Diff Ratio

[RAY]

I have joined the 3.46 club. She was a 4.08 but i wanted more of a quieter cruiser and that is exactly what i now have, in my opinion.
Still very acceptable on hills and pulling away, just put her into 2nd ( 1st ) if I'm pulling away on a steeper hill.
Auto trans, now does 60 mph @ 3100 rpm according to two different sat navs, perfect. Thank you to Colin Ferns for supplying and fitting the new axel, great job.
Ray

[JamesL]

Ray
Our cars probably said hello to each other : Having new springs and the engine serviced for the first time post-rebuild.

Mine runs at those revs and I cannot imagine going "back"

[RAY]

Hi James, might you make it to the Bromley Pageant of Motoring this year or any one else for that matter, half a dozen Pagodas there last year on the Mercedes stand. Good fun day out.

[TheEngineer]

Join the club! I put in a 3.27 axle about 10 years ago and after a few years sold my 4.08 axle on e-bay. I am very satisfied, it makes a nice driving car and we have many hills and steep roads here in Seattle. There is a steep road near my house leading down to the beach and to Starbucks. We drive there almost every day and it is always fun to chase the car up this winding and steep road. It is almost like in Switzerland in places where they do the mountain races. I leave it in 2nd, otherwise it shifts up. I have a small oil loss and every 5 years I check the oil level. Maybe, when I  feel like it, I'll do something with the breather someday. Have fun!

[Benz Dr.]

I would check it at the start of every season. 5 years seems a bit long to me.

[RAY]

Re - TheEngineer.  I was going to go for the 3.27 and then chickened out and settled for the 3.46. I wasn't quite sure if it would be just a step to far for an auto but you seem very happy with yours. Do you know what your revs are at 60mph just so i know what i could have had. Plan to head over to the Alps next year so i guess i,l be doing as you do !

[Flyair]

Ray
Les Alpes next year? Did you get any intelligence about the next year Pagoda Euro Rallye 2018

[RAY]

Hi Stan, no intel i,m afraid, its just a coincidence, i,m hoping to go in the early spring as well as the Rallye.
Best
Ray

[TheEngineer]

Marilyn: at 100 km/hr (62.5 Mph) my engine turns 3100 rim indicated. I have an automatic and this is on level ground.

[Benz Dr.]

That's right where my car runs with the ZF 5 speed and a 3.92 axle. I'd say it's just right.

[Harry]

So - how is the near gear ratio accomplished?  Did you rebuild your rear end to install the ring and pinion or did you just swap out a replacement rear end with the desired ratio?  I wonder if the ratio is stamped or otherwise indicated on the differential housing?

[Benz Dr.]

5 speed cars came standard with a 4.08 axle which will give a 3.46 final drive so I changed mine to a 3.92 limited slip with disc brakes. The slighter higher gear ratio along with 5th gear gives me something very close to 3.33 final dive.

[PIP1947]

Hi Marylin and The Engineer. I read your April postings with great interest. My January '68 280SL auto was a UK delivery car (York) with, what I have always assumed, is a 3.92 diff. The drivers handbook states the auto with 3.92 diff as having a top speed of 121 mph. At 6500 rpm, this translates to 18.6 mph per 1000 rpm in fourth (6.5 x 18.6 = 121 mph). This then translates to 3200 rpm @ 60mph and 3350 rpm @ 62.5 mph and is reflected exactly in my speedo and tachometer readings. What I don't understand, therefore, is why Marylin's 3.46 diff is only giving her 60 mph @ 3100 rpm? A saving of only 100 rpm over my 3.92 diff. The Engineer is getting 62.5 mph @3100, compared to my 3350 rpm. The differences seem to me to be too small. What am I missing? Why does the 3.27 diff give only 20.1 mph x 1000 rpm, a mere 1.5 mph advantage over the 3.92 diff, which gives 18.6? I have been considering looking for a 3.46 diff, but I would want a bigger benefit before incurring the expense involved. Regards
Pip1947
'68 280 SL
Tunis Beige Metallic/Dark Brown
Sydney

[66andBlue]

Take a look at the graph here: https://www.sl113.org/forums/index.php?topic=18239

[PIP1947]

Hi 66and Blue. Exactly. Your graph seems to illustrate my point. Marylin's new 3.46 diff should be giving 60 mph at 2750 rpm, not 3100 rpm! Similarly, The Engineer should be getting 62.5 mph at around 2700 rpm, not 3100 rpm. Your,graph is very helpful and convinces me to start looking for a 3.27 diff. One query though, what is the problem with breather pipes? Cheers
Pip1947

[A Dalton]

 Speedo  head is factory calibrated to the diff  ratio, so using it to determine speed with a rear change is incorrect....same goes for tire diam changes.

