Originally Posted By: ZeeOSix
So Mopars have 70~80 PSI of oil pressure at idle with hot oil? I'm not talking about "computer controlled" oil pumps, etc ... just old fashioned positive displacement oil pumps.
My 2008 4.7 hits the pressure regulator at about 1800 RPM on 0w20 oil- about 70 PSI. At idle, the pressure is of course less- about 25 PSI when hot. My Hemi runs lower pressure- 60 PSI max once the oil is hot, but it too reaches that pressure at about 1500-1800 RPM on 0w40. At idle it runs about 30 PSI, meaning that it has a higher volume supply pump AND a bigger relief valve flow than the 4.7 does. The whole design philosophy (I assume... I didn't design these engines) is a fixed oil supply that is nearly RPM-independent for assured lubrication under all conditions. The Pentastar takes it a step further by putting the computer in the loop to avoid the even the small amount of power wasted by the excess oil flow out of the relief valve.
But even with all that, there are complicating factors. The relief valve isn't a hard limit on pressure- it just puts a very sharp knee in the RPM/pressure curve, so the pressure rises very fast from idle to 1800 RPM, then only swells another 10 psi or so between 1800 RPM and redline.
The only thing idle oil pressure should be used to diagnose is an oil pump producing less than the minimum required flow, OR bearing clearances requiring excess flow in order to reach operating pressure. Apples to apples comparisons of how the oil affects pressure OR how the filter affects pressure can ONLY be made at a fully warmed-up state, and at an RPM above which the pressure relief valve opens.
Originally Posted By: ZeeOSix
Again, if the oil pressure gauge is down stream of the oil filter, you will never see the effect of the filter's delat-p on the engine's oil pressure gauge - unless the oil pump is in pressure relief. You would have to place the oil pressure gauge before the oil filter to see the filter's effective flow resistance.
This is EXACTLY the point. At cruise speed on a 4.7, he's in pressure relief by design. Constant pressure into the filter (at normal operating RPM the pressure relief valve is open), minus the filter delta-P = reading on gauge changes by a combination the filter delta-P AND the oil viscosity affecting how fast it vents out of the clearances. You cannot separate one effect from the other without changing ONLY the filter or ONLY the oil, and you CANNOT say "you can't see the filter delta-P at all."
Put another way- I agree that the only way to truly and accurately measure the filter's delta-P is with a gauge before and a gauge after. But in the 4.7 application, the delta-P is a COMPONENT of the total change seen on the pressure gauge when you put in both a new filter and new oil, because the supply pressure (the relief valve) must equal the filter delta-P plus the back-pressure from the bearing clearances (which is what the gauge reads). INcrease the filter delta-P, and the gauge pressure will drop- just do the math. The REASON it drops is that higher filter delta-P causes a slight decrease in total flow. Its exactly like calculating the voltage between two resistors in series in an electrical circuit. Increase the resistor nearer the voltage source, and the voltage between the two resistors decreases because the current (analogous to oil flow) decreases slightly.
The key point is that we're not talking 50's-DNA Chevy or AMC that runs below relief pressure all the time, we're talking an essentially constant PRESSURE supply (above fast idle, anyway) in a modern Mopar 4.7. This is why the only meaningful comparison is hot oil at operating RPM so that the pressure relief valve is open- and even then you should really look at exactly the same RPM since the relief valve isn't a perfect limit on pressure.
So Mopars have 70~80 PSI of oil pressure at idle with hot oil? I'm not talking about "computer controlled" oil pumps, etc ... just old fashioned positive displacement oil pumps.
My 2008 4.7 hits the pressure regulator at about 1800 RPM on 0w20 oil- about 70 PSI. At idle, the pressure is of course less- about 25 PSI when hot. My Hemi runs lower pressure- 60 PSI max once the oil is hot, but it too reaches that pressure at about 1500-1800 RPM on 0w40. At idle it runs about 30 PSI, meaning that it has a higher volume supply pump AND a bigger relief valve flow than the 4.7 does. The whole design philosophy (I assume... I didn't design these engines) is a fixed oil supply that is nearly RPM-independent for assured lubrication under all conditions. The Pentastar takes it a step further by putting the computer in the loop to avoid the even the small amount of power wasted by the excess oil flow out of the relief valve.
But even with all that, there are complicating factors. The relief valve isn't a hard limit on pressure- it just puts a very sharp knee in the RPM/pressure curve, so the pressure rises very fast from idle to 1800 RPM, then only swells another 10 psi or so between 1800 RPM and redline.
The only thing idle oil pressure should be used to diagnose is an oil pump producing less than the minimum required flow, OR bearing clearances requiring excess flow in order to reach operating pressure. Apples to apples comparisons of how the oil affects pressure OR how the filter affects pressure can ONLY be made at a fully warmed-up state, and at an RPM above which the pressure relief valve opens.
Originally Posted By: ZeeOSix
Again, if the oil pressure gauge is down stream of the oil filter, you will never see the effect of the filter's delat-p on the engine's oil pressure gauge - unless the oil pump is in pressure relief. You would have to place the oil pressure gauge before the oil filter to see the filter's effective flow resistance.
This is EXACTLY the point. At cruise speed on a 4.7, he's in pressure relief by design. Constant pressure into the filter (at normal operating RPM the pressure relief valve is open), minus the filter delta-P = reading on gauge changes by a combination the filter delta-P AND the oil viscosity affecting how fast it vents out of the clearances. You cannot separate one effect from the other without changing ONLY the filter or ONLY the oil, and you CANNOT say "you can't see the filter delta-P at all."
Put another way- I agree that the only way to truly and accurately measure the filter's delta-P is with a gauge before and a gauge after. But in the 4.7 application, the delta-P is a COMPONENT of the total change seen on the pressure gauge when you put in both a new filter and new oil, because the supply pressure (the relief valve) must equal the filter delta-P plus the back-pressure from the bearing clearances (which is what the gauge reads). INcrease the filter delta-P, and the gauge pressure will drop- just do the math. The REASON it drops is that higher filter delta-P causes a slight decrease in total flow. Its exactly like calculating the voltage between two resistors in series in an electrical circuit. Increase the resistor nearer the voltage source, and the voltage between the two resistors decreases because the current (analogous to oil flow) decreases slightly.
The key point is that we're not talking 50's-DNA Chevy or AMC that runs below relief pressure all the time, we're talking an essentially constant PRESSURE supply (above fast idle, anyway) in a modern Mopar 4.7. This is why the only meaningful comparison is hot oil at operating RPM so that the pressure relief valve is open- and even then you should really look at exactly the same RPM since the relief valve isn't a perfect limit on pressure.
