3.27 Diff and engine idle: observations on driving style

[awolff280sl]

My 280SL has a 3.27 diff and 4 speed shift.
It has never been in a better state of tuning (timing, AFR, WRD shut off etc.)
What I want to describe is this: the car needs to be driven as if it has its original diff in it for the idle to come to rest at 800. In other words, high revs, 4000-5000 before shifting. The car will idle at 800 when driven that way.
When I drive it for a while in a "quiet" way and shift below 3500, the car will come to idle at 500. The other day, after an hour on the highway at about 75mph (+/- 3300 rpm), when I pulled off on the exit and came to a stop, the idle was 300. Then a few shifts under load up 4500rpm had the idle back to 800.
I had previously been chasing my idle around, but I've come to the conclusion that I needed to adjust my driving style to a 3.27 diff.
Any thoughts?
 

[Benz Dr.]

I think your distributor is hanging up and the flyweights are not returning properly. Also, the drive gears could be worn and you will never get a proper idle speed if that's your problem.

[awolff280sl]

thanks Doc, but I'm using a 123 distributor

[Benz Dr.]

In that case I would look at the drive gears.

[ja17]

Didn't someone else on the site have this problem not long ago?  I think that they found that the fuel dampner had a bad diaphragm.

[awolff280sl]

Hi Joe, I did have a bad fuel damper diaphragm that I replaced.
Before replacing it, the car would often idle too low after rev-ing under load.
Now that it's been replaced, when I come off the accelerator the revs will drop quickly to about 1200 at which point the linkage hits the throttle damper. Then the revs ease down to below idle, but then quickly come back up to idle.
However, whether the revs come all the way back up to 800 often depends on my driving style. If I drive shifting under 3000 for a time, the car starts idling at 500. If I then drive in a more "Italian" style, the car idles at 800 again.
I will say that I can't tell the difference between the two idle speeds other than by looking at the tach.   
   

[nirmal]

I finally got my car back from the garage with the 3.27 diff. There is a huge improvement and the car is now smooth as silk. With the 4.08 diff the engine noise was annoying. I don't know what will be the effect on the speedo reading with the new diff.

Nirmal
Dubai

[Eminent]

You mean that it's more relax drving at higher speed?
My sedan is also waiting for a 3.27 and till then i still have to do it with a 4.08...
It is difficult to find a place where i can swap it. Do it at home on the garagefloor is not so good idea i think.

[jeffc280sl]

Hi Andy,

Glad you solved the problem.  I could not see how the 3.27 changed anything with idle rpm.  3.27 axles are the way to go with manual transmissions.  Noise improvement is nice also.

Br,

Jeff

[billhon]

It seems a consensus on this topic that a 3.27 is superior to a 4.08. I've heard there were also 3.60s and 3.71's installed. Why did Mercedes offer so many different ratios? Can I convert a 4.08 on my 1968 285sl ? There doesn't seem to be any lower ratio axels available, especially 3.60's and 3.71's. If I do find one what is the average cost to install it?

[Garry]

It all had to do with which market they were selling into.  Some markets did not like the high revving. USA unfortunately got the 4.08 and most changes have been made there since.
 I had a 3.46 in my original 280SL and that seemed to be a good compromise rather than to going to the 3.27 that some say is too low for the engine that liked to rev freely.

As my years of Pagoda ownership has gone on, I have come to like the note to the higher revs of my 230SL with the 4.08 diff.

[tel76]

Hello Garry,
Have a look your first paragraph you may want to re-asses what you have written.

Eric, needed another beer.  Totally A about

[rwmastel]

The 3.75:1 diff in my 230SL (originally delivered to Italy) seems fine.  Yes, the engine revs a bit high, but really only if you're above the local highway speed limits.  (Local speed limits are 65mph, but recently 70mph on some interstates).  Most people with the more powerful 280SL engine like their 3.27:1 ratio differentials.

Did the original poster (awolff280sl) ever find the root cause of the different idle speeds?

[awolff280sl]

Rodd, thanks for reminding me,
I re-did all my throttle linkages, and I ended up "tightening" the closure the throttle butterfly by an additional 1mm or so just to make sure it was fully closed at idle. I had originally set the butterfly to the "just sticking" point, as described (by Joe A. I think). Now it's just a tad more than "just sticking". This seems to have done the trick. I think that temperature was affecting complete closure of the butterfly because it was set too close.