Bruce In Philly

2009 C2S 161K miles manual transmission

Just back from a 2,111 mile trip.

So, a question: Should the oil you use or the miles on your car affect gas mileage? If so, how and why?

I am kinda amazed that I am getting such consistent 21.5 MPG in the last 65K miles of ownership (and on different oil). I would think that something would be deteriorating by now and MPG is a usual indicator of an ageing engine no? I am using a tad bit more oil now... maybe 1.5 quarts in 5K miles where I was around 1.25 quarts for same... but that is really hard to tell as my measurements are not all that accurate. I appear to use less oil with trips like this where I have solid tank-after-tank of highway driving.

Notes:
- For 2015 and 2017, I was using Mobil 1 0w-40 as I was from when new to me at 26K miles
- I ran a few tank-fulls of Motul 5W-40, with either Ceratec or MoS2 then switched to what I used on this trip, Liqui Moly Leichtlauf High Tech 5W-40 with Ceratec
- Engine is now due for a plug change at 160K, parts on order (I ordered the blue plugs ~gasp~)
- Car seems to like higher average speeds
- Car computer overestimates MPG
- Lowest MPG was when I was driving around Atlanta
- Why did I stop using Mobil 1, move to Motul, and now Liqui Moly? I am nuts. Seriously, I am unstable.

Comments?

Peace
Bruce in Philly

Iceter

I remember an old episode of Car Talk where Gom and Ray said that the overwhelming percentage of engine wear on a street driven car was during start up. Combining that with what I know about engines, I am not surprised that your high-mileage engine is still doing well. You obviously take care of hour car and have a great deal of mechanical sympathy.

Consider that at highway speed, the engine is at just about perfect operating parameters. Its up to temperature, oil pressure is high and steady and there isn't a great deal of load on the engine. Compression is high and you're in a band of rpms that the engine tune was optimized for. Even a worn out engine will seem healthy under those conditions.

And of course, engines run their best right before they blow.

voiceprint1

Seems pretty on target with the speed and oil use. I like the liqui moly 5W-40, feels smoother running to me, when I used the mobil 0W-40 I feel like I was adding much more oil in between changes. I get pretty bad mileage in the city around 15, agree the car computer seems optimistic.

Wayne Smith

135K on my 2010 C4S. Never has burned any oil. Highway still reports 28 mpg but that is about 5% optimistic. High teens and low 20s when I'm having fun, again 5% optimistic.

Did yours burn oil from your acquisition? Did the oil burn quantity increase with a change of oil type? What was your mpg 100K miles ago? Just curious on comparison since you are data driven, nuts or not!!!

Floyd540

First off, Yes you are nuts. Second off, we are all nuts. Now with that out of the way, here goes. Mileage should stay about the same until there is a fuel injector problem with spray pattern. Ignition issues will be immediately called out by the OBD system as misfires. As to engine wear, over time the internal friction goes down as the moving parts become better friends with each other due to polishing their surface finishes. This improves fuel mileage This is offset with a slight decrease in compression and increase in valve leakage with higher mileage which means slightly reduced power output, thus requiring more fuel input to achieve the same performance. In a healthy mid mileage engine like yours, these items will essentially offset each other. Given that fuel injector spray patterns remain correct (not dirty or leaking) , I doubt you will not notice much difference in fuel mileage for the next 100,000 miles. As to oil consumption, it is essentially a "leak" between surfaces ( you decide which ones) and as parts wear, the "leak" just becomes bigger. We all change brands of oil for no real reason other than we "think" something is better etc. The power of marketing. I am the engineer and I changed oil brands in an engine which had perfect oil history. (Wayne has my report) My advice is still simple "use a good oil, change it at proper intevals (say 5,000 miles) and be happy."
Glad you had a good and safe trip. Others will provide accurate information to you questions. 9A1 is a great engine.

Bruce In Philly

Actually, when I purchased the car with 26K miles, it burned more oil than today. I was a bit concerned (being nuts and all)... then it slowly used less and less so I believe that break in was a long process for this car. I posted this back in '04: https://rennlist.com/forums/997-foru...-and-less.html

I don't know what to make of all this... to be sure, I don't think my car uses more oil than it did by about 1/2 quart every 5-7K miles.... but it is so hard to know as my measurement protocol is pretty weak... like Sharpie lines on the sides of oil bottles. I can say the change over time has not been very significant if at all really.

I switched from a life of Mobil 1 to Motul about 20K miles ago. I put in Ceratek and then MoS2 IIRC. After exchanging emails with Motul, I was convinced I would use less oil (not that I was worried, but it was touted as a benefit). Well..... after two 5K fills, I am 99% sure I used more. Again, hard to quantify, but maybe I was using about 1/4 to 1/2 quart more per 5K miles. So then I switched to Liqui Moly and the usage returned to what it was using Mobil 1. I guess I use around 1.25 quart per 5K miles. But I am not really sure. This is more than back when I had 50K on the car but not much.

