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  #1  
Old 10-12-2006, 06:09 PM
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W123 gearing

I am just looking for knowledge about potential gearing options for the 300TD. Eventually I want to swap out the stock 4spd auto for a manual. Is there a 5speed that will fit? or is the 4speed the only option? Can other manufacturers trannys fit with an adapter plate? has anyone tried? What about an overdrive unit? I know volvo had one on their cars at one point.. The main reasons to switch to manual would be increased fuel economy, lower maintenance. I was also wondering if anyone has explored gearing options on these cars. I know other rear ends from other 123s have been messed with, but what about G-Wagen gears? I know there is a fairly strong knowledge base for this sort of thing with american cars, but I have yet to find it for MBs... Basically I want to be able to do normal highway speeds (80mph) without turning ridiculous rpms, and getting awful mileage... What is the cost of doing this sort of thing in the MB?
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  #2  
Old 10-12-2006, 06:13 PM
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These cars are designed to run 80mph without turning ridiculous RPMs. If you have a turbo model, you already have a 3.07 or 2.88 rear end, which is fine for the 4 speed auto or manual.
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  #3  
Old 10-12-2006, 06:25 PM
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I've always wonderd about bolting an overdrive unit on behind the tranny, would it work?
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  #4  
Old 10-12-2006, 07:03 PM
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Quote:
Originally Posted by Old300D View Post
These cars are designed to run 80mph without turning ridiculous RPMs. If you have a turbo model, you already have a 3.07 or 2.88 rear end, which is fine for the 4 speed auto or manual.
Im somewhere near 3500 at 80-85... to me that seems too high... I bet I could get better mileage if I somehow got it dropped it down to 2700ish... Also, my engine drinks oil much faster at those speeds, but doesnt exhibit signs of extremely excessive blowby via the cap test.. is this normal?
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  #5  
Old 10-12-2006, 07:26 PM
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ok i have an 83 240d and i really want a manual tranny. does anyone know the model trans that will fit. i hear there is a four speed and a five speed. also if i do the swap what rear gears would be optimal for city driving but still good mileage on the highway?? thanks for anyone input ahead of time.
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  #6  
Old 10-12-2006, 08:30 PM
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Quote:
Originally Posted by GREASY_BEAST View Post
Im somewhere near 3500 at 80-85... to me that seems too high... I bet I could get better mileage if I somehow got it dropped it down to 2700ish... Also, my engine drinks oil much faster at those speeds, but doesnt exhibit signs of extremely excessive blowby via the cap test.. is this normal?
You haven't said what your current rear end ratio is. Sounds to me like possibly a 3.46 -- is your TD not a turbo? You would certainly turn less RPM with a 3.07, but it's questionable to me whether you would see a significant change in fuel mileage -- the power needed to propel you through that air resistance remains the same, and the slight decrease in friction from the lower RPM may or may not compensate for the pumping losses in the engine.
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'89 Toyota 4x4, needs 2L-T
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  #7  
Old 10-12-2006, 08:32 PM
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Quote:
Originally Posted by carwithfarts View Post
ok i have an 83 240d and i really want a manual tranny. does anyone know the model trans that will fit. i hear there is a four speed and a five speed. also if i do the swap what rear gears would be optimal for city driving but still good mileage on the highway?? thanks for anyone input ahead of time.
You likely will not find a 5 speed. They are rare. But any diesel manual tranny from a W123 should fit, and possibly the units from the W115s as well.

