Does anyone out there know what would be the ideal rpms (for fuel economy / driveability) for a non turbo 300d?

It seems that mercedes designed these cars to highway cruise above 3000 rpms - I have a 75 300D with a m/t conversion and am thinking of putting in a 3.07 rear end but would like your imput first. Theoretically lower rps (ex 2000) seem like they would equate to better mpg but what do ya think.

Keep in mind these cars were built during the era of the 55MPH speed limit.

The issue is always a tradeoff between available horsepower and horsepower requirements.

If you're traveling down a level highway at 60 mph, the perfect rpm is about 2200............very quiet and more than ample horsepower to push the vehicle along.

However, if you encounter an upgrade, the available power...........about 40 hp.........will be insufficient to maintain 60 mph. Most folks won't accept this. Naturally, you could downshift to third and the rpm would climb to about 3300, with more than ample power to climb the hill.

The opposite problem is the fact that you only have four gears available. If you select a very tall ratio for the highway, the driveability in the city will be somewhat miserable. A ratio of 2.47 with the available torque from a 617 n/a would be completely unacceptable in terms of acceleration.

Therefore, it's always a bit of a compromise. On a 617 n/a, I probably wouldn't use anything taller than a 3.07 due to the difficulty of maintaining highway grades.

The SD has a 2.88 and it's taxed to it's maximum to maintain a speed of 68 mph on some of the relatively steep highway grades. There is just insufficient horsepower available at 2900 rpm from this engine.

Yes, but they were built in Germany, the land of the autobahn. It's safe to cruise around 3,500 rpm if all else is in order (i.e. valves in reasonable adjustment, etc) or more if you need to pass someone. My wife had a '78 300D before her '82 SD and it would run along happily at 85-90 mph.

__________________
Houston, Texas
1984 300CD 235K miles, Elvira, one owner
1987 300TD 180K miles
1974 TR6 78K miles Sarah Jane, one owner
OBK #27

The answer depends on where you live. If you live at elevation, an NA 617 could lose a lot of horsepower due to lower atmospheric pressure. Under those conditions, higher gearing becomes more problematic. Tom Walgamuth has a very high geared standard shift non-turbo 617 with which he is happy. He lives in Indiana.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

The 300D turbo and 300SD were never intended to drive on the autobahn. And the 300D N/A was intended to be a SLOW car on the autobahn, but not as slow as the US models, as they got manual transmissions. The people who wanted to go fast bought a 230, 280 or 450.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Sorry to tell you this, but NFW that a 617 runs at 4200 rpm.........nearly fully loaded.........and is characterized as "happy".

230's dont go fast!! If you feel the need for speed something with a 6.9 badge on the back is needed. I dont know if they made it to the USA though.

The most economical speed for an engine is the rpm at max torque.
Power = torque x speed (rpm) The rpm for max power in theory should give the max speed of the car unless it is over geared.
Brian is correct in explaining that its a compromise, If you gear too high, get ready for a trans rebuild, all the changing between 3rd & 4th is going to hurt the clutches in your trans. it will soon start flaring on change. The best way to get long life out of a trans is to get it into 4th and keep it there. It helps economy too.

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

Define Happy?.....

In my car "Happy" is right about 67 to 68 MPH (I forget the RPM's). The engine seems to be the smoothest and quiet at that speed It must be just below 3,000 RPM. Anything over that is loud (wind noise, engine noise, etc...) and just "does not feel right". But, I usually cruise along at 75 to 80 anyway.... Just so I don't get run over.



Edit: Oops... The above is with a 2:88.

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AJ

1985 300D (SOLD)

Exactly.

That 'happy' zone for my 84 300D seems to be anywhere between 72 and 80mph...just seems to be at ease with all the elements - all is well and she is smooth and seems to love it!

__________________
71Inka02

62 W121 190DB - Original CA/AZ Car - Excellent Example
99 E300 Turbo Diesel - 362,000...Awesome Daily Driver
01 E320 Wagon - 297,xxx (wife's work car)
01 E320 Wagon - 131,000
02 Land Rover Disco SE- 110,000
87 BMW 325is (E30) Full Restore - Son's
06 Mini Cooper 'S' - Son's
87 Toyota FJ60 Land Cruiser - Showroom Condition - 152,000

84 300D - 142,xxx :cool - "Sold"
83 300D-"Sold" - 211,456
85 300SD-"Sold" - 213,789

3500 rpm with 70% load at 80 mph.............

see post #7...........it applies to you as well.

Seems to be the case with my SD also.

I think a 2.47:1 and driving 3rd gear city would be plenty of RPM to dork around fine. Should be around 2300-2500rpm depending on converter slip/load around 45mph.

Almost all the roads where I am are flat or with a very slight grade so a 2.47:1 on the freeway would yield around 2400-2500rpm @ 65mph and would be plenty fine for me.

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

Brian,
I dont think they realize that you cant run a marathon at a sprinters pace!

The tortoise and the hare story!!

Maybe at 80 mph you cant hear the protests from the motor over the wind & road noise. The good sound insulation on our cars give us a false sense of security.

How many times have you heard 'its good for a diesel to be run flat out'. Well not my diesel !!
I am always saying to the young bloke when some one passes me doing 85 , 'dont worry, we will catch him down the road when he stops'. -- Normally with the hood up and steam & smoke rising!!

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

Yep............all these fellows who think they can run 80 all day long...........and the engine is "happy".

After 100K miles of such happiness, valves, chain, sprockets, tensioner, and possibly rings will be in their future.