vandal968

Rosie's on a road trip this week, went from Phoenix to LA, her longest trip so far with our family. While doing the usual prep of checking tire pressure, topping off fluids, etc. a few questions came up.

1. Porsche says to check the oil level with engine warm. I know that the distance between min/max marks on the dipstick is about 1.5qts, how much of a difference in reading have you guys noticed between warm and cold?

2. I usually drive lots of short trips and average about 14.3mpg. On extended trips, I've gotten 18+ mpg, I normally fill with Shell super. On this trip, I filled with Shell before leaving, reset the mileage meter and was getting between 16.5 and 18.2 depending on if it was up or down hill and I was averaging around 75mph, occasionally up to 93 or so for the first 150 miles. I made a stop at the Mobil station in Quartzsite (last AZ gas before the $$$$$ California gas). She took 10.5 gallons. After coming back onto the highway (same road, direction, speed, driver, etc.), my mileage slowly ticked down one-tenth at a time and eventually settled at 14.4 by the time I got to my destination (200 additional miles). There was a bit more wind in the second half of the trip, but that died off after a while. Is Mobil gas sh#te? Did the gas station in Quartzite dispense "regular" out of their super pump? I was surprised that the gas was only $0.18/gal more than I pay for super at home since roadside stations normally gouge badly. Obviously, I'll see what happens on the return trip in a few days (filled with Shell), but I was curious if anyone's got any thoughts about this. Car ran the same, just mileage dropped like a rock immediately after the Mobil top-off.

cheers,
c

djantlive

vandal968

Ok, like djantlive, I wasn't expecting much of a difference in cold vs. hot, but I just did some science on it and the difference is significant. I checked the dipstick cold (63F outside, overnight temp of 51F) and the oil was at the bottom edge of the ejector pin dot between MIN & MAX (top photo). Then I drove around for 20 mins, no high speed (50 max), including 10 minutes at the In-N-Out drive thru. Got home, ate a double-double and a shake, probably 5-10mins elapsed, then checked the oil level again. Oil level moved up to the middle of the "A" in MAX (bottom photo, took the reading twice and wiped the dipstick clean each time). I'd say that it's reading almost 1/2qt higher when hot than cold. That's pretty significant. Here are some (admittedly poor) pictures. It appears that if you fill your car to MAX when cold you will be 1/2qt overfilled when hot, and it may be even more pronounced when topped-up on a colder day since it's quite mild here. Porsche says to check when hot and I now believe them. Based on this, if I ever top-up when cold, I won't take it above the midpoint.

RE: Costco gas. When I lived in NorCal, Costco Super would make my 968 knock just off idle. I have never experienced knocking in that car with anyone else's super gas (been daily-driving it for 15yrs, I know it well) and I tried it many times over the years in-case it was a fluke, or just the winter or summer formula, etc . A friend of mine had the same experience with costco super in his ML500 (also in NorCal). The 968 has very high compression for a non-DI 25yo car (11:1). I am forced to conclude that either Costco 91 octane is less than 91, or that everyone else's 91 is actually greater than 91. I am making this claim ONLY for gas sold in California where they mandate a special formula, Costco's in other states may have better gas.
,

Dilberto

That dipstick "dot" between MIN and MAX is where all cold oil should be filled to. Thermal expansion will raise oil level to MAX... which is exactly a half liter, or 0.52 quarts. Any more and you're wasting oil....