The 240D already has an optimal rear gear, the 3.69. Your acceleration will suffer with anything lower.
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  #8  
Old 10-12-2006, 09:29 PM
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My car is a 1984 300TD with the 617.952 turbo motor. I gather this means it has a 3.07 rear, but I'll get under it just as soon as I can and double check. I have a 1985 300TD on blocks in the woods (rotted to hell and back but good everything else.. except moldy interior). Does the '85 have a 2.88???? I have read elsewhere on this forum that '85s have taller rears... does this apply to the wagons too?!!??? if so I might just slap that one in.. smaller improvement than I'm looking for but at least its something... might go to a local trans shop and ask them about overdrive options...
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  #9  
Old 10-12-2006, 11:11 PM
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yes the 85 has 288s. you would need to change the rear end and the speedo. but the increase in economy would me minimal. it is a swap i would only do if there was a failure in the diff, i think.

i am not aware of any od units that would bolt onto the back of the benz automatic. it is possible the volvo unit could be adapted. but that would be expensive unless you had the skill and tools to do it yourself.

going to a lower numerical rear end will normally increase the fuel economy significantly. in my opinion it is the only economical way to significantly increase economy in a benz diesel.

lots of folks have done it. with an automatic about any commonly available rear end will work, but with a stick if you go too high the results can go south for taking off from a stop.

tom w
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  #10  
Old 10-12-2006, 11:26 PM
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Quote:
Originally Posted by t walgamuth View Post
yes the 85 has 288s. you would need to change the rear end and the speedo. but the increase in economy would me minimal. it is a swap i would only do if there was a failure in the diff, i think.

i am not aware of any od units that would bolt onto the back of the benz automatic. it is possible the volvo unit could be adapted. but that would be expensive unless you had the skill and tools to do it yourself.

going to a lower numerical rear end will normally increase the fuel economy significantly. in my opinion it is the only economical way to significantly increase economy in a benz diesel.

lots of folks have done it. with an automatic about any commonly available rear end will work, but with a stick if you go too high the results can go south for taking off from a stop.

tom w

Where can I get a diff with lower ratios than 2.88? How low can I go before I lose driveability with the automatic? How much do I stand to gain mileage-wise? This is very exciting!
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  #11  
Old 10-12-2006, 11:36 PM
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Quote:
Originally Posted by GREASY_BEAST View Post
Where can I get a diff with lower ratios than 2.88? How low can I go before I lose driveability with the automatic? How much do I stand to gain mileage-wise? This is very exciting!
There is more info available if you would just search than your excitement could handle. At least 30 threads like this over the last couple of years. Few, it any have done anything. Somebody installed a 2.47 and wasn't very pleased if I remember. The 2.88's from the 85 would be cheap and rather easy. It is a good highway gear, and seems to do fine around town also.
Sounds like your best answer is to slow down from driving 85, but that may be too simple.......
Overdrive installation is just a pipe dream.......
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  #12  
Old 10-12-2006, 11:45 PM
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Well - I have found the MB diesel "easter egg"
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  #13  
Old 10-13-2006, 12:39 AM
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Originally Posted by JimmyL View Post
There is more info available if you would just search than your excitement could handle. At least 30 threads like this over the last couple of years. Few, it any have done anything. Somebody installed a 2.47 and wasn't very pleased if I remember.
Thanks for that. 2.47 was the magic number to type in the search box to get to those threads. I remember them too, just couldn't find them Anybody got a 2.47 they want to trade for a 3.07 or 2.88? I'll look around for junkyard specimens in the meantime... and maybe order up some flex discs.
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  #14  
Old 10-16-2006, 02:52 PM
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1984 W123 300td

Okay.. so I have been browsing the rear end swap threads... and it seems the way to go is keep the auto (which is a wonderful tranny btw), and swap in a 2.47 pumpkin, but I can't find one where someone swapped a 2.47 into a W123. Am I blind, or is this impossible?? It seems that all the swaps were into W124s or W126s. Can this be done as a bolt-on job? Do I need to fabricate a bracket? Do I need to get the diff with axles? What else? I want to do this, but I don't want to go ahead and buy things if it won't work... please help me out here
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  #15  
Old 10-16-2006, 06:04 PM
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the first one i remember was into a 300td. the words "diesel geek" is in the thread title. i think the 247 requires a bit of adaptation to do. different axle flanges i think it was.

they loved it.

tom w
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..I also have a 427 Cobra replica with an aluminum chassis.
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