w126 differential strength
 

greetings,
We're building up a high hp (some where north of 600hp) 91 560sec. We'll be running a 3.06 gear with a Wavtrac carrier. questions are how much horsepower can the differential housing handle. Or are going to spill it's guts the first time we step in it?
"regards to everyone"
Allen




1966 220SEb 4.6L 32v
1991 560SEc 5.3L turbo

I'd be much more worried about the axles. MB diffs are pretty damned tough, well the old stuff at least.

In addition to the potential axle/CV joint weakness mentioned by Mr. d, it bears asking, among other considerations, which of the two different size diffs is to be used, what transmission/clutch/torque converter is to be used, what the engine torque vs. RPM may be, what the maximum gear reduction will be, what the drive line configuration is to be, and how sticky the tires will be.

w126 differential strength
 

greetings,
the differential is a 210mm (8.26in) ring gear 3.06 ratio in a 91 w126. which I was assuming was pretty stout, I agree about the axles which will probably need to be improved. As long as axle angles on launch are controlled they'll last longer.
a 4L80E trans (GM)and (we're still on the fence about this) a 2400-2800stall convertor. 1st 2.48, 2nd 1.48, 3rd 1.00 4th 0.75
we'll also be using a one piece driveshaft, the center support bearing isn't going to hold up.


Allen

ac:

In the event that you are in the US, locating a 3.06/210mm diff, while not impossible, is a bit of a challenge. Some 450SL/SLCs, and some 450SE/SELs had the combination. If you have one, consider using the rear sub-frame from a pre-1986 W126 chassis; the diff will bolt in. The 1986 & later (Gen II) W126 used a resilient mounting for the diff, and as a consequence the sub-frame does not accept the 1st Gen style diff.
If you have already embarked on installation of the gearset into the Gen II housing, rock on. The source of an aftermarket diff carrier for the 210mm center section as used in the 107/116/126 chassis' would be of interest to many on this forum. Although a nominal 210mm diff was used in W140 and later chassis', it is a different design than the earlier ones.

From a strength standpoint, the 210mm case will be quite OK with an auto-box.

Re: Engine & trans
The 4L80 box is significantly larger physically than the MB 722.3 box. Anticipate some floor surgery.

The diff will not be an issue. The half axles will be fine also, unless you mount some serious drag rubber. You will need a better driveshaft, one piece, assuming the factory is two piece like the w124.

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Good question. When I put the M120 into the 107 I considered all this. I found that MB used the same drive shaft tube same axles and hubs. They made a change to the diff and called it the strengthened housing and changed the flex disks to thicker 4 web design. So it appeared that MB felt that the diff was the week link in going from the M117 to the M120. Beyond the M120 cars like the E55 started to get bigger drive shafts resplined hubs a newer 114mm wheel bolt circle with 14mm wheel bolts etc. So at 600HP you are really pushing the envelope especially on the diff which apparently was the first thing MB changed. For that reason and others I elected to go with the 129 strengthened diff at the time. I have since reinstalled the original 107/126 diff, and about to upgrade to a 2.82 gear ratio. So I also have concerns but I am only at 400 HP. I will certainly be watching this thread see where the week links are.

Incidentally HP means nothing in this equation it will be all about torque.

Also your front differential rubber mounts and probably the rear one as well will not cut it at 600HP with a 3.06 gear. You will probably need to make up urethane replacements for those.

I made up a set for my car because I needed thinner ones. I used 40 Shore A durometer rubber from Mcmaster because it seemed to be about the same durometer of the stock ones. Mine were not bonded together like the stock ones so that didn't help but they squeezed out like a water melon pit in about 10 runs with 400 ft-Lb torque engine running a 3.59 first gear and 2.47 differential. That rubber was rated at 800PSI. In doing the calculations, I found that there is a stress on those rubber pads of about 928PSI.

Force = Torque/distance

Torque = Engine torque* fist gear*diff gear = 400*3.59*2.47 3547 ft-lb at the axles.

distance measured from axle to mounting pads ~ 8.5" = .708ft

Force = 3547/.708 = ~5010 Lb

Area of mounting pads = 2*pi*1"^2 - 2*pi*0.375"^2 = 5.34 in ^2
Where 1" = radius of pad and 0.375" = radius of center hole in pad.

