[W126 Coupe] 2.82 Rear Diff for 84 500SEC

[Divov]

I've gone from 2.24 (stock 500SEC) to 3.27
This is fairly radical.
That means I only do 68.5% of the speed at a given RPM compared to the stock set-up.
My stock 500 indicated 160km/h @ 2900rpm. Now at 2900rpm I'm only going to do a 'calculated' indicated 109.km/h.
However, that's about par for other Mickey mouse cars - generally they gear to do 100 - 110 kph @ 3000rpm.
Now, on the rev limiter @ 6000 rpm, I'm going to be doing a "calculated" indicated  +- 225km/h. (probably a true 205 - 210km/h)
For short track racing, this set up is ideal for me and next Saturday we race and I'm itching to see what happens to my lap times.
Incidentally, running with my GPS in my car prior to the diff modification, I was disappointed at how inaccurate the SEC's speedo is.
It lies like hell!! - over-reads maybe 10%. 
Most cars speedo's lie but I would have expected better from Mercedes....
Alec D.
P.S. for Dawid Loubser - phone me to arrange to collect tickets for the race on Saturday @ 082 773 4287 - or I can leave them at the race-track gate in an envelope for you. (2 tickets)
Alec D.
  ----- Original Message ----- 
  From: Jonathan 
  To: Mercedes Coupes Mailing Lists 
  Sent: Sunday, July 22, 2007 7:58 PM
  Subject: Re: [W126 Coupe] 2.82 Rear Diff for 84 500SEC


  Interesting, as when driving around on a 2.82 I find the cars RPM on the highway livable, but it makes me less inclined to want to do serious highway driving.  I find the 2.82 makes a Large difference over the stock 2.47 acceleration wise, very perceptible IMHO, esp. when you have the opportunity to swap back and forth from stock to modified.

  Here are some plots using Dan's MPHvsRPM Excel chart:

  http://blueridgemb.com/stockdiffrpm.jpg

  http://blueridgemb.com/307diffrpm.jpg

  Now if I could run Eddie's Gear Vendors or once we get the 5/6sp conversion done I'll be all for a 3.07, or even 3.47:) 

  Jonathan


  Gerry Van Zandt wrote: 
    The 3.07 isn't all that bad in highway driving. At 80 MPH you're doing EXACTLY 3,500-3,550 RPM; at 65 it runs around 3,000 RPM.  Redline for a 560 is what, 6K or a little higher -- there's plenty of headroom.  I drive my 89 560SEC from Portland up to Seattle quite regularly (a 2.5 hour trip) and it's really not a problem.  


    I'm still getting upwards of 15 MPG average per tankful.  The last tankful, all highway driving, was 17.59 MPG with the 3.07.  16 and 17 MPG for tankfuls with all-freeway driving are not at all uncommon.


    The benefit at low speeds and off the line far outweighs the engine running 700 RPM higher at 80 MPH, compared to a stock 2.47 car.


    An 84 500SEC or a 380SEC is a direct bolt-up for a 3.07 from a sedan. The later cars (560s) require some slight mods -- the 3.07 gearset must be placed in a 560 pumpkin.  You do have to calibrate the speedo with an adjustment box.  Mine was about 10-12 MPH off.  This is easy to do and you can tuck it behind the instrument cluster back in the dash.


    A 2.82 is a good compromise in terms of ratio, but it's not enough of a difference, IMHO, to warrant the time and cost of doing a replacement.  The 3.07 is a large jump and the results are dramatic.  If one is doing the swap for better off the line quickness and such, I'd just go whole hog.  Those of us who have done the switch wouldn't go back.


    Cheers,
    Gerry






    On Jul 20, 2007, at 9:15 AM, mbcoupes-request at mbcoupes.com wrote:





      As an aside, if you do a Lot of highway driving you might find the 3.07

      from a 450 a bit Busy...granted it will feel Great off the line though!




      Jonathan






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