Quote:
Originally Posted by RPM90
That's what I've been saying, except, I disagree that the oil temp is the better indicator of that. The parts have reached the engine operating temp by the time the thermostat opens, that's the reason why it stays closed until that time, so that engine temp can be reached faster.
By the time the oil absorbs enough heat to get it to this magical 240F, the engine is well into being warmed.
The engine/coolant temp is the better indicator, but we don't have one, and that sucks.
|
The issue with using water temperature as a guide is that it only indicates how hot the top of the engine is, virtually all the heat is created around the combustion chamber. If you have ever had a water pump fail, you will know that the coolant in the cylinder head can boil in a matter of seconds, without circulation.
With cars I've had in the past with full instrumentation, the water temperature will reach normal temperature about twice as fast as the oil. At this point the top end of the engine will be pretty close to thermal equilibrium, but until the oil is up to normal operating temperature, it will be cooling the big-end bearings and preventing them from reaching equilibrium.
If the big-end of aluminium connecting rods are not at the normal operating temperature of about 100C, but at about 60C (likely the approximate bearing temperature when the coolant has just reached 100C, as no coolant reaches the bearings, they are oil cooled), then they will be running tighter than at thermal equilibrium.
Aluminium has a thermal expansion coefficient of about 22x10^-6/C, steel has a coefficient of thermal expansion of about 12x10^-6/C. This means that a 50mm diameter journal bearing will be 0.02mm tighter at 60C than at 100C. Engines with big end bearings of 50mm diameter require a bearing clearance of about 0.06mm when the engine is built at room temperature (nominally 20C), so at 60C the clearance will be 0.08mm and at 100C 0.10mm. With the oil not circulating as well at 60C compared with 100C (of course this can be seen by the oil pressure, which is directly linked to oil temperature at a given pump speed, ie engine speed for mechanical pumps), the flow rate will be lower and the pressure higher due to both oil viscocity and the tighter space to squeeze through in the bearings.
I will not run the engines of my cars at high RPMs until thermal equilibrium has been reached and the oil is lubricating the big ends at the optimum bearing clearance.