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Discussion Starter · #1 ·
This is a general automotive question and applies to all piston engines.

My possibly twisted view of Oil system and pressure basics.
Crank mounted oil pumps suck oil through a pickup tube from the oil pan, the oil is pushed through the engine oil passages and delivered to the cam journals, rockers, sprayed all around the valves and springs, sent to the rod bearings, crank bearings, into the pistons and rings. All these points are "leaks" in the oil delivery system.
Factory oil pumps are designed to supply enough oil volume to maintain desired pressure in the oil delivery system. Once the desired pressure is reached the oil pump bypass valve allows excess oil pressure from building up and recirculates excess oil back to the pan. It functions similar to the way a waste gate limits turbo boost or fuel pressure regulator sends fuel back to the tank.

Temperature, engine build clearance specs, higher rpm and engine wear can all effect oil pressure.
Temperature, build clearance and engine wear oil pressure can be compensated for by using different weight oil.

Running higher rpms is usually less of an issues with crank mounted pumps however there can a point at higher rpms when the oil pump tries to move more oil than the pick up tube can deliver and starves the engine of oil. A larger pickup tube allowing more oil to flow to the pump or a performance oil pump designed for higher rpms with different shaped rotors can solve this.

Aftermarket high volume oil pumps, these are usually fitted with longer rotors allowing more oil to be moved per rpm. (In turbo terms, it's like a larger turbo delivering more air at the same boost).

Aftermarket high pressure oil pumps, these usually have factory style rotors and deliver the same volume per rpm as the original pump, the bypass valve is set to open and maintain a higher peak oil pressure than factory, shimming factory spring or installing a stiffer spring in the bypass valve will yield higher peak oil pressure. (In turbo terms, same turbo with a higher rated wastegate, same air per psi but running higher boost).

Engine oil does several things, it lubricates seal (valves, cams, crank, turbo bearing (ball)...), components (cam lobes, valve springs, pistons, rings...), carries away engine heat caused by friction and most importantly it is liquid bearings for the cam, rockers, crank, rods and turbo shaft (sleeve bearing).

Liquids act as a solid when attempting to compress, it can be displaced with applied pressure but not compressed. Take the brake system for example, when left no escape path the fluid in the lines acts as a solid metal rod transferring pressure from the master directly to the back of the caliper pistons, at normal temp and in good condition the fluid in brake lines can withstand more pressure than any part of the brake system, if 10 times more pressure is applied than the seals will blow, if the seals where made to handle 12X the pressure the lines would blow, upgrade the lines and fittings and the calipers will split or crack or explode, upgrade the calipers and the seals will leak... but the fluid will not compress.

This is how engine oil under pressure acts as a liquid bearing that keep rotating components from coming into contact with each other.
The oils escape paths/ leaks/ openings are designed with specific tolerances to allow the oil bearings to keep moving keeping the oil from burning while maintaining a constant liquid pressure through the system.
So sleeve bearings and cam journals are just oil sleeves.

Logically in my mind all these "liquid bearing oil escape paths" allow the potential for one of the metal parts to squeeze the oil bearing out it's path of contact when the pressure the part exerts toward the other is greater than the oil bearings ability to resist pressure.

When engine power is increased so is friction and that creates more heat, the cooling system is designed to take car of it but the engine oil also caries away a lot of heat because it takes all the friction. The hotter oil gets the thinner it gets and it and the oil pumps ability to maintain liquid bearing pressure are reduced because the oil is flowing out the escape paths faster. More heat also breaks down oil faster. So keeping oil at proper temperature is very important to engine survival.
Using an oil temp sensor and gauge, looking up oil viscosity and oil temp charts for your selected oil weight and monitoring oil temps could be better than watching oil pressure and simply knowing when oil pressure has dropped because oil temp got to high. When oil pressure drops enough the oil bearing cant resist getting squeezed out and the rotating components wear, increasing the size of the escape path making it harder for the pump and oil to maintain liquid bearing pressure/ oil pressure.

An oil system that maintains factory oil pressure and oil temperature should maintain "solid liquid bearings" between all surfaces.

When doubling or tripling factory power output from an engine 2 or 3 times (at minimum) the amount of air must be in the cylinder, by increasing displacement, increasing cam duration, increasing compression ratios or forced induction the piston has to compress 2 or 3 times the amount of air into the same sized combustion chamber with doubles or triples the pressure against the piston and against the liquid rod bearing making it the most prone to be squeezed out from between the rods metal bearing and crank.

The Question:
At what point does cylinder pressure against the piston overcome oil pressure between the rod and crank?
Why can't I find any online calculators for this?
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