VirginiaF1

My question centers on oil vaporization rates / oil burn consumption..
(1988 base n/a 8valve @ 232k)

With my Valvoline VR1 20W-50, the Dino Flash Point is 478F (248C)...
( I assume that is at 14.5psi? )

With current Virginia temps at 100F, what is the cylinder wall temperatures at say an average of 4k rpm?

For those 944er's using oils with lower Flash Points, does that equate to a greater oil burn rate?

Lastly, does 100F summer ambient vs. 25F winter temps really make any difference in these high temp equations?
( ie: running 15W-40 with an even lower FP)

As always, Thanks!
mikey

ron.eddy

Found this that should shed some light. Looks like everywhere the oil would touch is well below the Dino oil flash point.
Book on Google Books

kombatrok

The oil that is getting burned off is what is getting past the rings and burning in the cylinder... nothing to do with how hot the metal that the oil is touching is.

Nothing that the oil touches will actualy cause the oil to flash off and burn.

944Ross

Sure it will. It causes the lighter fractions to vaporize, leaving the heavier residues and probably additives. This may not be "burning" but it is a loss of oil volume (hopefully picked up in the AOS and burned). The underside of the piston crown is a potential place for this, also near the top piston ring. That's where you see carbon buildup.

The choice of oil weight doesn't have to do with flash point, tho, it has to do with film strength at temperatures. You'll waste bearings long before enough oil vaporizes to matter.

That reference Ron linked to appears to show a heavy-duty diesel piston that is air cooled?? Must be a Dietz engine. Surely a worst case situation.

VirginiaF1

I was browsing a Porsche tech articles on Peli and found this line:

"The lower the flash point the greater the tendency for the oil to suffer vaporization loss at high temperatures and to burn off on hot cylinder walls and pistons."

So the oil that STAYS UNDER the rings suffers consistent sludging/carbonization from cylinder wall heat?
( I'm a newb just trying to learn so thanks for all answers.)

944_S_TYPE

just my .02..... A max temp of any liquid cooled engine's operating system should be between 180-210 degrees. As water boils at around 212 degrees f. No?

kombatrok

That makes sense, I hadn't thought about it like that.

A system under pressure boils at a higher temp. So the boiling point for the cooling system is actually much higher.

944Ross

Cooling water temperature is not the same as the metal temps in the engine. If it were, no heat would transfer from metal to water.

DarylJ

With a 15 PSI cap, water doen't boil until almost 250 degrees.

Although even the emission crazy motors don't typically run much above 220 on any kind of regular basis.