Real-World 617.950 Fuel Economy - 1,000 Miles a Week:

[Doktor Bert]

I have been driving this 1978 300SD for almost 20 years now. I think I drive it harder, on a constant basis, than most of you do. Forum member 'Hammerwerfer' has ridden in the car and will attest to it's incredible acceleration and how hard I push the car on a regular basis.

The 300SD has evolved to some extent, after the rebuild/upgrade of the engine and transmission back in 2004, but it is, for the most part, a very basic engine with nothing more than increased boost pressure (14.7 psi @ WOT and roughly 10-11 psi @ Cruise) increased primary fuel pressure, upgraded compressor wheel, ported/polished head, 3.0mm of shims under the ALDA (Atmosphärische Lade-Druck Anpassun (for MBZ trivia fans) Injectors set to max discharge pressures and advanced camshaft and IP timing. I am still running the direct-drive 722.120 transmission, the original 3.07 (awful) axle and the factory optional 205/70R14 tires.

I have several videos posted that show the boost gauge going to 10 psi at about 2,000 rpm with only about 1/2 throttle. I have several You Tube videos that show the cars 0-60 and 60-105 mph acceleration, as well as the tire-screeching 1-2 shift.

For 11 years, I commuted 110 miles a day from my home to the police department where I worked. I drove the 116 frequently, because it would net between 22 and 28 mpg, depending on how it was driven and whether or not you were running the A/C. After I retired last year, I took the 116 with us to So Cal.

Although I live in Palm Springs during the week, the past two weeks have seen me home every night at our home in the High Desert, a 214 mile daily commute. To me, the drive isn't at all bad, so I really don't mind it. I guess after my recent trip to Florida and back, nothing seems that far to me anymore!

Over the past 2 weeks, I have been watching my fuel economy really closely. I have discovered a number of things about the old 116 I thought I would share with all of you.

In all the years I drove the car, the low fuel light (a red triangle on the 116) never came on. I thought it was broken. In every instance, when the fuel gauge read empty, I could fill the tank with 16 gallons of fuel. Recently, I ran the car much lower on fuel than I had ever done before and the red triangle lit up. I set my trip meter and drove the car until it ran out of fuel.

What I discovered was the car would go 110-115 miles from the time the low fuel light came on until it literally started running out of fuel. Filling the car from this dead-empty condition took a full 21 gallons, which means there is 5 gallons left when the red light comes on.

My morning commute begins about 4:15am PST and my speeds range between 70-80 on I-215 all the way to the I-15/I-10 interchange. Once I get past San Bernardino, I frequently run between 100-115 mph, when my side of the freeway has minimal traffic.

Between last week and this week, with a little over 2,000 miles driven, and running the A/C on every return trip (no A/C in the morning) the 116 has yielded an average of 25.75 mpg, with the best numbers near 30 mpg (28.7) and the worst near 23 mpg. Ironically, being stuck in stop-n-go traffic hurts my economy more than higher speeds do!!!

I am going about 525 miles on a full (20 gallon) tank, but I prefer to fill up at the 'E' which takes about 16 gallons, which gives me about 425 miles to a tank of fuel.

As far as maintenance goes, I am a true minimalist. The first secondary fuel filter went 45,000 miles before I changed it. I usually change the primary filter at every oil change, but I don't change oil very regularly either.

Ideally, I like to change the oil at 5,000 to 10,000 mile intervals. I have used a lot of different petroleum based oils, to include Chevron Delo 400 15/40, Mobil's Delvac 15/40, Shell's Rotella 15/40 and, most recently, Valvoline's 15/40 Diesel Engine Oil. I get the most consistent oil pressures with the Valvoline.

I have run the engine about 15,000 on one oil change, partly due to a maniacal schedule and sheer laziness, although I did throw in a new OX-78D filter about mid-way through the cycle. I try to adjust valves at about the 15,000 to 20,000 mile mark, although the last 2 times they were still within the .006" and .016" setting where I like to keep them.

I serviced the 722.120 after about 50,000 miles, and I only. ever use Ford 'F' Type fluid in these units. I feel the more aggressive frictional properties in the Ford (MC233-F specification, IIRC) fluid makes the bands and clutches last much, much longer. I have been using F-Type in customer rebuilds ever since the early 1990's. There was nothing at all in the pan and the fluid still smelled good at 50,000 miles of use.

I'm not starting a debate on the merits of Dexron vs. F-Type, I am just telling you what I do on this car.

I have used all kinds of things in the fuel, from 2-stroke premix, to MMO, to ATF, Cetane Improver, Lucas Upper Cylinder Lubricant and even 6 months of running on 100% VVO. The engine runs quietest and seems to make the most power on VVO. As far as additives go, it has been tough to really feel a difference, although I can see the presence of the additives in the clear primary filter. I sometimes throw in some 2-stroke premix just to put a little lubricant in the system.

I have been, for quite some time now, planning a 2.47 rear axle swap, but I cannot take the car off the road long enough to work on it. My wife has taken a liking to my 2001 Supercharged Mustang GT, so I find myself driving the 116 again daily.

I have to say I miss being able to wring 155 mph out of a car every now and then. Interestingly, the Supercharged 4.6 Litre V8, running 7lbs of boost, laid down 27-28 mpg numbers all the way to Florida and back running the A/C 24/7!!!

I would like to be driving a more modern car, perhaps a mid-1990's 300TE, but having a kid in private school and saving up to buy a new house all mean keeping expenses at a minimum, so I keep on driving the ancient W116.

I'm not sure if these ramblings will help anyone, but perhaps the results of my fuel economy over the past two weeks may help to establish a real-world baseline for what the average 617.950 will churn out.

Regards,

Robert