oil pressure issue
 

i have a mark iv 515" big block chevy
-4' gm crank
-h beam rods
-wiseco aluminum mini dome pistons
-milodon oil pan 31557
-melling 10774 oil pump with milodon pickup roughly a little over 1/4" clearance from pan bottom
-block was line honed this winter and main bearings have .0025 clearance
-johnson +.300 tall hydraulic roller lifters
-comp 11-456-8 camshaft 242-248 @.050 and .566 lift intake and exhaust
-davis unified ignition hei distributor
-10w30 vr1 is oil used
-10 quarts of oil

long story short my engine has 55-60 lbs of oil pressure as long as i stay under 4000 rpm and anything over it drops as low as 25 lbs. engine has been run over 5 minutes in the water and maintains 55-60 lbs but drops at any higher rpms than 3800.

i can ride around all day long at any rpm under 4000ish and oil pressure is great!

IM AT A LOSS AND DO NOT KNOW WHERE TO GO FROM HERE!!! ANY SUGGESTIONS ARE GREATLY APPRECIATED

I wonder if your airating the oil or its just not draining back to pan fast enough, pooling in heads / valley. Not sure how to prove that out.

-Nick

did have my buddy pull breather out of valve cover and there is oil up top but nothing excessive. its draining back as it should and we were running at 3900-4000 when he checked

Does it consistently stay low or bounce around a bit? Maybe a thicker oil ?

it stays 35-50 at idle when warm after making a 5-10 minute run at 3800rpm. at 3800 rpm it stays 55-60 when running. anything over 4000-4100 it will drop quickly and sit at 20-25

i pulled the motor over the winter and went through it because of the same issue and its still happening.

Maybe cavitation in the oil pump, have you tried a thicker oil ? 20w50?

no cavitation in oil I've already eliminated that issue by running it until it quit by reducing oil level

Not sure what you mean by that ? Did reducing the oil level make the problem quit ?

Also 10774 is not an oil pump number. Which pan do you have ? I am thinking maybe you have the pickup a little too close to the bottom ? I like to see more like 3/8 to 1/2

oil pump is a melling 10774...

broke the engine in on a stand and it was definitely cavitating. so i removed a quart at a time until it quit.

my pan is a milodon 31557.

Okay, it’s listed as a ten quart pan. In case someone else uses that pan at sometime, wanna’ share how many quarts keeps you out of this oil psi issue ? :wink :)

what cylinder heads, as mentioned I have seen oil pool up top and drop oil pressure. My personal engine I had in my old boat was a 548. had V-1 AFR heads, 14 qt oil pan. Had great oil pressure w 20/50. , dynoed it once with 5w/30 or something, had great oil pressure. Attempted to run it in boat with a 5/30 or 5/40, when oil got over 200 degrees, oil pressure would drop as rpms went past 3800/4000. I was running closer to .003 mains and .0028 rods but did have priority main Dart big M block. Did a filter cut, no metal, went back to 20/50, 65/70 psi at 6300 hot.
Fast forward, ran it just playing around on my dyno without the blower for fun trying parts. First issue I had was with a Canton 10 qt pan (14 qt was swapped), when engine got upstairs after 5 or so seconds, oil pressure would start rapidly flickeriing/down 20/30 lbs from 60/70 , added a qt of oil, was slightly better. Pulled breather off at exact second it was doing it (with 2 people), heads were flooded with oil, again, V1 AFRs that had small drain back holes. The 14 qt pan evidently masked that issue previously. tore heads off on dyno, opened up drain back holes similar to the AFR V-2s, pulled pan to inspect bearings, bent the reallly stiff trap door springs a little and when I reran it with normal amount of oil, isuue went away.

Anyone think that the oil bypass valve is opening at the 3800 RPM. Maybe have the 11 psi valve vs the 30?

I wonder if he has a mark 4 block,,his roman numerals says so,unless confused.then he should have follow artic and I on removing those bypass's in gen 5 and 6 blocks.

https://duckduckgo.com/?q=roman+nume...or-9047698.jpg

block is mark 4 and the bypass it had was in the old line adapter that mounts where a filter would in a automotive application. i removed it and put a billet hardin marine oil block adapter on it that does not have a bypass provision any longer. the heads are merlin 1-043 and we did run it up to as high as 3900 rpm and pulled breathers while doing so and there is oil draining back and not pooling up in the heads

humm,,how close is the oil pump pickup to the bottom of the pan?you have that measurement?

the pickup I'm using sits higher in the front than it does in the rear slightly but the closest portion is a little over 1/4"

All that would do is keep all of the oil from being filtered. If anything it would help oil flow.

Correct, flow is inverse proportion to pressure. You eliminate the restriction therefore you loose pressure and increase flow. I believe he said he had a loss of pressure issue not a flow issue. I don't think he has checked flow as of yet.

the only bypass in the system is the bypass in the oil pump.... all others have been removed

You have not mentioned oil temps at all?? Have you checked them??
I also would not be using 10w-30 in any marine engine

milodon 32270 windage tray fixed the issue!

Did that clear a stock factory merc pan?

He was using a 10qt Milodon pan

Yup, aerating the oil .. I chased that problem for a long time.

Thanks for the update.