My friend inherited a 1985 500SEL. He likes it an would like to use it daily. However, he told me he's unhappy with the fuel consumption. He asked me whether there are alternative engines to substitute the 5.0L M117.

I often found people swapped the original engine of W126 to M104 together with a higher-ratio differential. So, is it a good idea to replace the M117 with M104 with a shorter differential? I believe the original 500SEL's diff ratio should be around 2.47 (Euro model). With M104, the 3.46 diff from M110 or M103 should match. Am I correct?

Or is there any better option? Please advice. Also, my friend doesn't often go beyond 75MPH.

Are you sure it's not a 380/560 SEL or a later model year.

The bottom line is that all S-class models are heavy, so they don't get great fuel mileage. Also, given the age it will probably cost more to buy an engine and pay for the labor to swap and integrate it in than the car is worth, and the fuel economy won't be much better, if at all because the smaller engine will have to work a lot harder. Also, If you live in an area that requires emission testing you're into a black hole.

The short wheelbase W126 S-class was available with the 3-liter M103 with a 3.46 axle and a four speed automatic. It was a good "town car", but most didn't like the 4000 or so revs at 70 even though that engine is butter smooth.

Some years ago a 300 SE owner swapped in a taller axle ratio and had to spend a lot of time in third gear, even in highway driving. He discussed it here. It was a total dog.

IMO your friend should choose to accept what he thinks is too high fuel consumption or sell the car and get something lighter that gets better fuel mileage.

I don't think most people understand that fuel consumption is more a function of vehicle weight than engine size. That's one reason a 495 HP Corvette can knock down about 30 MPG
at legal freeway speeds. A low Cd and frontal area and very tall top gear also contribute, but that 6.2 liter pushrod V-8 has stump pulling low end torque right off idle, so it can easily pull a tall gear in a relatively light car.

Duke

X2, well said.

My $0.02 USD,...either accept what it is or get a W126 300SD with an OM 617, preferably an '85.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

1) A friend should not let "My friend" consider an impractical swap.

2) As noted above, emission inspections will dictate a great deal.

3) What is the mechanical competence level of the "friend"?

4) The least labor swap will be to install an M116-4.2. The 4.2 is a direct bolt-in (including the trans). The camshafts can be re-keyed to a more advanced position. The Gen I fuel and ignition systems will be retained.

The 500SEL has similar HP as the US 560SEL, but runs on regular gas. That's your savings...

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

One other thing to consider. Using a 39 year old car as a DD comes with the insurance risk of being totaled for a minor fender bender. As a DD, your friend will need conventional insurance rather than stated value or collector car insurance. As such, keeping the total cost of ownership down is the most important thing. Food for thought.

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

if it's a euro 500sel, then mileage will be an issue and any swap will be expensive. the euros are not tuned for emissions and tend to be thirsty.

the u.s. got a 500sel in 84-85. 10 years ago, had 1 that a customer bought for commuting around orlando. sold it after 6 months because the mileage was approx 16 mpg/hgwy. not a good daily driver today.

it needs to be inspected for timing chain wear and other issues any 40 yo car would have.