Question: Oil preasure drop? And do I need to change pump for thick oil?

[Bolisk]

So a few weeks back I noticed something that has me concerned. When I really get on it. . .after the RPM crosses approx 4,800 the oil preasure drops considerably. WHY?!?!?! It never goes to 0, but it dose start to drop, then once my rpm's drop the psi goes right back up. Could this be the bypass in the filter. . .and would that have any effect on the PSI? Or am i sucking air? Is it possible for all that oil to be pumped up tot the top and not drain down enough to starve the pump?

Some background info that might be helpful.

1) My car is a L46 350/350
2) It is outfitted by the revious owner with a high flow / hight preasure oil pump (idle is 35 psi when oil is warm, normal running is arround 70psi)
3) I was running 10w30 oil when I noticed this behavior.

Next, I have recently change my oil from Mobil 1 10w30 High Milage. . .to Mobil 1 15w50. (the oil pressure drop still happens) The reason why I changed to that oil is because that is the oil that Mobil specificall recommeds for flat tappet lifters (lots of zinc and phos, over 1500 parts per million). My car has LT-1 guts, and is running solid flat tappet mechanical lifters. I was running the high milage because it had 1,000 parts per million zinc. So the next question is, is that weight way to thick? In other words is it safe to run? Next question is, if it is safe to run. . .is it safe to run on that high preasure / volume oil pump, and should i replace the pump back to stock?

 

[lars]

Drop in oil pressure at elevated rpm is caused by one (or both) of two things:

1. Windage. If the engine is not equipped with a windage tray, or has a poor quality windage tray, the air "slugs" being thrown downwards into the oil pan from the bottom of the pistons at high rpm will blow the oil out of the oil pan and way from the oil pump pickup. This aeration of the oil will cause pressure to drop dramatically. The solution is to drop the pan and install a good quality windage tray. Dyno testing I've done with windage trays shows that good windage control can produce horsepower gains in the 20hp range easily, and the windage control will save the oil pressure at high rpm.

2. High volume oil pump. If the previous builder installed a high volume pump instead of a high pressure pump, the pump can actually pump oil into the top end of the engine faster than the oil can drain back. This can cause excessive oil consumption and a drop in oil pressure at high rpm. The solution is to drop the oil pan and replace the oil pump with the Melling "Z-28" pump. This is a normal volume, high pressure pump for high performance use that will not flood the top end of the engine with oil.

The Mobile 1 15w50 is safe to run, but I doubt you need that much viscosity to save your flat tappet cam. I've been running Mobile 1 10w30 in my flat tappet engines for years, and I've had no problems. The thicker oil is causing more parasitic drag, and you're using more hp just turning the engine over in the thicker oil.

Lars

 

[Bolisk]

Thanks for the info. My car has the correct GM LT-1 windage tray. I think that is a good one. You opinion?As far as the pump is concerned. I know it is high pressure because it's running at 70psi. I don't know if it is high volume. I will ask the previous owner.I'm prett sure it's not an issue with the oil drain back holes. . .in the head gaskets.

 

[mr.beachcomber]

Thanks for the info. My car has the correct GM LT-1 windage tray. I think that is a good one. You opinion?As far as the pump is concerned. I know it is high pressure because it's running at 70psi. I don't know if it is high volume. I will ask the previous owner.I'm prett sure it's not an issue with the oil drain back holes. . .in the head gaskets.

My stock '72 LT-1 never had problems with the stock GM windage tray even when auto-xing. Your oil pressure is consistent with the stock LT-1 pump; however, I agree with Lars, your oil fluctuations point more towards a high volume pump coupled with a stock capacity oil pan. The net result is pumping the oil out of the pan into the heads. (Even if the oil drain back holes in the heads are free and clear, you can pump the oil upstairs faster than it can drain back.)

One other item you might consider is checking the oil pump's pickup to make sure its no more than 3/8 inch from the bottom of the oil pan. (Many people replace their oil pumps and just press on the oil screen/pickup instead of measuring it's clearance and then tack welding it in place.)

BTW, I used the Mobil 1 High Mileage (HM) 10W-30 in my LT-1 until I sold it to a collector. The HM version of Mobil 1 is an SL oil with roughly 1,000 ppm zinc in it. More than enough for a stock LT-1.

 

[Bolisk]

Any way to identify which pump I have. I'm pretty sure it's a melling 55, just don't know if it is an A or HV.If the pump is an HV, I know I have a spring that will reduce the pressure. Will that slow to flow down at all? In other words, I'm assuming more oil would get dumped back into the pan via the releaf valve, hence I may not pump all the oil out of the pan as quickly.

 

[Bolisk]

Ok it is a high volume pump. I just dug up the second spring that I got with the car. And the paper work with the spring says that the pump is a high volume high pressure pump, and that if I want to reduce pressure, I should replace the colored Spingarn with the non colored spring in the bag. So the question now is. Choukd I get a ne pump, or just change the spring? Or could there be something else going on.

 

[mr.beachcomber]

Even if you do change the pink pressure relief spring to the plain steel spring, you'll still be pumping 27% more oil at 45-50 psi than a stock pump. With a stock oil pan capacity, you'll still run out of oil above 4,500 rpm. Go for a stock oil pump and you'll be happy. You can always sell your HV unit on eBay or the forum. (Just let 'em know it's not for a stock capacity system.) Good luck!

 

[Crimson Thunder]

You could always leave the pump alone and just buy a higher capacity oil pan. This of course would be more costly than a pump change. It would also probably need a different pickup as well.