One must recal speedo when changing ratios.

The chart is correct, mathwise.

Look in parts book and you will see the different speedo part # for  different factory ratios.

[Shvegel]

Pip 1947,
The axle in your car has the breather located on a flat spot (Boss) on the left axle tube.  The 3.27 has the breather located on the center casting at the top.  The 3.27 axle was designed to be used in higher powered V8 engine cars so it was fitted with an oil cooler.  this was a steel tube that had oil thrown into the top of it by the spinning ring gear when the car war rolling down the road and the oil ran down to the bottom of the tube and back in to the bottom of the center housing.  The oil cooler doesn't fit in our chassis and really isn't needed with our lighter and less powerful engines so it is simply removed and the holes plugged with 2 engine oil drain plugs and copper washers.  The problem with all that is that the oil path off the ring gear and into the tube is now blocked and some of the oil being flung up into the top of of the axle gets pushed up and out the breather causing an oil leak.  The solution to the problem is to remove the left axle housing and add the threaded hole,  switch the breather to the left axle housing and plug the hole at the top of the center housing with a threaded plug. 

[PIP1947]

Dear Shvegel
Many thanks for this comprehensive explanation, all of which makes perfect sense. From what you say, I take it the problem only exists with the very high ratio diffs, as designed for V8 powered cars. Perhaps, therefore, installation of the 3.46 diff is simpler than the 3.27. Either way, modification of the 3.27 diff does not sound like a particularly difficult task. Thanks again.
Pip1947

[RAY]

Like i said, the two 60 mph readings were taken from two different sat navs and both said the same when checked against the rpm.
Having said that i am aware that the speedo needs to be recalibrated as it appears its 10mph out at that speed, showing 50mph when the sat nav true speed is 60mph, but that will be a winter job.
Can only tell it like it is.
Ray

[A Dalton]

 et al ,

 Using a tire diam of 25.5", these are rounded out  MPH @ 1 K RPM

 3,27 - 23 mph
 3,46 -22 mph
 3,92 - 19 mph.

So @ 3K, one should  see a  gain of 9 mph going form a 3,92 to a 3,46...and a 12 mph gain with the 3,27.

I like to break it down to 1K so you can drive with the tach and know your mph w/o speedo change/cal.
Same goes for Euro  speedo cars

If one wants to  get fussy, you can determine the tire diam exactly by loading car [ as suggested in the manual] , checking  for your normally used tire PSI in the rear and measure the Rolling Circumference..then divide by pi*
This is the exact real  driving tire diam , as it allows for tire wear.,chassis load, and tire PSI.   This is easily done by marking the tire and the pavement with chalk [ making sure this mark is coming thru the axle C/L ] , and rolling the car foward one complete rev, remark the pavement and measure the distance between the 2 marks..That is real  loaded circumference..do the pi math and then use that diam in your calculations.
A little over the top, but, why not ..............

http://www.bgsoflex.com/rpmmph.html

If one does not see gains close to these with a diff. change, even using GPS, then the reason may be a wacky tach, slipping clutch/TC, or the infamous loose flange nut ,  The speedo pinion is a  clamp/friction drive design , being sandwiched on the tailshaft, If the nut is loose , the pinion slips  causing the speedo to be incorrect
. A simple test for this condition is to watch the speedo at lower gears..if it bounces when excell/decell,then the suspect is loose flange nut,,,very common on 113's.


PS
I use 25.5  in the example b/c stock factory tires were SR profile, which equates to 25.65" diam,
whereas a replacement with 195/75  would be 25.52  [ close as you can get in replacement sizing]


A Dalton
BenzTechs