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Old 01-05-2005, 10:22 PM
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Oil Pressure questions

Hi again all. I have a question about the oil pressure in my 71 Nova. The motor is a 7.4L 454 2 bolt main that came out of a 86ish model Suburban. I have a Melling High Volume oil pump on the motor (HV-77). Anyways, here is whats happening:

WHen I start the motor up at first, the motor carries excellent oil pressure, in the 75-80 PSI range. After the motor is warmed up, it still carries good oil pressure of about 50-55 PSI, and when I rev it up while in park, the oil pressure will go up to about 75 with each rev. All this is as it should be (im pretty sure). Now heres the problem:

If im going down the highway turning say.. 2500 RPMS, the oil pressure is good and hold steady at around 60PSI, but if I punch the gas while at 2500 RPMS going down the highway, the oil pressure drops to around 25PSI, but doesn't go any lower then that. I did not think this was normal. I have checked the oil and all, and its exactly full.

I was thinking that if the motor is taching 2500 RPMS, alot of the oil is already in the top part of the motor, and if I punch it while taching 2500 there isn't as much oil in the sump of the pan to shoot around, so the oil pressure drops. Does this sound about right? If so, would it be ok for me to over fill it 1/2 a quart or something and it may stop the problem? As long as im still carrying 25PSI, I should still be good though?

As always any info you guys can provide is GREATLY appreciated.

Rob

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Old 01-05-2005, 10:34 PM
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oilpress

Sounds like your oil pan is not baffled correctly,letting the oil climb the side of the pan on acceleration,thus starving the pump. Just a thought,are you sure dipstick and tube are correct. Hope this helps
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Old 01-05-2005, 10:46 PM
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Thanks for your response Kelly. Yes im sure the dipstick and tube are done right, and a local VERY reputable machine shop assembled the entire motor, intake to oil pan, so Im fairly ceratin its all put on right. The oil pan that came off the motor originally was a 6 quart "truck" oil pan, because it came out of a suburban. With 68-74 Novas and Camaros, they take a very small sump oil pan because be the steering is behind the frame and there is usually a problem with the steering hitting on a bigger sump oil pan. Because of this I put a 5 quart big block oil pan on the motor out of a Camaro. Im just windering if switching from a 6 quart to 5 quart oil pan is causing this problem.

Rob
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Old 01-05-2005, 10:47 PM
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is the oil foamy?How far from the bottom of the pan is the pickup?
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Old 01-05-2005, 11:07 PM
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There seems to be nothing wrong with the oil at all. It doesn't burn any oil at all. As far as the pick up question, I honestly don't know. I had the "truck" oil pan on the motor, and when I realized the sump wouldn't clear the frame I bought the camaro oil pan, which came with the pick up for the oil pan. My mechanic put all of those items on, so I honestly don't know how far the pickup is from the bottom of the sump.

Thanks for your input guys.

Rob
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Old 01-05-2005, 11:14 PM
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re

If you put the pan on after the shop, they may have put 6 qt dipstick and tube on. I don`t envy you if you have to pull the pan,but no since trashing a good "high dollar engine". do whatever it takes to get it right.
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Old 01-06-2005, 06:17 AM
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IMO - It's either low oil level and/or pickup height. I have experienced both problems and it is exactly what you are describing. If you're pressure never dropped below 10 psi your probably okay. But you never have your eyes on the gauge 100% of the time. I got lucky and the crank only needed to be polished. I'm not saying that is what you will have to do, but you need to pull the pan and investigate.

My pickup clearance problem was compounded by using a one piece Fel-Pro oil pan gasket. That by itself moved the pan bottom an 1/8" more. When I looked at it more closely, I had a good 3/4" clearance. That was with a Moroso pan and pickup too.

Good luck solving your problem. Ed www.edgesz28.com
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Old 01-06-2005, 06:43 AM
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If this just started "all of the sudden" one day then one other possible thought is... The pickup housing foot (screen) is broken from the pickup tube... I had this happen to a 427 a few years ago and it did what your describing...?
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Old 01-06-2005, 06:58 AM
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OIL PRESSURE

A few things come to mind.

I would put 1 qt extra in and try it to see what happens. But i will take a guess and say it's not going to fix it. What happens with the hv pumps is they move so much oil you wind up with the by- pass in the pump pushing alot of oil out of it, Then theres a deal that happens where you get a rope effect between the by pass and the pick-up and you loose oil pressure. I have seen this happen on the dyno, and replacing the hv pump with a standard pump will fix it...

I would say that if the extra qt of oil fixes it then you have a problem with the oil not getting back to the pan fast enough. This would also be resolved by installing a standard oil pump. If the extra qt does not fix it then you are probably looking at the problem i decribed above.

You can drain the oil and use one of those 1/4 dia flex lights and stick it in the drain hole and see how far the pick-up is off the bottom of the pan. Something else that can happen with the hv pumps is if the pick-up is to close to the pan floor it can suck the bottom of the pan up against the pick up and cut off the oil flow. I always weld a stand-off on the pick up to keep this from happening.

The bad part here is the fixes require you to remove the pan and i know it's a real pain in the butt on your body style....

Keith
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Old 01-06-2005, 08:00 AM
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agree with Keith

all of his reasons are why you shouldn't use a hi volume pump on a driver motor, and add one more, it's costing you HP and mpg.
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Old 01-06-2005, 09:05 AM
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Sounds like the main bearings are worn out to me.
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Old 01-06-2005, 05:00 PM
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For a quick check on mains, I use on an auto, power brake until you get a good load on the engine but not enough to spin the wheels, record the RPM and oil pressure, then go to neutral and take the motor to the same RPM, good mains should have about the same oil pressure and a drop can mean main problems. If it is a manual just load it up to about 1500-1600 by slipping the clutch and record. Then do neutral again. Only slip for a very short time.
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Old 01-06-2005, 07:53 PM
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Today I checked my oil and it was at the full mark. I then put about another half quart of oil in the motor, and now the problem has stopped. Do you guys think there would be any problem just keeping the motor a half quart over full, until I can fix the problem right? Next time I drive the car I will do the Main bearing test described and tell the results. Thanks again guys.

Rob
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Old 01-06-2005, 08:18 PM
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I wouldn't think a 1/2 quart of extra oil would really have any adverse effect on anything.

Overfilling the crankcase too much will cause "windage" and if the level is high enought, the crank can become a big blender, whipping your oil into foam, causing an oil pressure drop and possible engine failure.

A half a quart should be okay, but since you've got an expensive mill, why not buy a deep sump drag pan for your chassis with the corresponding oil pump pickup. I would go for a 7 quart drag pan, and you'll be well insured of sufficient oil supply.
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Old 01-07-2005, 06:26 AM
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thanks 85406

for the mains test, makes sense, I learned something today!
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