oil pump help

[Ron Gusack]

Originally Posted by Adger Smith

Ron,
You seem to have a good grasp of what is going on and
you are close on the cause of pressure. (sorry this gets long)
Look at the way it works with your garden hose.
Yea, one of those cheap soft plastic garden hoses.
You have it hooked up to your house/city water supply.
Open the valve wide open at the house end.
You are now flowing the set volume (through the valve as a flow orifice)
Behind that valve/orifice you have a pressure, say X.
Then on the hose side of the orifice/valve you take a pressure reading and find out it is X minus. (lower on that side)
Your end of the hose is wide open and there is nothing on it to slow the flow, Volume. You screw on one of these twisty spray nozzles and the pressure on the hose goes up (the soft plastic also swells) because you have cut the volume of the free flow. As you cut back on the volume to an orifice the same size as the house valve the pressure on the hose side is less than the house side of the valve. When the volume of the spray nozzle equals the volume of the house valve the pressures on the hose and house will be equal, too.
How much pressure and volume will it be?
That all depends on the source. How much capacity the source has and if there is a bypass in the system.
Now we are at the oil pump.
You said"Pressure is determined by excess volume and the bypass spring"
Yes and No. For all practical purposes you can turn any automotive pump in a free flow situation and utilize 100% of it's volume and have 0 pressure and not have any bypass activity.
Now put tight bearing clearances and tight lifter bore clearances on the end of that free flowing pump. AHA! you have pressure, resistance to flow volume. Vary the clearances and you vary the "Require flow volume" which will limit the volume out of the pump and increase the pressure till it kicks the bypass in the pump open. The excess volume then bypasses and the output is held at what the bypass regulates the pressure and volume to.
Start by determining what your required flow volume is. Do you have tight or loose clearances? Then figure out what pressure you want or need. Don't pump any more oil at any higher pressure than you absolutely need. It will take more power if you go up on either one. It may not be 5 to 7 HP, but it is power.
The way I first started approaching what was required was to consider what the engineers designed for the original engine. They are going to over engineer whatever they do by a certain percentage. If you have reasonable clearances the flow demand will probably match the flow capacity of a stock pump. Keep in mind the flow capacity of the pump varies with RPM. Ever wondered why the pressure drops when you slow the rpm down? The pump flow capacity gets less and the flow required by the engine clearances stays the same so there is not enough excess volume to work the bypass.
The trick is to get the least amount of loaded pump to flow the required oil volume at a low pressure to keep the bypass from over working.
1. Why is a Hi vol pump needed. "Market Demand" The avg person has no idea what is going on, but they don't want the pressure to drop when the engine speed slows down.
2. Hi Vol might be needed in some extreme cases of clearance or "Required Flow"
3. The end user usually equates engine health with hi oil pressure.(it Ain't so)
Now to the anti cavitation. It is my opinion that when you have too much pump gear size and are working the bypass too much the gears will cavitate. They will build real high pressures on the outside of the gear teeth and the low pressure on the inside causes air bubbles. The cavitation slots in pumps are there to reintroduce higher pressure oil to the low pressure areas.
I think it is a crutch. If you have a pump that is matched to your system needs you don't need them. Again the customer doesn't know or understand why the pressure drops when you are at idle. So they do cause problems of misunderstanding with customers. At this point I might add I don't like a system that is over pressured or has too much volume. It can get you into trouble with oil aeration. Air in oil doesn't lubricate or cool the parts & air can negate some of the effects of chemicals that are in the oil. If you experiment with your pump system and run only what you need you can save a little power. (Each engine and combination has it's own requirements)

Great explanation Adger and it's clearer now. My clearances are on the tight side, almost .003 on the mains and in the .0025 area on the rods. I think I saw evidence of the pump gears causing air bubbles. When I spun the HV pump I got lots of air bubbles and I was shocked because this is the pump that puts the bypass oil back into the pan. I thought I might not have enough oil in the pan, so I added a second quart. It was better but there was still lots of air in the oil, although the bubbles were smaller I think. When I did the SV pump, with bypass oil staying in the pump, there was NO air bubbles in the oil. The pressure with the SV pump bounced and the HV pressure was steady. Alan Roehrich told me about putting 1/8 pipe plugs in the cap and drilling two 1/4 holes on the outside of the cap to allow bypass oil to go back into the oil pan. I have a spare pump that I'm thinking about doing this trick to so that the bypass oil won't stay in the pump. Is this worth doing in your opinion??

[Mike Pearson]

Originally Posted by Jim Hanig

Mike what do you think caused the shaft to break? I have raced chevys for 50 + years and never had a melling, chevy pump fail. Jim Blair does low pressure pumps Pierre s.d.

I have found over the years that you are much better off using a product that is specifically designed for a racing application instead of using a modified passenger car type product. I found this out the hard way. I use a Moroso billet oil pump a system one oil filter and racing quality engine oil just to name a few. Just Google Meling oil pump failures and you can get some info. The pump that broke was a Melling M55. Snapped off at the neck just below the bolt flange. It destroyed that short block.