Thread: Oil accumulator for low pressure?

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i have a mild 350 that once hot, has only about 13 pounds of pressure at idle. the psi starts off great when i start it but as it warms, goes down to 13. Upon acceleration, it climbs mormally to about 35-40. Would it be wise to install a moroso oil accumulator to keep the pressure up at idle or would i have to tear into the engine and see what the problem is? i have been running this engine like this for a while now. Do you think anything is severely damaged? I do get oil to the rockers but 13 psi is pretty low!!

An oil accumulator would help keep the oil pressure up, but only for a short time. 13 psi at idle with a stock volume pump is actually pretty good.

I tried one of the Moroso accumulators on our rock crawler with a 350. I originally thought that I needed to maintain oil pressure during extreme angles.....I took it off because there was no apparent change (although it was tough to watch the oil pressure gauge when you're about to roll).

The thing works good to precharge the engine on startup, etc. For you idle idea, you need to find out how much oil passes the pump in a given time. The Moroso accumulator only holds two quarts so it wouldnt support a very long idle period.

mike in tucson

I originally started with fossil fuel. quaker 10w-30 to break in the new engine, but have now switched to quaker high rpm synthetic blend. There wasnt much difference in the switch but the synthetic doesnt break down at high temps. You think i shoud try somethin thicker or thinner? I think the oil gets thinner as the temp goes up and in return the pressure drops the thinner the oil. Just a guess.

13 pounds at idle is okay, as long as it has 40 or so at speed.

That accumalator is to give ya nuff time to shut it down and save the engine.....

15 to 20 psi at idle is normal for a stock clearanced small block, as long as the pressure starts increasing as engine speed comes up. Your situation sounds completely normal, and the 13 psi is probably just gauge variation. If you want to be sure, hook up a mechanical oil pressure gauge to the port at the rear of the block by the distributor.

Let me start by saying that if you go to a tribology website, there will be no category of bearing wear that has to do with low oil pressure, only lack of lubrication. The oil pressure isn't what supports the crankshaft. When a cylinder fires, you have maybe 600-900 psi acting on piston with a surface area of 13 sq inches and it's being supported by a main bearing with a supported surface area of maybe 3 sq inches? The force on the piston creates a downward force of 8000-12000 lbs while the upwards force 30 lbs of oil pressure in the crank journal might be only 90 lbs (it's actually 0 because the oil pressure acts on all sides of the crank, therefore, cancelling itself out). The oil pressure generated by the pump has no contribution to the bearings ability to carry a load, it only ensures that there is more than sufficient oil supply to the bearing. Instead, think of a car hydroplaning. The way a wheel makes contact with the road forms a sort of a wedge infront of the contact patch. If you push that wheel through water at high speed, water gets packed into that wedge, creating tremendous pressure on the wheel. That pressure can actually lift the car off the road and that is when you have lubrication. Inside of your engine, that same wedge forms between the bottom of the crank journal and the bearing shell. As the crank rotates, friction between the crank and the oil pulls the oil down into the wedge much like water is packed infront of the wheel, pressure is generated, the crank is lifted off of the bearing, and lubrication is achieved.

Here comes the tricky part. As engine components reciprocate and vibrate, they may cause rapid pressure changes in the lubricating film. Entrapped gasses may come out of solution, forming bubbles, or the oil may even boil. When these bubbles pass through the high pressure region of the wedge, they collapse so rapidly that they will errode the surface of the bearing. This is cavitation. Putting the oil supply to the bearing under pressure helps keep bubbles from forming. The greater the speed, the greater the pressure differentials, and the greater the oil pressure required. Smokey's rule of thumb was 10 psi per 1000 rpm. Which says you're ok. GM says 5-6 psi at idle minimum for stock engines.

Note: Like my post on cams, these are all of my theories that I developed in the last 15 minutes, especially the cavitation part. Don't take what I say for granted, I may be wrong. But I do know that in mild street engines 13 psi at idle is ok. I ran my engine at 6 psi for over 20,000 miles and kept a close watch on bearing wear (pulled the pan and plastiguaged bearings periodically) and had no excessive wear.

Most people don't realise that a rod bearing isn't operating with pressurised oil anyway. The rods only get intermittent squirts of oil as the crank turns. Cross-drilling and grooved mains improve the flow, but its still not fully pressurised. The main thing oil does is cool stuff off, the slipperiness is important, too, but heat is the real enemy of any engine.

I did not realize that.

HMMMMM The GM engine assy. manual perscribes 40 psi. @ 2000 RPM's with oil clearances at .0007-.0028 on the rods and 0003-.0015 on #1 .0006-.0018 on #'s 2,3 & 4 and .0008-0023 on #5. These are for a 1970 350-370 hp engine. Measurements taken cold. Hope this helps.

Just another bit of info. An accumulator will only supply a given volume of oil at the highest pressure attained by the system during the charge cycle. This will diminish in pressure to equal that of the system pressure over the time it takes to equalize depending on volume used.