13 miles to the gallon

[RDM]

All,

I have a 1980 911SC that came to me at the beginning of October. When it arrived, it was almost undrivable, due partly to it sitting for four months in shipping lots and on a big boat. As time has gone on, I am sorting through what I have and beginning to understand it (with a great deal of assistance from this list).

First stop was the mechanic, who checked fuel pressures, replaced the WUR, and pointed me to a replacement CDI box (bought on this list, thank you).

The alternator went to pieces, and while the car was stationary, I checked for induction air leaks using the positive pressure method, and I didn't find any. Related- when I remove the oil filter cap, I get no change in idle, which is supposed to be a sign of air leaks. I also got the chain tensioner update done successfully).

With the new alternator in yesterday, I mistakenly set the timing well advanced, and was very surprised to find performance scintillating rather than grudging. So today I looked at the distributor advance, with the help of SC Distributor Curves - Little Long

With vac lines closed off,
5° at ~950
15° at 2000
18° at 3000, 4000, and 5000.
which is within spec of my Bentley (21° +/- 3) but the advance curve doesn't seem quite right. In fact, it's not even a curve!. So I suspect I have distributor disassembly and cleaning in my future. Distributor service (Clean and lube) real easy without removing the pinion gear!

But is 13 MPG what I should expect? I've got 5 tankfuls of Standard Oil's (S.O., or Esso here) best through my tank, and it hasn't gotten better. Ignition components (cap, rotor, plugs) are near new. Is the advance curve enough to account for this? Where do I look next?

[matt f]

That sounds about right.

[redstrosekNic]

If you drive your 911 like I drive mine, that sounds close.

[RDM]

Driving is mostly village to village, which means 30 MPH zones, 42 MPH zones, and 62MPH zones, with some traffic. So lots of accelerating/decelerating, but not extreme speed.

[redstrosekNic]

Originally Posted by RDM

Driving is mostly village to village, which means 30 MPH zones, 42 MPH zones, and 62MPH zones, with some traffic. So lots of accelerating/decelerating, but not extreme speed.

It's the accelerating and decelerating that'll get you. High speeds, not so much.

For city-like driving, 13mpg sounds normal...

[ClickClickBoom]

18deg total advance?
35 deg at 5000RPMs is what I set my car to. I regularly got 24MPG at "drive it like its stolen speeds" Experiment with a different advance number, Jerry Woods suggested 32 degrees as a starting point, dependent upon octane available/used. I tried as much as 38 degrees, but the engine was much happier at 32. It is called tuning, every engine is slightly different and will respond accordingly, but 18 degrees total advance is not enough. At 25 degrees my car felt like it was missing 2 cylinders.

[SLR]

Nop! that is not good mpg. I have a 1985 911 carrera in exalent condition so think it is a good guide line. I am now living in Germany and drove from Dorset UK to Nurenberg about 1130 km and I got around 8.7k per l, but the car was heavy and all though the roads are very good I was sitting on a speed of 90/130 mph and at one point 163 mph! (stupid) and a huge difference in full usage,
but normally about 25 to 28 mpg.

[Vereeken]

That is a lot even when no freeway in the mix.

Perhaps only small distances only on a cold engine?

Still high. My SC does not have that fuel consumption.

[yelcab1]

For an 80SC, at city driving, and if you lay your foot into it like I always did, 13mpg is right. Not a good comparison to that of an 84-86 carrera because of totally different injection system.

[OsoMoore]

I drive my 79 SC back and forth 4 miles to work in 30-45 mph roads with traffic lights. A few runs out in the country at 50-60 each week. I average 19-20 mpg. Driving all-day highway cross country I got 23-26.

Your mileage sounds really low, I suspect something is definitely wrong.

[Vereeken]

Originally Posted by OsoMoore

I drive my 79 SC back and forth 4 miles to work in 30-45 mph roads with traffic lights. A few runs out in the country at 50-60 each week. I average 19-20 mpg. Driving all-day highway cross country I got 23-26.

Your mileage sounds really low, I suspect something is definitely wrong.

That is also what I would see on my 82 EURO SC.

[mikesride]

Your getting almost 1/2 what I am getting on a CIS 82 SC.

[ClickClickBoom]

13 MPG is carb territory....

[RDM]

Originally Posted by ClickClickBoom

18deg total advance?
35 deg at 5000RPMs is what I set my car to. I regularly got 24MPG at "drive it like its stolen speeds" Experiment with a different advance number, Jerry Woods suggested 32 degrees as a starting point, dependent upon octane available/used. I tried as much as 38 degrees, but the engine was much happier at 32. It is called tuning, every engine is slightly different and will respond accordingly, but 18 degrees total advance is not enough. At 25 degrees my car felt like it was missing 2 cylinders.

That's 13 degrees centrifugal advance plus 5 degrees baked in plus vacuum advance (~10 degrees at no load) for a total of 28 at no load. But the centrifugal advance isn't as progressive as I would expect.

Originally Posted by OsoMoore

I drive my 79 SC back and forth 4 miles to work in 30-45 mph roads with traffic lights. A few runs out in the country at 50-60 each week. I average 19-20 mpg. Driving all-day highway cross country I got 23-26.

Your mileage sounds really low, I suspect something is definitely wrong.

Originally Posted by Vereeken

That is also what I would see on my 82 EURO SC.

Originally Posted by mikesride

Your getting almost 1/2 what I am getting on a CIS 82 SC.

So what's the difference between an 18 L/100km (13mpg) and 10 L/100km (22mpg) car? Some get 13mpg, but others do substantially better. I don't think this is all driving style. Where do I turn my attention?

[timmy2]

Hook up an exhaust analyzer and see what you are running for a CO output.
I'll bet it's fairly high in the 5 ppm range. Lean it out to 3.5 and your mileage should improve.

[maui44]

NO way, 13 MPG is what you should be getting. City driving more like 18-20.
+1timmy2, Something is wrong in your mix? Timing does not sound right.

[easelboy1]

I've had it both ways - 18-20 city and 13 city. It's all about the throttle. But that is a subjective thing - like Timmy2 I would look at CO output.