Oil Capacity - Myth or Reason

[boxermania]

My apologies to the moderators. I posted this thread in the parts for sale forum, please remove from there....sorry


Word of mouth has it to run the oil level in the air cooled engines, 1 qt low.

I have never heard the reason, nor understand, what supports this practice..........nothing changes at the pumps, scavenge and pressure, and will continue to do their jobs until they run out of liquid to pump.

On the other hand, if the scavenge pump looses prime momentarily (due to low level in the engine case) it can potentially aerate the oil (not good) and the air will have to come out at the tank before it is pressurized to supply the engine needs.

Another lesser known fact is that in addition to lubrication the oil also serves as a cooling medium and one less quart will in essence reduce the ability of the oil to remove the heat from the engine components by ~1/12

So, are we dealing with a myth here or is valid reason for running one quart low. Inquiring minds want to know.

[gerry100]

Don't disagree with your points.

Reason I've heard to run a quart low is to prevent oil from sloshing out the breather and making a mess in the air filter.

Only a problem during hard cornering. ie track use.

The quart you give up does reduce cooling capacity somewhat but is not significant when the car is moving consantly, as when on a track.

I don't see a reason to run low when on the street unless you're never in stop and go traffic and drive like a madman.

[Eagledriver]

The quart low has virtually no effect on cooling as the system reaches equilibrium when the engine is warmed up. The amount of oil in the crankcase has nothing to do with how much you put in the tank. The scavenge pump loses it's prime constantly as it drains the case. It has a higher capacity than the pressure pump (so it can stay ahead in volume). It sucks air much of the time. Putting more oil in will not prevent the things you are worried about and these things are not usually a problem anyway.

For track use I recommend running the tank full to prevent oil starvation when the oil in the tank sloshes away from the tank outlet. This is actually a problem and a full tank can help.

-Andy

[Rick V]

I run 1/2qt (or so) low for the reason of expantion. In the spring when temps are low in the morning then warm in the afternoon the level is higher going home from work than going to. In the summertime it is even worse. I live in the country, I do mostly mountian driving, when I get to town I loose airflow to cool the engine. (stop and go traffic). It is easier to run a little low than to clean up the mess, when it expands over the full mark. I don't worry about it. 13.5 qts of M1 will do just as well as 14qts of M1.

[HarryD]

The reason I run my oil level 1-2 quarts from full is that if the oil tank is full and I autocross, I slosh a bunch of oil into my air box. Besides if I have 11 quarts or 9 quarts, I still have plenty.

Now, is 2 quarts low a bad thing?

In my mind, NO, because my air cooled 911 is dry sumped (as is yours). This means that most of the oil is in the tank and the level difference between the full and empty marks does not indicate a lack of oil available to flow to the lubricating pump. In our cars, the lubricating pump suction is always flooded and oil flows to the engine.

On the other hand, our scavenging pump (did you know we have two oil pumps in our car?), moves the oil from the engine sump, through the filter and into the tank. This pump gets occasionally starved as the oil sloshes away from a pick up, but no big deal, the other pump is still moving oil to the engine where it is needed.

Do a search on the oil systems for a 911. There are some great diagrams and explanations.

[DinFtown]

I run about 2 qts low, but I have a front mount and big oil lines. So even running 2 qts low I think I am still running about 16+ quarts of oil... I run low because if not it gets sucked into the intake and gets burned or comes out the drain on the air box on to the motor, runs off the side of the motor and burns off on my headers... either way, running low is much better...

[boxermania]

Interesting experience based replies, however, Eagleracer (himself a racer), appears to disagree with the racing/auto-cross crowd.......