Originally Posted By: turtlevette
The bypass is open most of the time. Pretty much anything above idle. Otherwise the pressure would go to several hundred psi at redline.
I don't see a lot of mechanical aptitude in your posts. The last time we discussed this I had to remind you the oil pump has a bypass.
Just so we are all on the same page here:
Oil pump rotational speed, and by extension of that, volume, correlates with engine speed. The pump has a pressure controlled relief valve that begins to open when the pressure, observed upstream of the pump, reaches a predefined point.
An engine's oil system will only "consume" a particular volume of oil oil at a given temperature at a given pressure. That is, since viscosity correlates with temperature, back-pressure from the engine has an inverse relationship with that; as oil temperature rises, viscosity drops. Which means that as viscosity decreases, less pressure is created by pumping a given volume of oil through the engine.
On a cold engine with thick oil, equipped with an 80psi pump relief, it may very well hit the relief at 1,500RPM. This does not mean that the relief flies open like a flood gate on Niagara falls though, it simply means that the relief valve opens enough to regulate the pressure to 80psi. On some engines, even with the relief wide open (I've seen this on an SBC with an HV pump and 15w-40 in it) the volume moved by the pump overcomes the ability of the relief to flow it and you see oil pressure well in excess of the relief pressure on the pump.
As the engine warms and viscosity drops, so does the resistance provided by the engine. This means that resistance for a given volume (RPM) decreases. Which is why you see less oil pressure when an engine is warmed.
On a stock pumped SBF with a relief at 65PSI, it wasn't unusual for my car, with 0w-40 in it, to sit at around 60psi on a cold start high idle (1,500RPM) and you could literally watch the pressure drop as the oil warmed. This resulted in about 35psi hot at idle (~185F coolant temps) and you were into the relief by around 4,500.
With 0w-20 in the pan, the same engine was around 50psi on a cold start, 32psi hot at idle, and into the relief by around 5,500RPM. But to be fair, I never really beat on it with the 0w-20 in it, so I'm sure oil temp was never as high as it was with the 0w-40. The 0w-20 was my "winter" oil, so ambient temps were much cooler as well.
I've observed the same thing with SBC's, as well as BBM's and a Ford Y-block (only have one data point for that last one unfortunately). Where it wasn't usual to see somebody sitting on the relief was with the guys using 20w-50 in an SBC or SBF with a High Volume oil pump. This was on an engine with stock clearances. Never understood that one
So I don't agree with your premise that the pump relief is open most of the time. Perhaps on certain engine families that is the case (we've discussed this before on here and IIRC there were certain Mopar engines, running the spec viscosity oil that were on the relief frequently) but at least in my experience, running something in the neighbourhood of the spec viscosity on a warmed up engine, relief events only happen at the top of the RPM scale. So unless you are cruising down the highway at 6,000RPM, the relief is probably closed most of the time, LOL!
Of course that excludes warm-up where the relief will be approached more readily for a given volume of oil due to its increased viscosity and people running oil three grades heavier than spec, like 20w-50 in their F-body
