rpm vs efficiency vs displacement for MPG
 

so I am looking to build a high mpg mercedes. I will sacrifice local road performance (slooooow) and no power for good highway milage since my drive to work is 30 miles highway, 2 miles suface roads each way. I have been trying to think what would giive me the best milage. I am thinking a highway gear or a retrofit 5 speed for od. I am also thinking of swapping the 300D turbo for a 240D NA. I have a few questions. what is the normal milage for a 240D, is it better than a 300D turbo, if so how much improvement could I expect. what is the most efficient highway rpm for the 300D/240D engine? Can i run below that rpm sacrificing some efficiency but netting an increase in milage. does anyone with a 240D 4spd think it will be able to pull its own weight at 60mph with a 5 speed OD trans?

you should also think of some aeromods. I need to figure out the CD and all that jazz of the w123 sedan. I want to do some mods, reversible and non body harming, to my w123.

So far I want to remove the external mirrors and install interior convex mirrors as well as fab up some fender skirts


The front spoiler you have might help us ;)

the 2.88 gearing is pretty good for mpg's as well.

I think my cd has the 3.07 rear. I think the skirt would hurt to be honest. this car is purely my driver to work so I will do anything reasonable including swapping in a 240D and fabbing a 5spd if it will pull it. Im kicking myself right now for not pulling a 5 spd from a mercedes in the pnp. it was the one with the starter wart on the wrong side but I think I can make that one work.

You won't get better mileage with a N/A 240. I have a '77 240 automatic and it maybe gets 24mpg if I drive it slow. To keep up with traffic, you have to drive it like a UPS truck..... WIDE OPEN! I know... I have driven BOTH!!

To get the best mileage, you'll basically have to re-engineer the whole car. Start with the body. Lower it, install skinnier tires, things like that. Find out how much power it takes to move it down the road at your desired cruising speed. It will probably be something like 22-25 HP.

For the engine, a turbo isn't a bad thing but NA may net you a little better economy because of the higher compression.

You'll need to come up with BSFC (Brake Specific Fuel Consumption) curves for the various engines you're interested in. Find one with the lowest BSFC for the HP needed to move the car down the road at the cruising speed. The graph should also show the RPM where this takes place. Then set up the cars gearing to have the engine run this RPM at the desired cruise speed.

Since I'm not an engineer I would have to come up with this information by trial and error but someone smarter could probably calculate a few things and come up with the best engine combination. Perhaps there is a dyno database on the web somewhere with this info...?

Word.

If you really want the best economy, go with a 2.88 rear end, put a shift kit into the tranny so it has a firm shift between shifts and run Biodiesel with a 3* degree timing advance.

Keep your tires properly inflated and the usual. Having to re-engineer the whole car for better fuel economy is too cost prohibitive and you will spend less on fuel than you will replacing motors and the like.

Also, you can use refrigerants like Envirosafe and get great cooling out of your AC and in most states it is legal. This move will save you about 3 MPG alone. It's made in Florida and for sale and use their as well.

does anyone have the BSFC for a 300D turbo engine? Is this normally given for the engine or the entire car system? I feel with lowered RPM cruising at 65mph max I could realistically get in the 35mpg range. Right now the car cruises at 65 at 3000 RPM, i would like to get the cruise RPM down around 2200. I only have one gear lower to go 2.88. that change only gives me around a 200 rpm drop. right now Im thinking of adapting a 5spd from a 190.

As stated before, you need to figure out the BSFC for the ENGINE rpm (that's how they're rated) then determine what you'd like your target cruising speed to be. Then calculate what rear end ratio you'd need to have your engine be at the best BSFC for your target cruise speed. This is alot of work but if you're looking to max out your efficiency it really is the best scientific way to do it. Among skinny tires (less rolling resistance), light wheels (less unsprung weight) and aerodynamics (less drag).

from what I have been reading through from search finds on the subject I think the RPM band I want for cruising at 65 MPH is 2200 RPM. I cant get there with gears, (2.88 will yeild 2800 RPM) the wheels are very light and the tires are not the thinnest but I keep the pressure high. It really is looking like the OD trans is the best solution. this will be easier for me to accomplish in florida since the biggest hill I see is the bridge over the bay, besides with a 5 speed I can always drop to 4th for hills and still be the same as I am now.

Nah, you need a different engine, i.e. you need to reengineer the engine.

The current common-rail diesels can provide almost twice the power while getting around twice the fuel economy.

That's not going to happen playing around with 30-year-old designs.

The best bet is a 2005 E320CDI or if you're willing to go smaller, a VW TDI. The W123 is a money pit when it comes to trying to get close to these.

I'm running a 2.47 diff from an 83-84 380SE, in my 300CD. Although some have said it would be too tall, and would lack power that is not what I am finding at all. In fact I hope to find another 2.47 to put in my SD:D. I am also running the vogtland springs which as well as really improving handling has dropped the height of the car, and reduced drag... I hope.

I would look for a 190D 2.2 with a 5 speed.

You need a hydrostatic regenerative brake.

http://www.boschrexroth.com/country_units/america/united_states/en/news_and_press/press_releases/brm/Rexroth_Hydrostatic_Regenerative_Brake/index.jsp

This company says it can be retrofitted to existing vehicles but I think they have trucks in mind.


Seriously though, diesel engines typically get their best fuel economy between the rpm their torque peaks at and their power peaks at. (what a great excuse to go get a dyno run :D ) This should help you figure out gearing.

what kind of MPG numbers are you getting with the 2.47, those would get me in around 2400 RPM at 65? I also have a set of vogtlands, just have not installed them yet