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Oil Temp/Pressure: I need a physics lesson

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50
43
Exp. Type
HPDE
Exp. Level
Under 3 Years
South Carolina
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!
 

TMSBOSS

Spending my pension on car parts and track fees.
7,490
5,169
Exp. Type
HPDE
Exp. Level
10-20 Years
Illinois
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.
 

TMSBOSS

Spending my pension on car parts and track fees.
7,490
5,169
Exp. Type
HPDE
Exp. Level
10-20 Years
Illinois
It has been so long since I took physics that I thought some of those laws would have been repealed. ;)
Or 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. :ohdamn: :cool:
 
306
278
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
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!
You are _really_ over-thinking this.
 
98
113
Exp. Type
Time Attack
Exp. Level
Under 3 Years
So cal
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.
 
50
43
Exp. Type
HPDE
Exp. Level
Under 3 Years
South Carolina
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.
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.
 
306
278
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
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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