overdrive for W126? - Page 2 - PeachParts Mercedes-Benz Forum

pawoSD · #16 09-15-2011, 12:12 AM

A 300SD with a 2.47 drives just fine, as long as it has the high stall (2500rpm) torque converter, you barely notice the difference in the city/acceleration.

That said, a 3.07 diff with the high stall converter (technically never sold from the factory as all '85s had a 2.88) is a speed machine. That is the configuration I have in my 300SD.....it flies if you mash the pedal off the line. I rather enjoy it.

netboy · #17 09-15-2011, 12:19 AM

Quote:

Originally Posted by Brian Carlton

The answer to your question is found in horsepower. The engine produces 123hp at 4400 rpm. To achieve this rpm in the 1:1 gear (4th) requires a 3.21 axle ratio. Without that ratio, the vehicle won't make the peak rpm required for full hp.

Naturally, if maximum power is not your objective, as it is with most of us, a taller diff will result in lower rpm's on the highway.

However, this lower rpm comes at a price.

Once the engine drops below 2500 rpm, it's power drops substantially and the capability to climb a hill at the designated speed becomes impossible due to the limited horsepower available.

The 2.47 is only suitable for those with relatively flat terrain where the required horsepower to maintain vehicle speed is only 25 or so. Alternately, the driver can shift down to third and put the engine very close to peak hp and easily maintain the speed.

I replaced the 3.21 in the SD with a 2.88 and it will barely maintain 65 mph on a decent grade (7%). I don't have the luxury of a 4-3 downshift with the 2.88, where the 2.47 does provide that option.

The 2.47, however, will result in relatively poor accels in city driving, requiring the use of the turbo (rpm's above 2500) for most driving conditions.

Living in east Tennessee, we have a few decent grades around here. Maybe I'll try taller tires first. Very informative, thanks Brian.

daw_two · #18 09-15-2011, 07:03 AM

Quote:

Originally Posted by netboy

what did you get the diffs from and what does the mod consist of.

I know !!! Look in the Parts Forum !!!
FS: 2.47 differential

netboy · #19 09-15-2011, 03:30 PM

I might be interested. I am having trouble locating the gear ratio stamp on my present diff. I have scrubbed it clean and can't find the stamp. I would have to check on a road trip from Knoxville to Germantown. Going out of town this weekend headed NORTH to Louisville KY.

TnBob · #20 09-15-2011, 04:07 PM

I hear the diff stamped ratio is at bout 7 oclock on the bottom left (drivers side) of the differential.

pawoSD · #21 09-15-2011, 05:35 PM

I'd never try using a 2.47 on a 617 unless you're using the 1985 torque converter....the low stall would make for some slooooow acceleration.

R Leo · #22 09-15-2011, 06:54 PM

Quote:

Originally Posted by LandYaghtLover

I never understood why the gear ratio and diff was such a bad setup on these "autobahn cruisers". I brought this up in another thread asking about a trans swap. At the very least it would be nice to get the torque convertor to lock-up. That would be a nice boost in highway milage.

US market cars weren't built as Autobahn cruisers. At 55mph, as originally geared, the engines are running right in the middle of their torque curve.

Marvelicious · #23 09-15-2011, 09:07 PM

I feel like lower gears would be great paired with a VNT swap. As near as I can tell, the 722.3 and 722.5 use the same torque converter - too bad the lockup converter was so uncommon, lockup would be a nice addition to these transmissions.