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  #1  
Old 06-09-2019, 02:24 PM
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Join Date: Aug 2007
Location: Middle Haddam, CT
Posts: 250
W108/109 4.5 gas mileage

The 4.5 used exclusively for the American market beginning in 1972 is known for its lousy gas mileage.....around 14 mpg on average.
I suppose its a combination of things like lower compression (8:1 vs the preceding 3.5s 9.5:1), less than ideal camshaft and ignition timing, use of a torque converter transmission instead of the more efficient (but rougher) hydramatic style torus coupling and the inherent poor aerodynamics of the W108/109 body design.
But STILL.....
Any tips on how to improve on this? Without spending 20 years worth of potential gas money savings on extensive modifications?
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Berfinroy in CT
Present vehicles:
1973 300 SEL 4.5
1959 Rolls Royce Silver Cloud I
Past vehicles;
1958 Bentley S 1
1976 ex-Max Hoffman 6.9
1970 300SEL 2.8
1958 Jaguar MK IX
1961 Jaguar MK IX
1963 Jaguar E-type factory special roadster
1948 Plymouth woody
1955 Morgan plus 4
1966 Shelby GT350H Mustang
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  #2  
Old 06-09-2019, 02:52 PM
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I decided that a 4.5 was the way to go after experiencing the nearly-as-bad or even-worse mileage of dual zenith 6-cylinder 2.8 m130 cars.

A manual trans from a 3.5 will bolt to a 4.5 if you’d like to try that on for size.

My 1973 450sl r107 best ever mileage was 16 mpg. I think son of it may have to do with the 3.27 (4.5 w108/109) and 3.07 (r107 4.5) rear end final drive ratio.

If you find yourself an overdrive transmission, that too could improve the situation. I doubt you will stumble upon a 6.3 rear end for cheap.




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Past mb: '73 450sl, '81 280slc stick, '71 250, '72 250c, '70 250c, '79 280sl, '73 450sl, parted: '75 240d stick, '69 280s, '73 450slc, '72 450sl,
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  #3  
Old 06-09-2019, 03:32 PM
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Quote:
Originally Posted by berfinroy View Post
The 4.5 used exclusively for the American market beginning in 1972 is known for its lousy gas mileage.....around 14 mpg on average.
I suppose its a combination of things like lower compression (8:1 vs the preceding 3.5s 9.5:1), less than ideal camshaft and ignition timing, use of a torque converter transmission instead of the more efficient (but rougher) hydramatic style torus coupling and the inherent poor aerodynamics of the W108/109 body design.
But STILL.....
Any tips on how to improve on this? Without spending 20 years worth of potential gas money savings on extensive modifications?

If the car is used 12,000+ miles per year, an engine upgrade to 10:1 w/ appropriate valve timing, aftermarket ECU w/ wideband O2 feedback, and tall gears could be a win.
If the car is a hangar queen, upgrades are indeed a loser.

Last edited by Frank Reiner; 06-09-2019 at 07:19 PM.
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  #4  
Old 06-10-2019, 12:13 AM
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I don't know how many on this forum are old enough to remember the cars of the mid 70's-when technology had not caught up to the emissions requirements. Horrible gas mileage and poor performance were the norm. A friend bought a new '75ish Dodge and you could literally see the gas gauge go down. Low compression and retarded cam timing.
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111 280SE 3.5 Coupe
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  #5  
Old 06-10-2019, 10:34 AM
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Tony - I do remember the gas guzzlers from the 70’s. I also remember waiting in line for hours waiting to buy gas. Just prior to those lines I was paying $.025-$.035 cents per gallon for gas and diesel was even cheaper.
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  #6  
Old 06-10-2019, 11:58 AM
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I run 10 degrees BTDC ignition timing or that engine won't make good power. You can also adjust the manifold pressure sensor a bit which can really help. Make sure you have decent fuel pressure - some people have reported better performance with slightly higher than spec pressure.

