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  #1  
Old 07-26-2016, 01:53 PM
Shadetree
 
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Flex disk issue 617 engine/trans in 380 SE body.

I just finished setting the 84SD engine and transmission in the 85 SE body. While sorting through parts I noticed the flex disk were different. I had no intent on changing anything on the rear axle of the gas car.

The yoke on the SE rearend will not bolt to the SD drive shaft. What is the simpler remedy for this issue?
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  #2  
Old 07-26-2016, 03:02 PM
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If I am not mistaken, the 380 has a 3.46 different ratio and the 300sd is definitely a 3.07 ratio. The 617 will be turning some high RPM's at a higher speed with the 380 dif.
As far as your issue, the best way is to swap the yoke out. Either at the trans end to use the 380 driveshaft or the diff to use the 300sd shaft. Swapping out the yoke on the diff is a bit more trickier because you need to pay attention to the rotational torque. Do a search on this subject. Plenty of info on that here.
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Old 07-26-2016, 03:32 PM
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When I swapped a 617 into my 380SL I was able to use the stock driveshaft without any modifications. I did need to swap in the yoke from the gas version of the 722.3 transmission to match the flex disc and the driveshaft yoke. (I actually swapped the tailcone too so I could use the electronic speedometer sensor to match the one in the car rather than swapping in a mechanical gauge). Make sure you keep the spacer that goes with the yoke.

Another potential option would be to use the front half of the diesel driveshaft with the rear half of the gasser. I'm not positive that would work, but it just might. One thing to be aware of is that the two halves are balanced as a unit and are usually marked to keep the clocking....accordingly you may have to rebalance the mismatched driveshaft. (Personally I'd try it without just to see if I got lucky)

I think you're going to find that that 3.46 is way too high a ratio. My SL has a 2.47 and I'd consider that perfect with the 617. Yes, it's a little slow off the line but cruising at speed, it's just a pleasure. Because of the higher weight and drag, I'd suggest you try to find a 2.88 rear end - I think that would be perfect.
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  #4  
Old 07-26-2016, 04:50 PM
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I'm not qualified to tear into either the transmission or differential.

How much trouble to just swap the SD differential housing to the SE?
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Old 07-26-2016, 05:07 PM
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Bolt in job -

Info here - Mercedes-Benz Forum - View Single Post - Rear Differential Interchange
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Old 07-26-2016, 05:48 PM
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I thought the V8s had taller rear ends than the diesels?

-J
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  #7  
Old 07-26-2016, 06:06 PM
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Quote:
Originally Posted by compu_85 View Post
I thought the V8s had taller rear ends than the diesels?

-J
It is my understanding that as the number climbs, 3.13 vs 3.54 for instance, the higher number means the lower gear. Which translates to faster acceleration and higher RPMs at all speeds.
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Old 07-26-2016, 06:17 PM
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No you have that reversed. Lower number rear ends means fewer turns of the input for one turn of the output. In this case 3.13:1 vs 3.54:1. I believe the 560s have a 2.8:1 rear end.

My ECOdiesel Jettas had a 4.25:1 differential

-J
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Old 07-26-2016, 06:22 PM
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Quote:
Originally Posted by mach4 View Post
So the differential's dimension which is indicated by the '185,' and '210,' is not going to be a problem?

I'd also know who I might get to swap the gears. I think the 247 gear is the way to go here, what say ye?

I do not travel much but I'd hate to not be able to keep up with traffic on the highway.
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Old 07-26-2016, 06:24 PM
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Quote:
Originally Posted by compu_85 View Post
I thought the V8s had taller rear ends than the diesels?
Check your diff - it should have a 2:47 already

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Flex disk issue 617 engine/trans in 380 SE body.-image.jpeg  
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Old 07-26-2016, 06:52 PM
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My bad using off the top of my head. I was thinking the op had a straight 6 cylinder 300se, which does have a 3.47 ratio diff.
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  #12  
Old 07-26-2016, 07:16 PM
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If you use the SD's differential, you will need to take the axles as well. The SD and SE are different widths.

Personally, I would keep the SE's differential with the better ratio. It is what I converted to and I enjoy a much quieter drive.

Exchanging the yoke on the differential isn't that major. You will need a 30 mm deep socket and an inexpensive in/lb bending bar torque wrench to measure the bearing load, before removal of the original yoke. Also, there is a locking cap.
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  #13  
Old 07-26-2016, 10:11 PM
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I need a DYI article folks.
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Old 07-27-2016, 10:26 AM
Shadetree
 
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Quote:
Originally Posted by compu_85 View Post
No you have that reversed. Lower number rear ends means fewer turns of the input for one turn of the output. In this case 3.13:1 vs 3.54:1. I believe the 560s have a 2.8:1 rear end.

My ECOdiesel Jettas had a 4.25:1 differential

-J
I think we are saying the same thing but using different terms. To achieve a speed of 70 mph a 247 gear will require an engine to produce an rpm of 2750. With a 3.07 gear a speed of 70 mph would be produced at an rpm a little lower than 2700.

If that is true then I certainly prefer to change the yoke and keep the SE's differential. What part of the FSM will cover this specific project?
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  #15  
Old 07-27-2016, 10:34 AM
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Quote:
Originally Posted by Clemson88 View Post
I need a DYI article folks.
You should leave the rear end alone for now. Get the car on the road and drive it for a few weeks and if you don't like the performance, then worry about the diff swap.

To swap the yoke on the trans you need to remove one nut...I think you can handle that. You'll need a deep socket and then you'll need to "stake" it, but that's literally a hammer and center punch and two taps.
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