View Full Version : Oil Pressure 101

08-20-2011, 01:59 AM
After all the replies about PlastiGuage, I realized I really don't understand how oil pressure is generated. Is there someone who could briefly explain this to me? For instance, why would bad bearings result in poor oil pressure? Just remember you are talking to a novice.

08-20-2011, 04:53 AM
Mornin' Kev. :savewave:

It's all hydraulics. Fluids cannot be compressed (at least not in our "world"). As you confine/restrict a volume of oil via bearing clearance the pressure in the closed system increases. The pump is pushing the oil into a restricted galley which feeds the crank first and travels up to the cam area and finally the rockers. As the clearances on the rods/mains increase from wear or other factors, the oil can "escape" at a greater rate thru the bigger space and pressure in the system lowers overall. The trick is to find the balance between enough clearance to keep the top-end supplied with sufficient pressure and the bottom end supplied with enough to keep the babbett material from excessive heating (friction). Too much clearance at the bearings and the pressure never gets to the point where oil gets to the top-end. And there's a point at which too much gap between journal and babbett causes excess impact at detonation, the bearing is literally beaten thin and baad things happen kinda fast after that ("rod knock" is the first outward indicator).

That's the simple answer. Other factors are involved, like pump condition (the vane/rotor clearance), the pressure relief valve (to balance the pressure so not <span style="font-style: italic">too</span> much is allowed), but someone else can elaborate if they feel froggy. :wink:

08-20-2011, 02:37 PM
Where is the oil pressure being taken? I understand hydraulics and the fact that fluids can not be compressed, but I do not understand oil pathways in an engine or where in an engine has the oil under pressure. I think if I understood this, it would take me a long ways in understanding oil pressure. So, if I had bad oil pressure in a 948, the oil pump, main bearings, rod bearings, and cam bearings would all be factors in oil pressure. Is there anything else that would cause low oil pressure?

08-20-2011, 05:09 PM
Pressure relief valve seat and/or spring, missing or badly seated/sealed galley machining access plug(s).

Did you notice those holes at each main bearing cradle in the block? THAT's where the oil goes first. the bearing shells have matching holes to pressurize the galley in the crank, the oil is then supplying the rod bearing shells. All is contained in passages drilled in the block and crank at the time of manufacture. Generically, some (most) connecting rods will also have passages to supply the wrist pins, some cams are hollow or drilled and pressurized similarly to the crankshaft. Some main bearing shells have a narrow shallow channel in them center-circumferentially (izzat a real word?) to keep a supply applied constantly to the crank. Again that is a simplification, the filter housing is also part of the equation. It (and the relief valve) actually gets the oil from the pump first, then to the crank and around the engine.

The primary reason to "boil" a block before machining is to insure all the passages in the galley are open and clean... not just to make it pretty. :wink:

Anyplace in the galleys, the oil is (or should be) under pressure. The "open" parts of the crankcase are how/where it has been pushed out past the various spinny bits (because they have those "clearances") and allow it to drain back down to the pan to be recirculated thru the pump pickup and galley again.

If I'm not mecunicating this well, one of you other guys feel free to jump in here!!! :jester:

I haven't a good diagram to use as show-and-tell, either, but surely there are a number of them avaiable.

08-20-2011, 07:17 PM
Kev, think of worn bearings as leaks.

Let's look at a capped of garden hose. Turn the water on and you have full pressure. Now go stab the hose about a foot from the begining. You still have pressure, but it's not as high due to the leak. Now go stab it again about three feet from the last place. Pressure is still in the hose but it has gotten lower. The first leak will spray higher than the second. Now go stab it again three feet down from the last place. Pressure drops again. Do this for the entire lenth of then hose. You still have pressure in it, but it is subtantially lower than the begining. Does this make sense?

08-20-2011, 07:25 PM
That'll work, Billy. Like a buried soaker hose. :laugh:

08-20-2011, 09:20 PM
This is the best description I have ever seen on the oil system for the A-series BMC engine.
Try this:
Kevin, it will be likely more involved than what you need, but it should answer the technical questions about how the engine supplies oil to everywhere.
Scott in CA

08-21-2011, 06:08 AM
That's good, Scott. Thanks! I figger'd one of th' Sprite nuts had done something along those lines.

Steve did a fine job of documenting it. :thumbsup:

08-25-2011, 08:39 PM
Thanks, everyone. And that article was perfect, Scott.

08-25-2011, 11:20 PM
:iagree: Especially the lawyer part at the end. :eeek: