Oil pressure

[eschaider]

This is an age old question that has more urban legend and mystery surrounding it than Big Foot does. The phenomena of fluid pressure (including oil pressure) is very old science and actually quite straight forward.

With a known unregulated fluid volume supply (hydraulic pump) and an orifice you can determine the changes in pressure by simply by changing the orifice and / or the volume of the fluid supply.

The pressure rises or falls as the square of the change in orifice area. Lets use Bobs garden hose example because it is a very good illustration. If our spigot is capable of delivering, lets say 10 gallons per minute, and we have a nozzle diameter of 1 square unit of area measure (pick your favorite units square millimeters, centimeters, inches etc.) reducing that area by one half will increase the pressure by a factor of 4 compared to what it originally was.

Fluid flow and pressure have a similar relationship. To double the flow at the nozzle the pressure at the nozzle will rise by a factor of 4 compared to what it originally was. Similarly if we maintain the same fluid pressure at the nozzle and double the area, the flow will become 4x what it was. So far seems like pretty straight forward common sense other than the math

When we include a pressure relief valve it gives the fluid system designer the ability to tailor the low speed pump delivery or the high speed pump delivery. Additionally the block and head castings grow as their temperature increases. The difference in bearing clearances between cold and operating temperature will amount to a whisker over 0.0005" (a half thousandth) of an inch in an aluminum engine, less in cast iron. That additional clearance will increase flow and reduce pressure unless your oil pump is undersized.

Lets say you want to increase low speed oil flow but have a maximum higher speed engine flow you do not want to exceed. This is where pump capacity and the pressure relief valve comes into play.

By going to a higher volume pump you will increase oil flow to the engine bearing surfaces at all engine speeds but most importantly at low speed where you were most interested in increasing flow. The rub comes in as engine speed increases.

By doubling the engine speed and therefore the pump speed, you will quadruple your high speed oil pressure. But I only wanted 60 psi oil pressure at 5000 rpm and unregulated the bigger pump will produce over 200 psi at 5000 rpm. This is where the pressure regulator, what we call the pressure relief, comes into play.

By using a pressure regulator with adequate flow capacity to bleed off unneeded volume and setting the pressure relief opening at 60 psi we can have the increased low speed flow and also limit the 5000 rpm (or wherever you pick) pressure and flow to our preferred 60 psi target.

That's it in a nut shell. Here is an excellent 2 minute video from Melling where George Richmond shows the effect of changing nozzle size by changing bearing clearances. Click here => Melling Video

If you are using an OHC engine (like I do) something you need to be aware of is that, your cams are at the end of the oiling food chain and get oiled last. Because an oil system is a controlled leak, putting additional clearance at the rods and mains will take oil away from the cams and put you at risk of seizing a cam in it's saddles, breaking your timing chains and using up a lot of expensive parts.

Even if you have an OHV engine design you still have to oil the heads for rocker trunions and valve spring cooling. If you reduce the oil to the heads you are cheating the rockers, pushrods and springs. At some point in time, probably sooner rather than later, you will have to pay the piper — and it will not be cheap.

Originally Posted by SBSerpent

What is 'normal' oil pressure on a stroked 392 Ford Racing motor (430HP). This is the motor on my 2011 build (#983)?

Stroker cranks do not change the engine's oil appetite, it remains the same as the stock stroke version of the engine.

Originally Posted by SBSerpent

I just changed the oil (9 qts of 10W-30), added two bottles of the zinc additive and using the WIX 51515 filter (5" long filter). While warming up and at idle, the guage was reading 3 1/2 bar (~50 psi).

Warm up pressure and operating pressures will always be different no matter the oil you use. The important pressure you want to pay attention to is the pressure at normal operating temperature and as Bob already stated heavier weight (higher viscosity) oils will produce slightly higher oil pressures.

Originally Posted by SBSerpent

I have seen lower readings at idle after driving the car around for awhile. Just wondering what are the parameters that cause the oil pressure to rise and fall....and what is considered 'normal' at idle and at higher revs.

As oil temperature comes up to engine operating temperature, oil viscosity will decrease. Absent changes in volume, increased temperature reduces viscosity and at constant volume it will also reduce oil pressure. Additionally, as the engine comes up to operating temperature the block and heads will increase in size because of the increase in temperature.

This increase in engine temperature to normal operating temperature will add about a half thousandth (0.0005") to the bearing clearances because of casting growth attributable to the temperature increase. The effect of the increased bearing clearance will be a reduction in oil pressure unless the oil pump has adequate reserve capacity to make up for the new found oil clearance.



Ed