Fuel economy questions - PeachParts Mercedes-Benz Forum

Happycal · #1 02-07-2021, 03:47 PM

I have a 1991 300D that has a bad motor. I traded the 190d that I bought on this form for a 1995 E300 Diesel. That car was hit in the rear and I want to transplant that motor into my 1991. I'm not looking to do any rocket ship improvements. My main goal is safety and fuel economy. Hopping in the car I'm driving 300,400,500, 600 miles 24 hours is nothing to me . I don't do it all the time but I do it enough . I'm tired of asking what kind of mileage someone gets and they say I don't know I just put the nozzle in and go. I don't have that luxury. I think I had more money in my pocket when I was 16 years old then I do now.

When I do the transplant, the motor and transmission will be out of the car. The 606 will go into the 1991. It is a non-turbo. I believe that 1991 and 1995 had a 4-speed non lockup non overdrive transmission. I have several Transmissions in the shop. If I find a 5-speed Auto, what part number should I be looking for? Driving a stick is fun I had a blast driving that 190d that I bought, but I want an automatic with overdrive and lock up converter.

I have not had either 91 or 95 up in the air yet so I don't know the rear end ratio is. I have a 2.65 and a 2.82 rear end now, both limited slip I will beleive.

What is the sweet spot as far as RPMs go with the 606 for the best economy? I guess that will determine what transmission and rear end I will choose. I gladly welcome any input and feedback.

Felching · #2 02-07-2021, 05:11 PM

Both the nonturbo 606 and turbo 602 have nearly the same power and torque. Pushing a vehicle through the air is 90% of your economy on the highway, changing the engine won't change how much power it takes to propel the vehicle. The 606 is more thermally and volumetricly efficient but loses some mechanical advantage to friction driving that second cam, 14 extra valves and an extra cylinder.

Mercedes' diesels generally don't respond much efficiency-wise to RPM reduction. For example, when I swapped from 3.07 to 2.88 and no other changes, my economy averaged exactly the same at 26-28mpg as it has for years past.

The problem with the 5-speeds is they require an electronic controller for that 5th gear and a locking converter is only in the 722.6, not in the 5-speed 722.5 or any 4-speed 722.3/4.

IMO, the best route for a non-turbo 606 is to stick with the 722.4 in the 300D already and swap in a 2.65 diff. Being non-turbo, torque won't be an issue for that "weak" transmission.

Happycal · #3 02-07-2021, 05:54 PM

Quote:

Originally Posted by Felching

Both the nonturbo 606 and turbo 602 have nearly the same power and torque. Pushing a vehicle through the air is 90% of your economy on the highway, changing the engine won't change how much power it takes to propel the vehicle. The 606 is more thermally and volumetricly efficient but loses some mechanical advantage to friction driving that second cam, 14 extra valves and an extra cylinder.

Mercedes' diesels generally don't respond much efficiency-wise to RPM reduction. For example, when I swapped from 3.07 to 2.88 and no other changes, my economy averaged exactly the same at 26-28mpg as it has for years past.

The problem with the 5-speeds is they require an electronic controller for that 5th gear and a locking converter is only in the 722.6, not in the 5-speed 722.5 or any 4-speed 722.3/4.

IMO, the best route for a non-turbo 606 is to stick with the 722.4 in the 300D already and swap in a 2.65 diff. Being non-turbo, torque won't be an issue for that "weak" transmission.

Does the 722.4 have lock up? Is it a difficult to lockup the 722.6? So you were saying that the RPMs don't matter with the Mercedes diesels? Because every other diesel I've had is definitely related to RPMs.

Felching · #4 02-07-2021, 06:19 PM

No Mercedes 4-speed has lockup. As long as RPMs are between 2500-3500rpm on the highway, economy doesn't change in a meaningful amount. These are not truck engines, they are most efficient in that RPM range.
To control a 722.6 you would need this https://ofgear.dk/

Happycal · #5 02-07-2021, 06:49 PM

Thank you for the link. That works out to about $1,000.

h3ffe · #6 02-07-2021, 10:06 PM

For more data points: my experience with Mercedes diesels has been different. Lower RPM has netted me better fuel economy with every vehicle. 616, 617, 617a, 602, 602a, 603, 603a, 606, 606a.

Happycal · #7 02-07-2021, 11:21 PM

All the Diesels I've had seem to like 2000 RPM. Except for my concrete truck, which was a 12.9 l diesel. That was 15 years ago and I don't remember the specs exactly but Peak torque was it about 11 to 1200 RPM.

BirthdayBenz · #8 02-08-2021, 06:47 AM

Dieselpumpuk makes a transmission adapter for the 606 that allows you to bolt it up to a BMW 5 or 6 speed manual transmission.

tjts1 · #9 02-08-2021, 08:58 AM

Quote:

Originally Posted by Happycal

What is the sweet spot as far as RPMs go with the 606 for the best economy? I guess that will determine what transmission and rear end I will choose. I gladly welcome any input and feedback.

I don't know but I can tell you my 97 om606 NA 722.6 factory diff is 3.47 and it's noisy AF so try to go taller than that.
https://i.imgur.com/CjpIGYg.jpg

t walgamuth · #10 02-08-2021, 10:13 AM

Quote:

Originally Posted by h3ffe

For more data points: my experience with Mercedes diesels has been different. Lower RPM has netted me better fuel economy with every vehicle. 616, 617, 617a, 602, 602a, 603, 603a, 606, 606a.

agreed. Putting in a taller diff will always help economy in my experience and I have done so on at least 6 vehicles. Gas engines do not correspond to change as consistently. If you put in a od where the factory equipment did not have it you'll see about 18% improvement in consumption. Same if you put in a taller diff.

I would expect you can get into the low forties with speeds of 70 or less.

martureo · #11 02-08-2021, 10:26 AM

Just another FYI, the 606na is going to be a bit of a dog around town. I had a 95 E300 and the thing would cruise on a highway, effortlessly pulling along. But from a stop it felt a bit sluggish and that engine wanted to rev a bit.

So depending on where you're driving, the turbo might be a better option. If you're doing plenty of highway miles then the 606na is a really good choice.

t walgamuth · #12 02-08-2021, 10:58 AM

None of my turbos got off the line any better than their na siblings. The reason the cars get off the line poorly is that mb does not value that ability like many Americans do.

Happycal · #13 02-08-2021, 04:13 PM

I was looking at the diesel pump UK website. Will it be heresy if I put a Chevy transmission with overdrive and lock up with their adapter kit?

pawoSD · #14 02-08-2021, 04:29 PM

I'd say if your going to that extent to save fuel it probably wasn't really about saving fuel.

The most economical thing would be to make sure its running as best as it can be in stock form....and drive economically. Any change you make that costs $$ is sending you into the other direction....and would take many miles of significant fuel economy improvement to recoup.

If you want higher economy look at the later cars like the W210 606Turbo or W211 OM648.....the newer you go the better the economy.

tjts1 · #15 02-08-2021, 05:10 PM

Quote:

Originally Posted by t walgamuth

None of my turbos got off the line any better than their na siblings. The cars get off the line poorly is that mb does not value that ability like many Americans do.

My OM603 turbo jumps off the line a lot better than my OM606 NA. Might be due to the fact that the 603 is living the no ALDA lifestyle or that my OM606 is choked by the drive by wire throttle. I have a spare 603 injection pump that will be going into the 97 606 NA soon. The 606 head flows a lot more air that the 603 and but it's fuel limited by the IP.