Make sure that all of your ignition wires are metal core. I use NGK BP5ES spark plugs which seem to warm up quickly and work well in these older engines.
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  #7  
Old 06-11-2019, 08:27 AM
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Benz Dr:
10 degrees BTDC with vacuum retard line disconnected, I presume?
Not familiar with the "manifold pressure sensor" but I have a shop manual so I'll find it. Which way to adjust for the better?
I would think increasing fuel pressure would richen the mixture which would decrease mileage, would it not?
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Berfinroy in CT
Present vehicles:
1973 300 SEL 4.5
1959 Rolls Royce Silver Cloud I
Past vehicles;
1958 Bentley S 1
1976 ex-Max Hoffman 6.9
1970 300SEL 2.8
1958 Jaguar MK IX
1961 Jaguar MK IX
1963 Jaguar E-type factory special roadster
1948 Plymouth woody
1955 Morgan plus 4
1966 Shelby GT350H Mustang
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  #8  
Old 06-11-2019, 09:33 AM
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Personally, I wouldn’t mess with 14 MPG. As I’d said before, 16 mpg with an 3.07 rear end 1973 450sl is as good as it gets.

You aren’t driving this thing across the country, are you?

If you decide to mess with the MPS, I’d do so with a spare, not the original to your car.



If there’s one thing I’d like to enhance on my older MB, it would probably be adding a catalytic converter for a cleaner exhaust smell. The worst thing about the oldest cars I’ve got is smelling like fuel after starting them up. I find that there’s a big difference in the cars that have catalytic converters. This might end up choking the exhaust a bit, maybe decreasing fuel mileage, but increasing the likelihood of using the car for more formal events, like taking the 4.5 to a wedding or on a date night, etc. Who wants to have their clothes smell like an exhaust pipe... ever?


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Past mb: '73 450sl, '81 280slc stick, '71 250, '72 250c, '70 250c, '79 280sl, '73 450sl, parted: '75 240d stick, '69 280s, '73 450slc, '72 450sl,
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  #9  
Old 06-11-2019, 09:56 AM
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I am not sure a catalytic converter would help with your exhaust smell. You might want to have a look at the air/fuel ratio of your intake system. Unfortunately, our vintage Mercedes cars did not have oxygen sensors or computers to manage the fuel system. I do know that catalytic converters emit a rotten egg smell when the air/fuel ratio is too rich.
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  #10  
Old 06-11-2019, 06:50 PM
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Quote:
Originally Posted by berfinroy View Post
Benz Dr:
10 degrees BTDC with vacuum retard line disconnected, I presume?
Not familiar with the "manifold pressure sensor" but I have a shop manual so I'll find it. Which way to adjust for the better?
I would think increasing fuel pressure would richen the mixture which would decrease mileage, would it not?
I always adjust everything the way it runs in the car so in this case all lines connected.
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  #11  
Old 06-12-2019, 07:48 PM
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Location: London, United Kingdom
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A truly different time. I had to chose between a W108 280SE or a W115 240D 4 speed manual. While the former is a grander, arguably better looking vehicle, I wanted to avoid the poor fuel economy and the shrinking spare part availability. The 240D nets 30+ mpg.
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  #12  
Old 06-15-2019, 12:01 AM
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Back then a lot of 4 cylinder cars only got mid 20s mileage, so compared to that the 4.5 is not out of line.
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1972 280SEL 4.5, silver
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  #13  
Old 06-15-2019, 08:22 PM
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I usually got around 13MPG with my 4.5 and I have always had a heavy foot. Worst was 8MPG all-city, best was 15.5 on a trip to Atlantic City.
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  #14  
Old 06-16-2019, 04:25 PM
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Maybe one of the reasons for poor mileage is the lack of vacuum advance. Well, there is one, but it used for an AC idle step-up device, not as something to improve mileage. Maybe if someone connected it directly to engine vacuum it would improve highway mileage. On my Fiats a non-working vacuum advance will drop mileage 2-3 mpg (those cars get 27-30mpg on the highway).
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