6.9 vs 3.5 rear end question - Mercedes-Benz Forum

 
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post #1 of 6 (permalink) Old 09-24-2009, 11:05 AM Thread Starter
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6.9 vs 3.5 rear end question

I've got a '71 300SEL with a more or less shot 3.5 engine. Sometime in the future, I'm looking at swapping my 108's 4.5 into it, but wouldn't mind finding a 6.9 engine/tranny to put in instead. The question is, can the 3.5 rear end take the torque of a 6.9 engine?

What made the 6.3 rear ends different than the ones for the 4.5's?

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post #2 of 6 (permalink) Old 09-24-2009, 01:00 PM
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Only the ratios.. I believe...

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post #3 of 6 (permalink) Old 09-24-2009, 09:20 PM
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The 6.3 had a reinforced homokinetic joint to transfer the movment from the differential to the right wheel shaft. It also had a LSD. I saw some 3.5 and 4.5 with and some without that reinforced joint. LSD was an option on the 3.5 and 4.5. To check the type of joint you have you can push the rubber between the right wheel tube and the center casing. If you see a connection like a trans shaft universal joint it is not reinforced.

You can fit a 6.9 but this will be a lot of work. A 5.0 or 5.6 M117 will be much easier and lighter. I got both a W111 Coupe fitted with a 6.9 and another one with an euro 300 hp 5.6. The latter is nearly a straight fit while the 6.9 required a lot more fabrication (engine and trans mounts relocated, custom exhaust headers etfc.). with a 5.0 or 5.5 M117 the best is to use 560SL exhaust manifolds or 500 SL/C tri-Ys if you find some.
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post #4 of 6 (permalink) Old 09-24-2009, 10:33 PM
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Only the ratios.. I believe...
You believe incorrectly. There are major changes.

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post #5 of 6 (permalink) Old 09-25-2009, 01:24 AM Thread Starter
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You can fit a 6.9 but this will be a lot of work. A 5.0 or 5.6 M117 will be much easier and lighter. I got both a W111 Coupe fitted with a 6.9 and another one with an euro 300 hp 5.6. The latter is nearly a straight fit. with a 5.0 or 5.5 M117 the best is to use 560SL exhaust manifolds or 500 SL/C tri-Ys if you find some.
OK, sounds interesting. I'm not too familiar with other Benz engines - what model has the 5.6 engine? The 560SL's?
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post #6 of 6 (permalink) Old 09-25-2009, 05:19 AM
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Warren,

apart from the homokinetik joint and the LSD that the 3.5 and 4.5 may also have, and of course the ratios, I do not see other differences in the rear axles.

The M117 started as the 4.5, and went on evolving with an alloy block, bigger bore and stroke. But basically it's the sam engine. You keep your 3.5 engine mount brackets and trans mount, 450 SL exhaust manifolds and you are in business. Depending on the version you chose, the challenge may be more on the side of the elctronics.
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