This weekend wired and plumbed the sending units for three new gauges: oil temp, water temp, and oil pressure. I was somewhat surprised that the water temperature rose to between 150 and 180 before the oil temperature moved off the peg at 140, but whatever. What really confused me, though, is that the oil pressure in a cold engine is around 70 psi; as the oil temperature rises the pressure drops. Once the oil reaches around 180 the pressure settles at 35 psi, which is where I expected it to be. I recall from 50 years ago that Charles' Law (I know, I know, it's for gasses) holds that the volume of a gas rises with its temperature. Since the oil system is closed, meaning the volume is constant, I would have expected the oil pressure to rise as the temperature rose (Boyle's Law? also for gasses). Will someone explain this to me like I'm three years old? Why is this so counterintuitive? Thanks!
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Oil Temp/Pressure: I need a physics lesson
- Thread starterwdrlaw
- Start date
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PatientZero
@restless_performance
Oil thins as it heats up.
TMSBOSS
Spending my pension on car parts and track fees.
Oil thins with heat. A thinner oil requires less pressure too move through a given space. This drops oil pressure as temps raise. Secondary to this, steel/metals expands with heat increasing some clearances where pressurized oil is present. And yes, you are attempting to apply laws which pertain only to gases, to liquids.
Also the gas laws don't apply here since oil doesn't meaningfully expand as it heats, at least not in relation to how much gases do.
- Thread starter
- #5
It has been so long since I took physics that I thought some of those laws would have been repealed.
TMSBOSS
Spending my pension on car parts and track fees.
Or changed by some politician somewhere. Lol.It has been so long since I took physics that I thought some of those laws would have been repealed.
I must have been lucky to remember this. High School was “A Few Days Ago”. Ford (Not the Car) and Carter come to mind. For some reason.
You are _really_ over-thinking this.
NixonOr changed by some politician somewhere. Lol.
I must have been lucky to remember this. High School was “A Few Days Ago”. Ford (Not the Car) and Carter come to mind. For some reason.
Think of oil pressure the same way you would blood pressure the thinner your blood the less restriction the easier it flows through your body the less pressure. Seems your relating it more the pressure you would experience with a sealed container , example a propane bottle.
How hot did your oil get? i just put a gauge in as well but have not got to the track yet.. driving around town trying to heat it up never got past 175-180F max w me trying to heat it up. Gen 1 coyote, with piston jets, 5w-20. setrab 72 row cooler.
- Thread starter
- #11
My commute is < 2 miles each way, so oil temp has barely been off the peg at 140-150. I hope to drive a little more this weekend and will furnish more info. Mine is a 4.6L 3V, mostly stock, also with 5W-20. I don't have an oil cooler. The pressure sender is mounted remotely and the oil temp sender is screwed into a sandwich plate from GlowShift.
Ride a bike or walk. I bet it's not 10 minutes difference. Honestly, you're not doing the engine any favors by only running it cold. Get the oil hot every week or so to burn off all the condensation and raw fuel you're getting in the oil.
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