Question about the temperature of oil

Patman

Staff member
Messages
21,989
Location
Oakville, Ontario
I found this question on one of the f-body message boards, and was interested in the answers from you guys.
quote:
Noticed that the oil heats up much slower than the coolant but when the car has been off for an hour or so the coolant is still 160-170 but the oil seems to be cold again, I am assuming that the temp of the oil directly relates to the pressure. Like on a cold start in the morn its about 50 psi and after and hour or 2 its back up there but the coolant is still warm. Dont get it... I always thought if something heats up slowly it cools down slowly as well, was it called specific heat? What am I missing?
 
Messages
47,638
Location
Duvall WA - Pacific NW USA
I would think it's an "ultimate temp", volume/mass and a conduction issue. I too, have noticed this but have not made any scientific measurements, draw cooling curves, etc. Temperature at coolest spot on a fully warmed engine: Coolant = 212°F + under pressure Oil = 220°F + (what kind of car gets oil the hottest?) Volume: Coolant = 10-12+ qts Oil = 4-5 qts Location after shutdown: Coolant = under pressure throughout Oil = drains down to bottom of pan The oil with less volume, and not trapped under prssure near hot metal combustion areas will cool quicker - the coolant especially in a water coolant turbo continues to do it's job via convection and conduction even after shutdown.
 
Messages
334
Location
Detroit (Rock City)
I think a big part of the issue is 'where is the fluid located'? The oil sits in the pan after shutdown, where it's free to cool off independent (mostly) of block temp. OTOH a large amount of coolant is restricted to inside the block, keeping it hot longer, and then too as soon as you start the car and the thermostat opens the hot block begins heating the coolant back up again. Same is true of oil of course, but it heats up more slowly... R
 

MolaKule

Staff member
Messages
21,591
Location
Iowegia - USA
I think what he's referring to is "thermal lag." The heat energy input to a mass of material is governed by the equation: Q = mc(Tf - Ti), where m = mass, c = specific heat, and Tf and Ti are the temps at the final time and beginning time period. And heat always travel from the hotter object to the cooler one.
 

Patman

Staff member
Thread starter
Messages
21,989
Location
Oakville, Ontario
True, it is a lot more accurate, but he drives an LT1 f-body like myself, and I too can basically read my oil temperature from the oil pressure gauge. I can't tell you an exact number mind you, but I can tell when the oil is fully warmed up, since it will read 20psi at idle. When cold, it will read closer to 50psi at idle. So it gives an approximation anyways.
 
Top