overdrive for W126? - PeachParts Mercedes-Benz Forum

lwbdriver · #1 09-13-2011, 09:24 PM

As I was listening to my engine scream along at 3300rpm at 75mph, or so, I was wondering if anyone makes an overdrive unit that can be attached aft of the transmission. There are a couple of companies that do that for musclecars and RV's. It would be great to see the tach only doing 2500 or so on long drives. Great for mileage and engine wear too. Ideas?

pawoSD · #2 09-13-2011, 09:36 PM

Its a lot easier to drop in a 2.88 or 2.47 differential and lower the rev's overall. We did a 2.47 in one of our 300sd's and it was an excellent highway cruiser, and did fine in the city, could do 35mph idling!

Hampered acceleration....a bit. With a 2.47 its about 2500rpm at 75mph.

lwbdriver · #3 09-13-2011, 09:54 PM

Hmmm...that's an idea. What was the difference in fuel mileage?

netboy · #4 09-13-2011, 09:54 PM

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

pawoSD · #5 09-13-2011, 09:59 PM

Quote:

Originally Posted by lwbdriver

Hmmm...that's an idea. What was the difference in fuel mileage?

The same mpg, maybe .5 better in some cases, .5 worse in some cases. Couldn't really tell. Barely enough to notice. Main improvement is the quietness/lower rev's.

pawoSD · #6 09-13-2011, 10:00 PM

Quote:

Originally Posted by netboy

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

Diff can come from a 1981-1984 500SE/SEL or 380SE/SEL Got ours off a 500SEL in the junkyard that had 330k on it.

Diff was still good. Primary reason for the project was the original diff in the 300SD was toast and metal shavings/whirring were occurring.

netboy · #7 09-14-2011, 12:04 PM

I have a 83 CD that is primarily a interstate car and would love to lower rpm at 75mph. Its got to be healthier for the engine and better mpg because your engine is not working as hard. Lower diff with taller tires will lower rpm. Do those diffs mount right up or is an adapter to be made?

LandYaghtLover · #8 09-14-2011, 12:55 PM

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.

destroy · #9 09-14-2011, 02:02 PM

it's my understanding that it had to do with the 55 mph speed limit in the US at the time. no roads had speed limits over 55, and having the bottom end acceleration from the dead stop is pretty important too.

I remember reading a while ago that some said the euros had different ratios as well. could be wrong though.

gotta like the way the SD handles on the highway though. 55 is way too slow for this thing. 80-90 (or even faster...

) is where things feel nice. the revs do make me a little nervous though for sure.

compu_85 · #10 09-14-2011, 02:18 PM

Quote:

Originally Posted by LandYaghtLover

I never understood why the gear ratio and diff was such a bad setup on these "autobahn cruisers".

The 300SD was never designed to cruise on the autobahn. It was designed for the US market where the speed limit was 55.

-J

LandYaghtLover · #11 09-14-2011, 02:49 PM

Ok, that contradicts the info I was originally given. And that was that the transmissions were the same in the US and Euro cars. So they are not then?

compu_85 · #12 09-14-2011, 03:12 PM

The frist Diesel S-Class to be sold in Germany was the W140. Transmissions are the same, but rear end ratios might not be.

Having the engine sit at high rpms shouldn't really hurt it. Heck my old ECOdiesels sit at 4,000 RPM when going 80!

-J

mike-81-240d · #13 09-14-2011, 03:13 PM

Quote:

Originally Posted by LandYaghtLover

Ok, that contradicts the info I was originally given. And that was that the transmissions were the same in the US and Euro cars. So they are not then?

The transmissions are the same, the rear differentials were different.

netboy · #14 09-14-2011, 09:46 PM

The SDs definately out handle the Ds and CDs at 75-80mph. My 83 SD seems to float above 60mph. I am now in search of a lower geared differential for my CD. Maybe lift it a few inches and put some 245 tires on it.

Brian Carlton · #15 09-14-2011, 11:43 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.

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.