in stronger shaft but also increases surface speed in bearing, higher friction losses, higher temperatures in bearings, higher fluid leak-off. Build area with wider bearings have weaker shafts and slower fluid leak-off causing higher temperatures in bearings.

EVERYTHING in an engine is compromise, has to be built to the application. Top fuel engine makes a crappy grocery getter, grocery getter makes a crappy towing engine.

Do a web search on articles on hydrodynamic boundary layer lubrication. Fascinating topic.

 

10 lbs for the '54 331, factory shop manual