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oil pressure drops with RPM

N

nsxxtreme

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On my 383 I have a melling M55HV oil pump. Stock oil pan.
I stuck mobile 1 10W-30 synthetic oil in there. I say this because I have heard mobile 1 will have slightly lower oil pressure. I get 80-100psi cold.

Once warmed up I get about 60 psi all the way up to about 3700 rpms. Then the oil pressure begins to drop to about 45 psi by the time its 4500rpm. I've read you want 10-15psi per 1000 rpm.

I was thinking of sticking 20W-50 in there to see if I can get the pressure up a little more.
 
80-100 psi ?

why do you want such a high O/P ???

Sounds to me like you may be pumping all the oil to the top of the engine with that H/V pump and running the pan dry...


I would try to add 2 qts and repeat the test.....


how fresh is the motor???
and what clearances did you set the bearings up at ??
 
Heck my little 302 small block Ford I just rebuilt runs 44 lbs hot at idle,and 55 lbs at highway speed.....

and that's with a bone stock pump and pan w/ 10w40
 
wasted wages said:
80-100 psi ?

why do you want such a high O/P ???

Sounds to me like you may be pumping all the oil to the top of the engine with that H/V pump and running the pan dry...


I would try to add 2 qts and repeat the test.....


how fresh is the motor???
and what clearances did you set the bearings up at ??
Click to expand...
Brand new motor .0018 rods .003 mains.

Find hard time believing oil pan would be sucked dry.
http://forums.hybridz.org/index.php/topic/3012-oil-pressurehow-low-is-too-low/
 
Nothing wrong with using a high volume pump.

Sucking air or oil not returning to the pan quick enough are the first things that come to mind. Is the pick up tack welded? Any chance the weld blew a hole allowing it to suck air at higher rpms?
 
nvrenuf said:
Nothing wrong with using a high volume pump.

Sucking air or oil not returning to the pan quick enough are the first things that come to mind. Is the pick up tack welded? Any chance the weld blew a hole allowing it to suck air at higher rpms?
Click to expand...
No its not welded.
 
cuervo said:
Especially with a stock pan... you could be sucking it dry.
Click to expand...
From what I have read seems to be a myth. If it were true no matter how big the oil pan was it would be sucked dry given enough time at a given rpm. Good info on HV pumps http://forums.hybridz.org/index.php/...ow-is-too-low/

The only thing that will allow and oil pan to be succked dry is if the return oil passages are somehow clogged. There are a LOT of return holes I just dont see that happening.
 
Typically HV pumps also have taller gears, dropping the pickup closer to the bottom of the pan....did you measure the distance ( clearance ) between the pan and pickup ?



Brand new motor .0018 rods .003 mains.
Click to expand...

mains are a little loose for my taste....

Oil volume through a bearing clearance at a given pressure is X.... that about doubles with every .001 additional clearance.

So if you are running .0015 main and rod bearing clearance a stock pump is fine for 40psi.

If your running .003 main and rod bearing clearance, then it won't work very long, you need almost 2x, and your max oil pressure will be lucky to hit 35psi.
 
wasted wages said:
Typically HV pumps also have taller gears, dropping the pickup closer to the bottom of the pan....did you measure the distance ( clearance ) between the pan and pickup ?
Click to expand...
Pickup has bumps on it to keep it the correct distance from the bottom of the pan.




wasted wages said:
If your running .003 main and rod bearing clearance, then it won't work very long, you need almost 2x, and your max oil pressure will be lucky to hit 35psi.
Click to expand...
Not sure where you get your math. My max psi is 60+ish at around 3k rpm it doesn't begin to fall off until 4k rpm.
 
Cam bearings can bleed off oil pressure too,if they are on the "loose" side....Seems to me I recall chevy V8's needing some kind of restrictor installed in the oil galleys up front under the timing cover when using a high volume oil pump,or else you would risk losing oil pressure due to excess flow to the lifters??....been 20 years since I was in an auto machine shop/parts store,but I remember a few customers having issues with high volume pumps & oil pressure,when this mod wasn't made...
 
My 2 cents are you overfilled the pan. If the oil level is 2 high the crank will foam the oil and loose pressure. I have seen if quite a few times. Dad did it last harvest on the combine when he forgot he filled the filters and added in that amount to the pan. Got low enough on hillsides to turn on the light. Pulled to the end of the field and drained it in a bucket and the problem went away. I have had many HV pumps and never sucked a pan dry or had any bearing failure. It isn't like he is turning it 8 grand for 4 miles.
 
my 406 has the same oil pump. On 30wt oil it ran 95psi when cold if you got on it a little, 75 at a cold idle. Warm would be 50-60 and 18 at idle. I now run 10-30 and have 50-60 psi at 800-6000 rpm, 14 on a hot idle. I am running a stock pan and the engine is late 70's design. I don't know how tight my builder set the bearings.
 
Problem solved that oil pan and pickup I bought made a HUGE difference. Stock pan is crap for a performance motor. Rock solid oil pressure now up to 5000 rpm. I've got about 80-90 psi at 5k.
 
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