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  #16 (permalink)  
Old 09-10-2012, 03:15 PM
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Lotsa good ideas. Check the easiest first.
1. Foamy, air entrained oil.
2. Check the gage and the location of the connection.
3. Replace the oil filter.
3. Check the oil bypass valve.
If this doesn't work (and it likely won't), Pull the engine, remove and replace the main bearing caps. Plastigage the mains and then the rods, Clearances should be per the range in your engine manual.
Check the installation of the bearings and location of the holes. If this is the problem, have the crank turned and get some correct bearings.
A new standard oil pump should be fine. A high volume and high pressure pump can empty the pan during extended high rpm running. This is why NASCAR and other endurance race cars run enlarged pans with kickouts. A pan like this might be something for you to consider.
Let us know what you find.

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Old 09-10-2012, 03:27 PM
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One more thing, You didn't say where the oil pressure was at cruise or speed after it had dropped to 5 PSI at idle. If it's 20 to 30 pounds or so at 3000 RPM that should be enough.
In 1967 I worked in the Roadway Express shop and it was common and acceptable to live with almost any oil pressure on the gage at idle as long as it would pick up with RPM.
What I'm saying is that if it were my truck engine, I have lots of time, money and skills so I can afford to be a perfectionist. My old boss would have accepted the risk and run it since repairing a major failure is nearly the same cost and effort as making it perfect as a preventative measure.

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Originally Posted by gongchuan View Post
Just to clear up a few things and possibly unearth whole new questions,

#1 I was running 10w30 at original startup and break in. Had the same problem as with the 20w50. I went with the valvoline 20w50 for the zddp for the flat tappets.

#2 even if we stop and swim for 15 min the psi still acts funny (which would make me think its not the oil not returning to the pan). Only after sitting overnight does it run in normal range again. Also, its a ski boat and there is a pretty big angle at which the engine sits. Dont think oil can sit up in the heads so much, it all runs to the back of the head pretty quickly (found this out while adjusting the valves with cut off V.C.'s). Also after returning to over 2000 rpm's the pressure is right back up to 40 psi which would indicate there is plenty of oil in the pan, only after dropping to idle does it drop out to nearly nothing.

I am in no way "trying" to discredit what you are saying and I appreciate the feedback, but these are the reasons Im not certain if what you are thinking may be exactly right.
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Old 09-10-2012, 03:33 PM
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As I was saying, the oil might be climbing up the rear of the pan and getting whipped-up by the crank assy., especially if the boat is tilting back. The air in the oil will lower the pressure as it will displace some of the oil that should be entering the pump and compress in the output side, where oil won't compress. They make baffles just for this problem. Not saying thats your problem, just a possibility, that you can test by draining some oil.
There are two bypasses that I know of on a sbc. One is in the oil pump cover itself that can sometimes stick if the hole its in has a machining burr, or other irregularity. Its a valve that dictates the max oil pressure by swapping springs. Sometimes they can get weak if the spring wasn't removed before the pick-up is welded in, but that doesn't sound like whats happening.
If it sticks open it will lower your pressure.
The other bypass is the one on the fitting adapter that your oil filter screws onto. Its there to bypass the filter somewhat when the oil is too cold or thick, or the filter is clogged. If it opens you should still have close to if not better than full pressure from the pump to your gauge, only the oil won't be filtered.
FWIW IMO
ssmonty
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Old 09-10-2012, 07:31 PM
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Thanks a bunch everyone! Lots of good things to look at. Ill let you know what i find in coming days/weeks. Thanks alot again!
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Old 09-10-2012, 07:47 PM
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One thing that hasnt been mentioned, especially for a marine motor, what are you running for an oil pickup? Is it a flex hose? I have seen even braided ones collapse under load. The early Chris Crafts were notorious for that.
Ditch the high volume pump, get a Melling Select Series, with the larger base, made specifically for racing and off road use. I think it comes with an intermediate shaft. Use a high pressure spring for 70 psi or so. If your bearing clearnces are right you ll have 20 psi when hot and 10 lbs per 1000 rpm.
Check your pickup and oil level is critical. If motor is at an angle, too much oil could whip that oil to a froth.
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