Got a nice 72 220D, Whats a better differential ratio? - Mercedes-Benz Forum

 
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post #1 of 8 (permalink) Old 04-17-2011, 06:24 AM Thread Starter
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Got a nice 72 220D, Whats a better differential ratio?

I just bought a nice unrusted 72 220D and of course is gutless and winds up at 70. Because I'm a flat lander and plan on driving it on the highway I want to change the rear differential. Mine appears to be a 3.92 ratio. I'm interested in putting a 3.07 ratio from a W116 300SD or a 3.46 ratio from a W123 300D in this car. Are they directly interchangeable? After doing some gear ratio\ratio , ratio x engine speed x car speed calculations The engine at 70\4th gear is going at 3650rpm , with a 3.46it would be about 3200. 3200 rpm is about the speed that the 3.92 turns at 60, which seems to be a nice steady cruising engine speed. I was leaning on putting the 3.46 in the car and if I felt I needed(or could) lower the engine speed abit more, I could put abit taller tires on it..also, I'd use the GPS to find the real speed for speedo differences. However from my calculations with the red line in 3rd gear would be 64, I haven't did the calculations, but with a 3.07 ratio, I could use 3rd gear more from 55-65 and use 4th as an overdrive. I realize, I stress the clutch much more with a 3.07. Anyway , can I interchange the W123 300D 3.46 ration rear end with the W115 220D , and in the extreme, could I interchange the W116 300 SD 3.07 ration with the 220D. I got my data from the ADSITCO site.
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post #2 of 8 (permalink) Old 04-17-2011, 06:46 AM
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You can also feed all that data into a speedometer shop and have them change the gears as appropriate...
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post #3 of 8 (permalink) Old 04-17-2011, 03:33 PM
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I think the 3.07 ratio will be too long for the low-powered 220D, but who knows.

By the way: 70 mph is too low for the 220D, that is caused by something else than the differential ratio. It should reach 85 mph. Maybe you should adjust the maximum rpm (adjusting screw on the throttle).
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post #4 of 8 (permalink) Old 04-17-2011, 04:39 PM
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Your motor is governed to 4350 RPM. So your top speed should be about 80MPH with the stock 3.92.

Have someone floor the accelerator pedal, then you need to look at the linkage on the injection pump, is it hitting the stop?? If not adjust the linkage so that it hits the stop when the pedal is floored. DO NOT adjust the stopper on the IP, if you do you might damage IP internals.

I would install a 3.46, a 3.07 might be to tall for a OM615.

With a 3.46 you should be able to top out at 92 MPH.

'79 280CE with 226,000 miles
'76 240D 4 speed with 190,000
'85 300D Turbo with 266,000 miles


GONE but not forgotten
'76 300D with 195,300 miles
"83 300D with 174,000 miles

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post #5 of 8 (permalink) Old 04-17-2011, 05:12 PM
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Originally Posted by 79Mercy View Post
Have someone floor the accelerator pedal, then you need to look at the linkage on the injection pump, is it hitting the stop?? If not adjust the linkage so that it hits the stop when the pedal is floored. DO NOT adjust the stopper on the IP, if you do you might damage IP internals.
The 220D has a pneumatic governor. There is linkage going to the IP, but it does not have an influence on the top speed.
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post #6 of 8 (permalink) Old 04-17-2011, 07:26 PM Thread Starter
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I appreciate the reply about the governor. The main information I could use is whether the 3.46 ration differential out of a W123 300D will bolt into my 72 220D. As far as the governor, the car will do 85. When I first got the car, I noticed that when accelerating with the throttle fixed the car would start to govern down, I thought it was a intake filter restriction. Out of curiosity I timed 0-60 with the rubber transition piece off the intake and the time wasn't any better , but I never got the governor effect. A few days ago I read some other threads on this site and the vacuum governor thing made sense. The governor effect is annoying some times, the car is going the rated speeds in the gears marked on the speedo, so I don't want to monkey with the governor. guess I just want to know if a w123 differential from a 300D fits a W115 220D?? thanks for the replies
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post #7 of 8 (permalink) Old 04-17-2011, 07:32 PM Thread Starter
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Jipees, I like the Red 300D, sweet ride. I've just had this 220D a month, it's nickel and dimeing me but it has a nice floating ride for a small simple car, glad I bought it.
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post #8 of 8 (permalink) Old 04-18-2011, 12:43 AM
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The early W123s had the same rear differential housing as the W115 according to the EPC (part number 1153501420), so I think you can bolt a W123 differential to a W115.

There are some differences in appearance, mainly the axle shaft seals.

I know that the axle shafts are interchangeable between the W115 and W123.

If the car reaches the speed marks than the air throttle is adjusted properly.

Last edited by Govert70227; 04-18-2011 at 12:46 AM.
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