I have come to terms that my oil level indicator can be flaky.... especially if the real level is about at the hash mark transition. I have had some very odd readings immediately after an oil change and have learned to trust my rough measurement of what comes out goes in followed by a few cycles of checking.

I wish I could sound more sure of myself.... but so it goes with a nut case.

Peace
Bruce in Philly

cringely

I've calculated the difference between the car computer and actual mileage on both my previous 996 (IMS—aargh) and present 997.2—MT. The offset (computer reading high) on the 997 is about 1.63 MPG. The 8 data points that Bruce in PA (green triangles) showed indicates an offset of about 0.79 MPG high.
This indicates that the 997 mileage computer offsets is not identical between 997's. If anyone want to send me tables (private messages work best) of indicatef vs. calculated mileage, I'd be happy to add their data and calculate how much the mileage offsets are.


My other comment is that Bruce's MPG (fill sequences 5 & 6) seems low for an average speed of 66 mph. I assume that those were not at a constant 66 mpg, but alternating between near 80 mph and much lower speeds (as my 997.2 base Carrera gets an actual 31.8 mpg at a steady 65 mph.

Bruce In Philly

Max cool. I think we should have some sort of competition for who is more nutty.

Your mileage is higher because you have PDK. I have a manual transmission. MTs are just way lower... I think because we are less efficient.. we shift later. No? I also like using my shifter... I down-shift on the interstates quite a bit wanting torque if I need it.... just my habits. I have a supposition... that PDK owners keep it in auto way more than they use the levers thus being very efficient with gas mileage.

Peace
Bruce in Philly

Wayne Smith

Regarding mpg ... My commute was 25 miles one way. I would get single digit mileage for the first 5 miles as the motor warmed up (which actually required 10 miles). I would generally be between 26 and 28 mpg by the time I reached work. This was mostly freeway driving against the commute at speeds from 65 to 95. In other words, mpg can change drastically if your trips are short.

ManoTexas

Bruce, I admire your **** approach lol. But you now scare me. Had oil changed by great indy once/yr, approx 5-7k mile due to time constraints last 4 yrs. No clue what Oil they used, I could look at receipts but too lazy. Now with no full time travel due to CV decided to do all my own maintenance, and changed oil using Motul kit from FCPEURO. To date and 26k miles never burned a drop of oil (please lord don’t jinx me). Now I’ll live paranoid worrying I’ll burn oil using Motul.
I also admire people who get Blackstone oil analysis done, but on flip side figure it either says your engine about to blow, sell NOW to a sucker, or is added $$ to whomever buys car. Since never selling I’ll skip the analysis. Just change oil yearly and enjoy driving. Had many cars that went many, many miles with yearly oil even at 15k, and think we’re way over the top worrying about 5k oil changes lol.
But your analytics are amazing!

OKB

my 09 4s PDK was consistent at 24.5 per gal on the interstate

Petza914

Mano, the real purpose of UOA is to catch things before they go wrong and address them. For example, in my case on my 997, my Zinc and Phosphorous numbers were reading a little low for the type of oil I was running st the change interval I was using, so something in the motor was causing the anti-wear additive package to break down a little sooner than it should. Exploration revealed a leak in the AOS plumbing that was causing a slightly lean condition (fuel trim numbers below 1.0000) which makes the engine run hotter than it should and the spark plug electrode wear and tops of the piston crowns support this finding. Also found some bore scuffing in #6 - it's so light it can't be called scoring yet.

So, I've fixed the intake leak, am going to shorten my oil change interval from 4,000 miles to 3,000, run a slightly cooler spark plug, and hopefully, prevent having to rebuild this engine for a few years yet since it only sees about 5,000 mikes/year. Not discovering any of this from the initial UOA results, I would likely have more damage inside the motor and much worse scoring.

ManoTexas

Good info Petza. Having .2 I’ll just drive like crazy, change oil yearly or 5-7k and take my chances and not worry about it. But do admire people into oil analysis.

Floyd540

From a purely engineering standpoint , a great technical summary is given above in reply #13 by ManoTexas. "Drive em, change the oil and be happy".

JustinCase

^This. It is also better to study the numbers yourself (with practice, and maybe a little guidance from a pro) and rationally do your own analysis. I agree that the analysis you get from the UOA company may often be a little too optimistic, but the numbers are the numbers, and the trends are the trends, which can alert us to problems maybe in time to perhaps do something about them. One UOA is good, a consistent time series of reports is much better. Highly experienced experts like Charles Navarro or Jake Raby are very good at picking up issues from a combination of subtle changes.