srf506

Yeah, I'd hate to be the guy having to read the dipstick while its rolling! ;-) Seriously, the oil in these things is a learned trait, and as a new Porsche owner you'll be learning a few more traits about tire pressures, gearboxes, clutches, brakes, steering, suspension, and whatever. There are a lot of VERY knowledgeable guys on this board. They are also very helpful and patient with anyone who has a question. Do a search on this board, see what pops up, then ask away.

veggiecar300

Thanks again!
So I warmed the car up by letting it idle. Checked the oil (both by meter and by dipstick) and it was definitely high - didn't notice any smoke though.

I got a hand pump and pumped out more than a gallon - maybe a gallon and a half. Checked again and the level was just below the lower marker on the dipstick. So I added 1 quart and now it's between the markers.

BTW here's a picture.

veggiecar300

Thanks again!
So I warmed the car up by letting it idle. Checked the oil (both by meter and by dipstick) and it was definitely high - didn't notice any smoke though.

I got a hand pump and pumped out more than a gallon - maybe a gallon and a half. Checked again and the level was just below the lower marker on the dipstick. So I added 1 quart and now it's between the markers.

BTW here's a picture.

JackOlsen

I don't think the oil expanding is the reason the tank level changes so dramatically. We'd be talking about an expansion rate of 10-15% because of a hundred-degree temperature change -- I think we'd see a lot of burst oil cans and drums if that were the case. I've been told that it has more to do with the viscosity changes of heated oil -- how warmer oil is able to travel more efficiently along the complicated network from the tank, through the engine, and back to the tank.

I'm no engineer, but I'd be surprised if motor oil even expanded 1.0-1.5% when heated by 100 degrees. Maybe someone who knows better than I do could set me straight, though.
veggiecar300 was talking about checking the dash gauge, not the dipstick. As a 1972 owner, however, I can tell you that I could probably reach the dipstick while driving if my windows were down, since it's right behind the passenger door handle.

LaughaC

Yeah, I usually tie my steering wheel so the car idles in a tight circle as I folllow the car around to check the oil.

Just kidding -- DO NOT TRY THIS!! (disclaimer)

Mike Murphy

Believe it or not, I think motor oil is between 4-5%, surprisingly...

jack, I became curious about this myself, mainly because someone on this forum thought that their oil tank was leaking back into the engine through the oil pump. I don't recall who it was, but he ended up taking measurements of his oil in 15-minute increments from 200 degrees after shutting down for 3 or 4 hours until the temp dropped well below 100F.

What he found was that the oil level continued to drop as measured on the dipstick. I became curious, so I posted this on the forum: https://rennlist.com/forums/showthre...rmal+expansion

You must have been out racing that day, Jack, tearing up the track while I was at home on the computer, but I'm sure you had more fun

Larry T

you wrote << I warmed the car up by letting it idle>>
You should always drive the car to warm it up. Allowing it to idle for such a long period is hard on the engine - carbon builds up, etc. and it will take a very long time to get almost 3 gallons of oil warm by idling. When you change the oil, the specs call for replacing 2.64 gallons. And I hope you will continue to use Mobil ! 15W50.

theiceman

scoll down to volumetric correction .....

http://www.preferredutilities.com/engineering_data.html

you can not use that imperical experiment to back calculate the expansion of oil , far too many other factors.

if you are curios here is the real answer....

bourgeois911

According to this formula, murphyslaw is correct. It looks like the expansion rate is between 3.5 and 5% per 100 degrees F depending on the API

Peter Zimmermann

A look at the 911 lubrication schematic is useful. Following start up oil in the crankcase is scavenged, and sent on its way to the full flow oil filter. The cold, thick, oil can't get through the filter and backs up in the lines, all the way back to the crankcase. As the oil warms, more of it can find its way past the filter, and into the reservoir (tank). At this point, if the crankcase was glass you could watch the oil level inside it drop, as the oil level in the tank increases. Of course, oil in the crankcase robs horsepower, so the goal is to get as much of the oil out of the crankcase as possible, which means that it's in the tank, and measurable.

srf506

IMHO the reason we check while the car is warmed up and idling is so that all of the oil system thrmostats and components are open and flowing oil so the system is at its "dynamic" level. Thermal expansion, viscosity, and the volume of oil trapped in various coolers, sumps, and "40 feet" of oil line are all taken into account doing it this way so you don't overfill the tank and it doesn't expand into a huge mess in the vents, airbox and intakes.

"Verrrryyy clever these Germans!"

Mike Murphy

Pete, these are good points. But I was under the impression that the oil pump is a positive-displacement type, meaning that all oil coming in must go out. I thought that any oil that cannot go through the filter gets bypassed via a pressure relief valve. Is this not correct?

Peter Zimmermann

Now you've got me thinking! It's always been my understanding that the 911 "oil pump" is actually two pumps that work in tandem. There is a high pressure side that is responsible for getting oil to all of the lubrication points in the engine - that pressure is controlled by a pressure relief valve and backed up by a safety relief valve. The pressure relief valve, when activated, dumped into the crankcase until sometime in the mid-'70s, after that it dumps into the pressure pump's inlet side without circulating to the tank and back. The other pump is a scavenge (low pressure) pump that picks up oil from the bottom of the crankcase and sends it to the filter, where it is cleaned and returned to the tank. The pressure in the scavenge side is never sufficient to open the pressure relief valve inside the filter, that is only there in case the filter were ever to become clogged, but it's volume is calibrated to be able to keep up with the oil in the crankcase.

Mike Murphy

That sounds right to me, Pete. Good thing the filter has the relief valve, otherwise we could have an exploding oil filter in rare cases.