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  #1  
Old 05-16-2004, 11:23 PM
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240d/300d gear ratio

I just put an '80 300d motor in my 240d. I'm guessing gear ratios are different for the 240's, since the motor seems to screaming. (I never drove the 240, bought it with a blown motor. My only point of reference is the 300d I bought for the motor, but I did drive it ~100 miles.) Should I be concerned with the 5 cyl. bolted up to the lower gears? I'd like to think I could drive at 75 for a few hours without worry, but still have time to swap the rear from the 300d car if I need to.

Thanks much.

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  #2  
Old 05-17-2004, 05:33 AM
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The 300D rear end is a bit taller... I don't know the ratios off-hand, it's not a huge difference, but noticeable. I'd say about 150-175 (max) rpm difference @65mph.

My brother also installed a 300D in his 240, he was happier with the taller diff.
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  #3  
Old 05-17-2004, 07:11 AM
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I would most definitely swap the rear differential out.
I just completed the same conversion a week ago, transplanting a 617.952 into a 1981 240d. I did drive it with the 240 3.88 (I think) gears before I put in the diff from the 300 series. The car screamed at 75 - not good for the motors long life IMHO.
I switched out the diffs to the 3.08 that was in the 300, and what a difference!
Good luck,
Adam
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  #4  
Old 05-18-2004, 12:18 AM
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any idea as to the strength of the 300 diff's?
i.e. how much horsepower and torque they can withstand?
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  #5  
Old 05-18-2004, 02:37 AM
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I believe the 3.07 gears were only used in turbo 300D's. It sounded like he put a naturally aspirated 617 in there, since it came from an '80 123.

I don't know what n/a 300D's had in the rear end. I originally thought they used the dame ratio that the 240D's used, but that's been proven wrong
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  #6  
Old 05-18-2004, 07:14 AM
R Leo's Avatar
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Quote:
Originally posted by lucforce
any idea as to the strength of the 300 diff's?
i.e. how much horsepower and torque they can withstand?
The only way to kill one of these pumpkins is to run it without any lubricant.

3.07 gears were used in the 617.952 engined 123s until 1984. 1985 models had 2.88 gears.

From thisthread, here are the final drive (pumpkin) ratios for several models.



R107
---------
380SL,SLC RATIO 1:2.47

380SL,SLC,560SL RATIO 1:2.47 ABS

450SL,SLC RATIO 1:2.65

450SL,SLC RATIO 1:3.06

W116
----------
280S,SE RATIO 1:3.69

300SD RATIO 1:3.07

450SE.SEL RATIO 1:3.06

450SE.SEL RATIO 1:2.65

W123
----------
230,240D RATIO 1:3.69

280E RATIO 1:3.58

300D RATIO 1:3.46

300DT 85 RATIO 1:2.88

300DT TO 84 RATIO 1:3.07

W126
----------
300SD 85 RATIO 1:2.88 ABS

300SD TO 85 RATIO 1:3.07

300SDL RATIO 1:2.88 ABS

300SE,SEL RATIO 1:3.46

380,500 RATIO 1:2.47

380,500,560SEL RATIO 1:2.47 ABS

420SEL RATIO 1:2.47

W124
----------
260E RATIO 1:3.27

300D 1987 RATIO 1:2.65

300E RATIO 1:3.07
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Last edited by R Leo; 05-18-2004 at 07:28 AM.
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  #7  
Old 05-18-2004, 07:31 AM
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Diff ratio in my NA '80, '82 & '84 300Ds (In Oz) is 3.46
This allows cruising @ 110 km/H (around 67mph) without excessive engine noise.
Even when overtaking at speeds up to 130 km/H (~80mph) the engine noise is not a problem. Getting there is a bit slow for my preference though.
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240d/300d gear ratio-fatmobile-diff-2a.jpg  
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Lucille '87 W124 300D non-turbo 6 cylinder OM603, Pearl Grey with light grey interior


Various parts cars including 280E, 230C & 300D in various states of disassembly.
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  #8  
Old 05-18-2004, 10:15 PM
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Thanks for the tips. Apparently I was using the wrong search terms, because I couldn't find that thread. Anyway, a change from 3.69 to 3.46 doesn't sound worth the effort.

These things really buzz at 70mph, and I hear of others that steady state cruise at 80. Isn't that near redline? You all really drive that fast all the time? The cruise still works on my car, so I thought of just setting at 70 and forgetting about it, but am having trouble with that last part.
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  #9  
Old 05-18-2004, 10:55 PM
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Quote:
Originally posted by kchristopher
These things really buzz at 70mph, and I hear of others that steady state cruise at 80. Isn't that near redline? You
It definitely takes some getting used to. I used to own a Chevy S-10 Blazer that shifted into high OD at 42 mph. It lugged to about 1600 rpm at that speed and spun at a paltry 2300 at 70mph.

In comparison, my Bride's sedan (84 300D) turns about 3000RPM at 65 mph...nowhere near the 5500 RPM WOT limiter in the IP qnd my 75 240D really honks at 75mph (no tach on this one but I'm guessing its turning in excess or 4000RPM) but, it's been doing it on and off for 29 years and still runs fine.

Basically, diesels are made to work and running them under load at the upper part of their RPM range doesn't harm them.
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  #10  
Old 06-14-2004, 12:10 PM
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k,

If you change the differential ratio, the speedo and odo will both be off by the ratio of the difference of the gear ratios.

You may be able to correct this by replacing your speedometer head with the one from the car that you took the replacement differential from.

P E H
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  #11  
Old 06-21-2004, 08:39 PM
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P.E.,
The speedometer head swap would likely not change the difference observed in the speedometer. The gear that drives the speedo cable would need to be swapped for adifferent one, or a speedometer transducer would have to be screwed in to the cable.
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  #12  
Old 06-21-2004, 08:44 PM
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Luc,
Providing he changed out the tranny tp put the proper tranny in as well for the 300d, all he would have to do is change out the speedo head and rear differential ratio. There are no provisions for changing ratios in the tranny, because MB does this with the speedo head as PEH suggested.
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  #13  
Old 06-21-2004, 08:46 PM
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I'm leaving it as is, running great!
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  #14  
Old 06-27-2004, 12:58 AM
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hmm,
I have swapped speedo heads from various models back and forth every which way from models with differing differential gear ratios. Never have had an issue with speedo calibration.
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  #15  
Old 08-09-2004, 04:15 PM
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Picked up a 300D turbo diff for my 240D

I got a diff from a local u-pull-it this weekend. I was looking for a 3.07 to put behind my 240 getting the 617.952, and got lucky - it was a 2.88. The donor turned out to be an '85. I'm psyched now!

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