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Old 07-24-2002, 04:19 AM
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Post Small Block Chevy Oil Pressure

My son and I built a high performance 350 small block chevy for his 67 Nova. (desktop dyno at about 425 horse) We used all new parts. New block (standard bore, new casting), new forged steel crank and forged pistons, new Dart iron eagle heads, Moroso 6 qt pan and matching oil pickup etc. All parts purchased new from Performance Automotive Warehouse (PAW) in California. We plasti gaged all the rod and main journals during assembly, and all were well within spec. The problem is that the oil pressure drops under wide open throttle (WOT). Hot idle pressure is about 20 psi, moving right up to about 50 psi at 2500 rpm or so. However, under WOT, the pressure drops from about 55 psi down to about 35 when the rpm gets up to about 4500. However, if we gear down and run the engine at a constant 4500 to 5000 rpm, there is no pressure drop. It only drops under WOT. We thought that the problem might be too large an oil pump (we first used a high volume pump) so we changed the pump and pickup, using a new GM high pressure, standard volum small block oil pump for a 69 vintage Z28 Camaro. But the problem did not change. The engine runs and drives normally except for this problem. My son's got about $6000 wrapped up in this engine, and I'm stumped. Someone suggested that we tear down the engine and check for excessive bearing clearances, but I don't understand how that could cause a pressure drop only under WOT. Please help.
Thank you
Tom

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Old 07-24-2002, 05:24 AM
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I've had similar problems with my wet-sump sprint car. The pump moves so much oil at high rpm that most of it ends up in the rocker covers and valley...the oil can't return to the pan fast enough to replace what the pump has pushed upstairs which leaves very little oil in the pan. I installed restrictors in the oil galleys alongside the cam at the back of the block and the problem was solved. Also, check the clearance between the pickup and the floor of the pan. I believe this should no more than 3/8".

[ July 24, 2002: Message edited by: SJFast ]</p>
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Old 07-24-2002, 08:00 AM
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[ July 24, 2002: Message edited by: 78 monte ]</p>
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Old 07-24-2002, 08:04 AM
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[QUOTE]Originally posted by 78 monte: [QB]As far as 20psi at idle I wouldn't worry 55-60psi is normal driving. As far as 35psi at (WOT) its probably oil starved.Maybe a windage tray would help.Not sure. Have heard of guys opening up the oil returns.???
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Old 07-24-2002, 09:05 AM
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I had the same problem with my engine after it was rebuilt. They put three different pumps in but it didn't fix the problem, finaly the fourth pump worked. I really don't belive the Mechanic knew why it was doing that, there were many things that could have been causing it. To this day I still don't know what it really was. I know this wasn't much help to you, sorry.
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Old 07-24-2002, 11:08 AM
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never had that problem myself but the first thing that came to mind is windage tray but all the answers so far seem like good possibilites good luck
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Old 07-24-2002, 11:44 AM
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Did you check or install the oil passage plug under the rear main in the block ? If this plug is missing you will have low press and the oil will not run to filter and it will do funny things,if this was a new block it did not come with this plug, it blocks the passage between the two to the oil filter and rear main feed.I have seen this many times and i have over looked it myself.
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Old 07-26-2002, 09:53 AM
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Update:
Still trying to get to the bottom of this problem. The post by ROYS63 concerning the plug that needs to be installed in the oil passage of new blocks is one that I didn't know about, and am investigating. I'm trying to gather all the info I can before I tear down the engine. If anybody out there knows ANYONE who has experience with building newly casted green blocks that might be able to help me, please let me know. This board has been a big help to me already, and I appreciate all of your inputs. I'm gonna stay on this one till I get it solved. Thanks guys!
Regards
Tom
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Old 07-28-2002, 03:17 AM
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I have had similar problems with high performance engines and it should be cured by looking carefully at you oil pickup and oil pan configuration. Chances are that at WOT your engine is frothing the oil excessively and/or the pickup is to far off the bottom of the pan. Check into windage trays and a differnet oil pan {side kick-outs}. Does the situation improve if you run 1 quart less oil? Using a 7 quart pan at 5 quarts always helped me. Good luck.
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Old 07-28-2002, 06:40 AM
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I agree with SJ, restrictors in the lifter galleries are a must with any kind of radical engine. Those high bleeddown lifters are notorious for causing this as well, if you are using them, toss-em. A windage tray is a good idea also. The only other thing I might add is I have seen this problem only once before, I traced it to a cracked weld in the oil pickup. Seems the crack would open up at WOT/heavy load and would allow the pump to suck air, I would pressure test your engines oiling system with the pan off and see if there are any leaks or porosity on the suction side of the pump. This may need some ingenious engineering to test but I would be highly suspicious at this point and check every component over very carefully.
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Old 05-30-2007, 01:22 PM
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I will start my own thread. sorry

Last edited by ztwntyn8; 06-01-2007 at 01:15 PM.
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Old 05-30-2007, 08:57 PM
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Quote:
Originally Posted by roys63
Did you check or install the oil passage plug under the rear main in the block ? If this plug is missing you will have low press and the oil will not run to filter and it will do funny things,if this was a new block it did not come with this plug, it blocks the passage between the two to the oil filter and rear main feed.I have seen this many times and i have over looked it myself.
With tha plug left out once the oil filter is full the oil will bypass the filter as the oil will go in the path of less resistance.
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Old 05-31-2007, 07:55 AM
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Quote:
Originally Posted by ztwntyn8
I have a 94 Z28 6 spd with 80K miles on the clock. One day I was passing on the highway in 2nd gear. Out of the corner of my eye I saw the oil psi guage gain psi when I let off. I thought huh. After that I started to observe the guage during acceleration and it realy was dropping. It seems to happen after about 3K rpm and ONLY during a very heavy load. For example: it can happen on flat ground at wot or up a hill at about half throttle. I am using a ported MAF (which I believe is a mistake) throttle body bypass and other than a flowmaster it's stock. When I changed the oil it was like water but didn't smell like gas. The enging progressively looses oil psi as the oil life decreases. In the winter it's at it's worst. I feel that the porting of the MAF has made it super rich (no black smoke) at an idle and cold weather makes the problem worse. So, I change the oil and get some oil psi back but never what I used to have. I've never put a mechanical guage to it but if you take it upstairs at lite throttle it dosn't drop psi. I switched from 5W-30 to 10W-40
and have about normal psi now (maybe a little high). I believe there is a clearance issue now in the bottem end. A fellow once told me to expect the engine to sieze soon but it hasn't with another 15K miles or so. He told me with a stick that the input shaft attempts to move the crank against the rear main bearing via pilot bearing and ends up moving the crank away from an oil passage which bleeds off psi. I have done alot of WOT shifting and I still beat on it but I'm scared. lol This is the only forum where I have seen my exact problem. Oh, and the engine has never been apart. So, let me know what you think guys. Thanks alot!
You should probably start your own thread instead of interjecting in this one. It gets confusing for people to understand which vehicle we are talking about etc.
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Old 06-01-2007, 01:16 PM
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cool, I'll start a new thread... I'm new.
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Old 01-28-2008, 08:03 AM
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I had the same problem with a 383 stroker i built. tooling down the road it was fine but as soon as i jumped on the gas at 3000 rpm the oil pressure fell off if i let off the gas the pressure would come back up. so after pullin the engine out the second time we tested everything and finnaly came accross the one thing people tend to over look in an engines oilling system. the lifters. the lifters are a check valve times 16 in your engine so even if 3 or 4 are not working properly then this is a hole in the oil system and at rpm can and will create pressure drop fast.
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