Final drive for '83 SD ratio's.
 

Customer was asking about a lower numerical final drive( diff'.) for his 1983 300 SD.

At present engine turns 4000 rpm at 70 mph.
Was thinking a gas motor diff' might work....any ideas ?

Have tuned the engine for more grunt and performance is okay for a better ratio.

4K at 70 mph..........thats some low gears.........

Yep...That the reason he's looking at taller gears....Engine is very busy sounding at 75...he drives to southern Texas from OKC a lot.

MB reference literature state that all W126 300SD's from 1979-1984 had a standard Rear Axle Ratio of 3.07. The US-versions in 1985 went to 2.88 -which was the final year for the 300SD in W126 form. All 300SDL's from 1985-1987 had standard 2.88 RAR's.

Gear Ratios for all W126 300SD/SDL were:
I. 3.90:1
II. 2.30:1
III. 1.41:1
IV. 1.00:1

Factory specs for an '83 300SD calls for 4500rpm at 100 km/hr (approx 62mph). And engine speeds at 100km/hr for US-versions vary/differ from '82-'85.

That doesn't sound like stock gears to me.............that sort of RPM's are more than 3.07 gears would give you.....

Bone...I don't know the EXACT rpm he runs at 70 mph...I do know it is around the 4k mark though.

The engine is intrusive and a better cruising gear is what we are looking for.

The TECH' input from our friend "TheBenzBar" ( Interesting site, Benz..) is good...however, I need to know what chassis' I can look for to find a taller final drive.

Ie: Would a later V8 car work...needs to be a simple direct swap out.

This must mean nearly 5000 at 80 mph....a little too high for sustained cruising.

Do you have info' on a donor gas motored chassis that would work.?
What is lowest numerical I can hope for. ?

Neither my 300D nore my 300SD turns that many RPM's at 70....scrape the crud of the lower drivers side rear face of the differential....just outside the cover at the 8 o'clock position from the rear.....you should see a number and a 3.07 or some other number that number is the rear end ratio. Tell us what it is.

Supposedly, W126 rears can be swapped from one chassis to another but be aware to diffirentiate between the short (SD) and longer wheelbases just in case (SDL). I cannot attest as I have not seen one actually performed but only heard thru the local MB W126 techs near my area...

The lowest, according to literature, may be from a W126 V8 car such as a 81-91 500SEC/SEL - the standard RAR was 2.24 for the SEC - good highway cruising gears there!
A US-version 500SE/SEL ranged from 2.24 to 2.47.
The 380SE/SEL's had RAR's of 3.27 (but the US 1981 380SEL had a 2.47).
A 420SE/SEL had 2.47. The 420SEL's had 3.46 from 90-91.
A 560SEL had standard 2.47 for US-versions.

As you can see there are a variety of choices... The rear tracks from the V8 W126's to the OM617/603 W126's differ too - so there lies part of the answer... Maybe a MB Tech who has had 1st-hand experience in a swap could better relate the details... :confused:

beg your pardon, but I think this is mistaken. My SD's drivetrain is stock and I'm going more like 100 mph, not 100km/hr at 4500rpm. It's at about 4000rpm when I reach the limit of my very stupid 85 mph speedometer.

other than that, I think you're right on.

I'll thinking he doesn't have the stock rear end in that car and its been swapped with something far lower in it. THats why I asked him to confirm whats currently installed.

hmmmmm.....maybe he got the rear end out of my once and former '60 Willys pickup! :eek:

heh heh....those had some awefull low gears didn't they......5.38 makes for a screaming top end of about 50-60 mph depending on how hard you were willing to push it with the stock flathead

This is what the MB factory specs stated, as to whether they (MB) are 100% correct or not - I can't answer that. :confused: Here I posted the page - sorry it's a little hard to read at first but it should be legible enough:
http://www.angelfire.com/al3/alfie/w126300sdspecs.JPG

Oh BTW, there were 20,291 units of 1983 300SD's out there (mostly here in the States)... the most of all the production years of W126 300SD's from 1979-1985 (in '79 there were 3 cars produced! Rare!). :)

The chart is in error. There is a statement for "maximum engine speeds" on the left side. Apparently they printed the chart erroneously and the listed "maximum engine speeds" appear next to the "engine speed at 100 km/hour".

The 300SD with a 3.07 axle ratio and 195-70-14 tires will turn 3125 rpm at 70 mph.

If you are actually turning 4000 rpm with this vehicle, I strongly encourage you to do some further checking to see what the problem is. It would be very unlikely to have a differential with a ratio of 3.93:1 on this vehicle.

Please perform the following test:

1) Make a mark on the driveshaft
2) Jack up one wheel off the ground
3) Rotate the wheel 10 times and count the number of turns of the driveshaft while rotating the wheel.
4) Divide the result of the number of turns of the driveshaft by 20.
5) Report back with the result.