Big Block oil pressure drop

[Josh Noser]

Sorry for taking awhile to get back to the board on this issue, but I have been too busy the past few weeks.

As a reminder the problem I have is oil pressure drop off. I have good pressure at idle and through most of the RPM curve but at 4500-5000 RPM the oil pressure drops off steadily, however if I let off the gas it jumps back up to normal very quickly (~ 1 second). The motor is the original L-71 engine, which has been completely rebuilt. The heads are the original square port and they where hot tanked and completely rebuilt with hardened seats.

I looked back at invoices and found the oil pump in the car is a High-Volume, Standard Pressure pump made by sealed power (SLP-224-4153). Joe Lucia had said in response to my last post that generally high volume pumps work well in street big blocks. When looking for pumps recently I found you have to search to find a standard volume pump so it is no wonder I wound up with a high volume pump in the car.

Today I got under the car drained the oil and knocked on the pan and I do not think the pick up fell off, plus I talked to the guy who built the engine and he said he welded the pick up on. I also see that dropping the pan without a lift will be a chore. I really wish I had a lift!!

The question is should I change out the oil pump? Also what type of oil should I be running (currently 10-w30)? I only drive the car in warm weather. This problem is driving me crazy. I do not want to drop the oil pan if it is pointless because it will be a pain, as well at it currently does leak not any oil. On the other hand I need to get it figured out because I do not want to damage the engine.

Additional information if needed: The gauge I am getting the readings from is the original; the oil filter is Fram (not the old canister system)

Thanks for all the help with this, everyone has been very helpful!

Josh

[Mike McKown]

Re: Big Block oil pressure drop

Maybe your pick up screen is too far off the bottom of the pan. I have no idea what the book calls for. I shoot for about 3/16's"

[Roger Legge]

Re: Big Block oil pressure drop

This is certainly a remote possibility, but I'll mention it anyway. I have heard that if an engine came from the factory with a windage tray, and the engine is rebuilt/whatever without the windage tray, the crankshaft can whip the oil into a foamy sort of condition at high RPMs and then there is a loss of oil pressure.

Roger

[Scott Marzahl]

Re: Big Block oil pressure drop

Have you tried plumbing in another temporary mechanical guage at another location to use as a comparison? I would buy a generic guage with the plastic line and plumb it in at the plug above the oil filter so you could see both guages at the same time. Also, have you tried swapping out the Fram filter with a new A/C filter, however, if it was clogged up with assembly lube I would think the bypass valve would be actuated and your pressure would go up.

[Michael H.]

Re: Big Block oil pressure drop

Josh,

I think Mike and Roger are on the right track on this. There are some other possibilities but the most likely is oil starvation caused by an incorrectly positioned or no longer attached pickup screen. It's also possible that the windage tray could have something to do with this but I lean toward the screen. As Joe Lucia mentioned in the last series of posts on this, it MAY be possible that the overall oil flow is exceding supply but the fact that the oil pressure returns as soon as you back off the throttle would suggest that that's not the case. It sounds like oil slosh back to the bottom of the pan when you back off immediately picks up the pressure again. I see no way that the oil pressure guage could cause this problem, especially at the same RPM/pressure each time you make a test run. I would definitely not make any more test runs as it only takes a few seconds to create an engine desease called the "black death" and it's not an inexpensive repair. If you do make another run, I would definitely overfill the crankcase by at least one quart for the test. I still don't recommend it but it would probably answer the question about pump pickup height.

I agree, removing the oil pan on a garage floor is no fun but it's way more fun than replacing a crankshaft and several connecting rods.

Michael

[Josh Noser]

Re: Big Block oil pressure drop

Michael,

I agree I do not want to do any more test runs. I also agree that it would be a very slim chance that it is the guage. I really dread droppig the pan but I guess I have no other choice. What is the best way to disassemble the steering linkage? The tirod ends and boots are all new so I do not want to frag them. Also as far as I can see I will have to remove the starter motor, corect? I hope I will be able to do it, I was under there today and there is like no room. Less room than I remember when worked under there when I was restoring the car.

Thanks for you advice on this issue,

Josh

[Michael H.]

Re: Big Block oil pressure drop

Josh,

You should be able to lower the steering linkage far enough by just removing the nuts/bolts that attach the idler arm to the frame, if I remember correctly. The starter and oil filter have to be removed also. My favorite part after the pan has been removed is the constant oil drips that all seem to hit directly in alternating eyes. Good luck with your project.

[Clem Z.]

make sure to place the TDC mark on the damper

at 6:00 position to get the crank counterweights in the best position to slide out the pan

[frank mccracken]

Re: Big Block oil pressure drop

Don't worry about dropping the pan, it's pretty easy with the idler arm dropped. Just get the car up on stands high enough that you can work.
Doesn't this sound like a classic case of a Hi Volume pump sucking the pan dry?

[Clem Z.]

Re: Big Block oil pressure drop

my new stock 66 425 HP 427 would see the oil pressure head south when run up in RPMs if i did not run the engine 1 quart over full. i think the oil did not return to the pan fast enought because the piddle valve lifters allowed too much oil upstairs. we solved the problem by using edge orifice lifters in cars used for racing. try a quart over full and see what happens

[Chuck R.]

Re: Clem, what's the exposure being a qt. over?

Does the extra quart place any kind of added pressure to the engine internals?

I would think that if the oil pump was set to run at 40lbs. pressure, it wouldn't matter how much oil was in the base. Of course the more oil the more the chance of gasket leakage I suppose.

Of course this is what I get for thinking again

Chuck

[Scott Marzahl]

Regardless of pressure, the volume stays

If I understand your statement correctly Chuck, reducing the ouput pressure (function of the relief spring) of the pump is independent of the volume output (function of the gearing) of the pump. I've only used a High Volume pump once and it was in a blown big block with a nine quart pan so I had enough supply reserve where the output volume didn't drain the pan. Did the big blocks normally come with a HV pump?

[josh noser]

Re: Big Block oil pressure drop

I have tried running one quart over and that did help, but only a very small amount.

[Clem Z.]

Re: Clem, what's the exposure being a qt. over?

at higher engine speed the windage from the rotating assy keeps the oil from getting back to the pan in the proper places. at 6000 RPMs there is a 120 MPH wind blowing off of the assy caused by the spining of the crank and the pumping action of the pistons. that is why they call it a windage tray, the item installed between the spinning assy and the oil pan sump. put some water in a flat pan and blow into it with some compressed air and you will see what i mean. at higher RPMs the extra oil is being held in the uper part of the the engine by G forces and that is why even with the new LS engine GM recommends that you run 1 quart over if you do track days with your C-5 or C-6

[William O.]

Re: Big Block oil pressure drop

Pull the pan and take a look, take a very close look at the pick-up screen, if the screen is clear and free from any debris, (I had metal shavings from a rebuild, and the builder forgot to clean the screen!)
If clean, replace the pump.

Hope all goes well.

Bill