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Old 07-11-2006, 11:19 AM
jfikentscher's Avatar
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Differential exchange?

Can a 1987 300DT take a diff from a 300D or 300E? What problems would be presented if the switch is made?



1987 300TD 440K - My car
2005 Jeep Grand Cherokee Laredo 107K - Tim's car
2000 - CLK320 100K - Deb's car
1994 C230 150K - Josh's Car
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Old 07-11-2006, 11:28 AM
rrgrassi's Avatar
mmmmmm Diesel...
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You need to make sure the replacement will be a direct bolt in. You may need to also replace the driveshaft as well. Any modifications to the driveline can have dire results if everything is not "squared" up.

You need to be careful with the gearing as well, and you will need to get the speedo recalibrated. Also, it can affect your fuel milage and drive train wear. Higher gearing makes the rear wheels harder to turn, which puts more stress on your u joints and transmission, lower grearing makes the engine rev faster to go say 70 mph, than your current diff does, which makes the engine work harder.

Pro for higher gearing is fuel savings once up to speed. Engine has lower RPM. Pro for lower gearing is more grunt at take of from a dead stop.

70's Southern Pacific #5608 Fairmont A-4 MOW car

06 VW Jetta TDI 297K miles

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 195K

90 Dodge D250 5.9 Cummins/5 speed. 400K
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Old 07-11-2006, 01:36 PM
dieseldiehard's Avatar
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Location: Bay Area No Calif.
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Originally Posted by jfikentscher
Can a 1987 300DT take a diff from a 300D or 300E? What problems would be presented if the switch is made?


The diffs will marry up no problem. The same unit will fit a 201 as well as a 124 chassis with a cover swap.
A diff with a LOWER # (TALLER GEARING) will raise your acceleration (zero to sixty will be quicker) but the engine rpm will be greater as you go down the road, using more fuel and wearing the engine a little more than if it were the original version.

90-92 300E: RATIO 1:2.87
93-95 300E 2.65
93-95 499E 2.24

neil ke6dcj says the 2.87 is a good compromise between highway cruising and performance in his modified gasser 124 wagon (300TE) with a V8. Expect to pay at least $125 for a used one.

As for the speedometer, a speedo shop can change the readout using the original K factor, shown on the old instrument and recalibrate another instrument to the same factor. All it takes is $
'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

Last edited by dieseldiehard; 07-11-2006 at 02:35 PM.
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Old 07-11-2006, 01:47 PM
TheDon's Avatar
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Originally Posted by rrgrassi
which puts more stress on your u joints
the w123 and the rest dont use U-Joints.. im sure of the w123 not using U-joints.. they use flex discs
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Old 07-11-2006, 01:49 PM
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Check your differential case for ratio used

It may or may not apply to your car, but the 86-87 300TD wagons came with a 3.27 diff I believe, this diff was used only in these cars and not the gassers of the same years/class. I'm not sure if the diff was shared in the diesel sedans of the same years or not but if so, swaping out to a taller diff from say a 300e will likely cause some serious performance drops and speedo error. I believe that the diesel's benefit from the shorter diff, given the extra pig factor with diesel.

Just a though, if you have your owners manual it will probably also tell you what diff ratio you have.

Good luck
Stable Mates: 2 and 1/2 Benz
1994 E320 Cabriolet 96K mi (Kloud)
1987 300TD 307K mi (Hans)
2008 Jeep Grand Cherokee OM642 150k mi (Benzrokee)
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Old 07-11-2006, 02:20 PM
sailor15015's Avatar
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Originally Posted by TheDon
the w123 and the rest dont use U-Joints.. im sure of the w123 not using U-joints.. they use flex discs
True, they do use flex plates at the connection to the tranny and diff but there are u-joints as well. There's one in the middle of the driveshaft and I believe at each of the half-shafts.

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo
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Old 07-11-2006, 07:41 PM
t walgamuth's Avatar
Join Date: Apr 2005
Location: Lafayette Indiana
Posts: 37,599
there is a u joint in the center of the ds. the things on the half shafts are cv joints. they perform a similar function but are a bit different.

and dieseldiehard has confused a few terms. generally a higher numerical ratio is called a lower gear. so a 410 is a lower gear than a 323. and a lower gear will give more mechanical advantage and quicker acceleration or more towing power.

a taller or higher gear such as a 323 vs the 410 again will reduce engine speed at highway speeds, reduce fuel consumption, reduce engine wear and the trade off is more leasurly acceleration. the part about stressing the ds and u joints is theoretcially true with the higher gear, but it is something that is not a problem. it is minor enough that folks generally dont consider it in the equation.

hope this helps.

tom w

[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

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