Stress = Force /Area = 5010/5.34 = 928 PSI

So I exceeded the stress rating for the rubber and it didn't help that they weren't bonded in.

I have a new higher strength rubber from Mcmaster rated at 1500PSI and I will be bonding them with a Loktite epoxy I found. I will be just north of 1000 PSI when I go to the 2.82 gear set I'm putting in so I should be OK.

I was also concerned about temperature. My diff gets too hot to touch so I will be measuring the temp using temperature dots. The old rubber was only good for 170°F the new rubber is good to 300°

Cool calcs. Thanks. But ultimately you are not going to be able to apply more torque than you have traction for. Still, you might get there in 3rd gear at full trot.

That is true but my V12 107 with 389HP and 2.47 ratio LSD with MT 225/60-16 drag radials will not spin the tires on a well prepared track. It was actually slower than an open 2.65 diff with the same tires with 60' times down about 0.1 sec. Consequently I crushed the rubber bushings. The open diff spun the left wheel a lot but at least put the engine in the power band. Could really benefit from a higher stall converter here but I'm trying to do the best I can with a factory like ride. My next run will be with regular street tires which will spin hopefully just enough to get the engine in the power band.

I now have the drag radials up for sale and am preparing a 2.82 LSD diff. Plan is to put a set of smaller Nitto 275/50 -15 tires on for drag racing. That combination should run me right through 3rd gear at @110MPH. and hopefully enough wheel spin to get the engine up to power.

I could be interested in the drag radials. I already did the 2.82 swap and wavetrac install.

-edit- sorry - looks like they are 18s. I am on 16x8s.

Seems like you need a turbo on that thing, or just a 5.5 kompressor motor. Those things rock at the strip.

They are 16" 129 wheels I'm asking $400 for wheels and tires. PM me if interested.

I cant seem to upload the pictures here so PM me with an email address.

As I stated in one of my previous posts. I had a differential mount failure caused by too much torque and perhaps just not being bonded. See first two pictures. So the new high strength elastomeric material came in and I was just about to bond them but found that my industrial Loctite epoxy (the lower one in the picture) requires a special tool to apply it. Its not like the kind you buy at Home Depot (upper one in picture) with the plunger installed. And of course the plunger is a different size. So this will be another week.

In the mean time I have found a source for these machined washers and if I can get about 10 people that need them. I could probably have them available at about $50.00 a set. They should be better quality than the ones shown with the center spacer integrated into the lower washer like the factory original. Anyone running over 400HP should consider this.

For me these center spacers will just be a tube.

Also I decided to keep my 16" wheels as I have found a Nitto drag radial that will fit 245/50-16. So only the MT 255/50/-16 drag radials are available for anyone interested.

New differential mount washers have come in. These washers will work with 1/4" Buna N or poly urethane. The center section can also be shaved down about 1/16" and be used with 3/16 rubbers. I had to go with an overall thinner dimension to bring my shaft to diff angle within the specs of the later model 129 flex disks, which I am using in my 107 P/N A 230 410 01 15. The 129 requires an alignment of 10' to 50' . That's 0.17 to 0.83 degrees. I cant find specs for the 107 but both of mine measured out to ~1.5 degrees. The stock 107 has about 12mm thinner and therefor allowing more angular tolerance.

I assume these work with the 126 as well.

In any case for me to bring my angle down to ~1/2° I will have to grind down the center section and go with 3/16 rubber in between.

I also performed a temperature survey on the gearbox driving 65mph for 3 straight hours. Temp never exceeded 160° F in the vicinity of the pinion bearing. It was 50° F ambient for that test so temps as high at 200° F are to be expected and higher when climbing hills. but that pint is far enough from the mount to never see that temp, meaning the shore 60 Buna N rubber is fine for the application. I have been warned that using shor 80 Poly urethane will make the cabin noise excessively loud.

I made 5 sets of these and will be keeping two sets so I have 3 sets available at $50. a set for anyone interested.