Originally Posted by 62civa
Engine 101 student here. Why does it cause the oil temps to rise?
Well Im certainly no expert and am not offering to teach anything, but I will give you my perspective.
Its a bit different than cr74hbs but he may be right.
In my opinion when you partially fill the cooling jacket you obviously decrease the contact area the coolant has with the cyl walls. I agree that the vast majority of the heat is generated and absorbed in the head and the top of the cyl, but the lower cyl wall is also dissipating piston heat through transfer from the piston/ring to the wall. With the jacket filled that part of piston cooling is lost and (in my opinion) causes an increase in piston temps (especially under severe load). Since the inside of the piston as well as the lower skirt are exposed to quite a bit of both directed and splash oil flow there is quite a bit of heat transferred to the oil.
It seems that with proper adjustments to tuning the increased piston temp is capable of producing a bit more power in some cases.
That along with the stiffening of the cyl wall (better ring seal) is probably worth a significant power increase.
My only concern is that at times the filler will shrink, possibly unloading the cyl wall.
Remember this is just my thoughts and probably not close to correct, but I would appreciate your thoughts on this.
I just find this an interesting topic.