Rear axle gear change from 4,08 to 3,27

[Peter]

I have a 280 SL automatic with a rear axle 4,08.

I’m considering changing the 3,27 to bring down substantially the rpm. Question if of course if the 280 SL engine with automatic gear can handle this. 
Does anyone have experience with this?

Thanks, Peter Brunklaus 

[cfm65@me.com]

Hi Peter,
I think that is a big step down for an auto, especially because it pulls off in 2nd gear. I made the mistake with my E Type, cant remember the numbers, but I dropped about 500 rpm in top at about 120 km. It is really nice at cruise but too low for a nice crisp pull away. And remember the E Type is a 4.2 which was said to delivered 220+hp😉 (Take that with a pinch of salt)
I have a 3.69 in my 250 SL
Regards
Chris
Cape Town

[rgafitanu@gmail.com]

Hello Peter,
I did this exact swap a year ago and it is probably the best thing I did to the car. My case was a little worse, I have a W111 which is about 300 Kg heavier than the W113 with a 230 engine. I had the 4.08 and I can say that acceleration from 0 was no sharp pull like in my R230. I can feel no difference in the acceleration however, highway driving is a dream. Especially when passing no-brain youths in their mutilated Subaru's WRX or Civic's at 85 MPH (136 Km/h) in a 50 years old MB. I ended up selling the 4.08 to a W113 owner who wanted to restore his to specs so I got all my expenses back.
The only inconvenience is the fact that gear change 3-4 and back happens now around 35-40 MPH (60 Km/h) which is the speed that I am forced to do in town behind blind senior citizens and soccer moms. The car appears to be undecided on the gear. I blame it on the slow drivers in front of me anyway.
In my opinion do it, it's worth it. Other members here have done it before and there is a lot of info on the forum especially the problems that you will run into. Like the breather tube that you will have to replace with flexible. It is something that you can do in a weekend.
Good luck.

Radu

[UJJ]

I do not have an auto - gearbox but a 4 speed manual. I changed the diff to 3.27 10 years ago and it was one of the best things I have done to my 280 SL. I live in Duluth MN and the city is located on a fairly steep hill. But no problems getting up (and down) with this diff. I enjoy driving long distances which includes driving on freeways. 80 mph (130 km) and 3,500 rpm is just perfect.
Urban

[cfm65@me.com]

Radu,
Is your W111 manual or auto?
Chris

[rgafitanu@gmail.com]

Chris,
It's an automatic. The W108 that I got it from was automatic too.

Radu

[Cees Klumper]

I changed from 4.08 to 3.69 and like that a lot. I would not go all the way down to a 3.27 if I were you, these engines are designed to rev high.

[rgafitanu@gmail.com]

I have seen this "engines designed to rev high" statement several times in this forum. I remember from my machine design university classes that rpm is limited by the process of dynamically balancing a non-symmetrical system such as pistons-rods-snakelike shaft. Centrifugal forces make minute mass imbalances huge vibrations. Also the capacity of the gas-air mixture and exhaust to do their travel at high cycle rates, but that's not the case here at 5000 rpm. On the other hand you need high rpm for more power and speed.
I believe that the germans mastered the art of balancing in such a way that higher revs would not be a problem like before. I can say that my 230 engine feels better at 65 mph and 2800 rpm than 3500 rpm and I have a lot of reserve power to accelerate. I think I read here that MB offered initially the 4.08 ratio in US because of the 65 mph federal speed limit (they took it literally), while in Germany there was no speed limit on the highway where they offered lower ratios.

[cfm65@me.com]

Well Peter,
There you go. It seems that a 3.27 will work in your 280SL and Radu says his 3.27 diff came out of a 108. Sorry gents, I was mistaken, believing that this combination would be too low in an auto.
Regards
Chris

[Benz Dr.]

A 230SL auto is a poor choice for a 3.27 while a standard might be just capable.

 Everyone seems to be forgetting about engine torque which is maybe 150 ft lbs on a 230SL and really not suitable for starting off in second gear. My own car was 4.08 which is the standard ratio for a 5 speed. I changed to a 3.92 LSD unit and find that gearing to be perfect. My engine easily runs within the torque band in any gear without really spinning too high.

 Even the usual 3.75 gearing that I find in most 230SL's really starts to affect performance. You won't feel it that much in the lower gears but you will in fourth or high gear. Once you are up to road speed, meaning about 3,000 RPM, then axle gearing won't matter as much and you will only see different RPM's or engine noise relative to axle ratio. Lowering engine speed after you obtain a high cruising speed is a much different thing than starting out 500 - 600 RPM lower in all gears. That's why over drive transmissions are so good at delivering performance and economy at the same time.     

[rgafitanu@gmail.com]

Dan,
I didn't forget about the second gear start. It is a little bit more sluggish than before but I chose to live with it for the benefit of high speed. It wasn't a rocket with the 4.08 either. The fluid coupler has to work harder at start I think. My firewall is original and the original insulation is not keeping any noise away anymore. The engine at 4000 rpm is quite loud inside.
What is interesting is that half the time my transmission starts in first gear for a second or so. The linkages may not be right or the modulator solenoid is lazy.

[Peter]

Dear Gents,

Thanks for your experiences and views, this will help to take the right decisions.

Peter Brunklaus

[Howard Long]

I have a 3.27 ready to install when I put my 280SL 4-speed back together.  I think that will be fine especially since 1st gear in the 4 speed isa really a lower gear than you need for the 4.08.  The engine noise with the 4.08 on the freeway was just too much for me.

I know that the automatic starts in 2nd gear in the Drive position, I assume that you can select 1st gear if you have a very steep grade.

I am just an old hot rodder and I wonder if anyone has modified their automatic to start in first gear in Drive to be more compatible with the 3.27 axle. Sounds like a thing to try if I had an automatic.

Howard
280SL 4-speed

[rgafitanu@gmail.com]

Gernold in NH (I think) had a solution with a switch in parallel to the full throttle/downshift switch to make it start in first but you would have to manually switch it back every time to allow it to upshift further. Being a fully mechanical car (almost no electric controls) it's not easy to modify it.

Radu

[Benz Dr.]

Something I thought about a long time ago:  a 4.5 is about 3,800 lbs and a 113 is about 3,000 lbs. A 4.5 is about 230 HP and a 280SL is about 180 HP. If you do the math, and I did back when I did this mod many years ago, you will find that the power to weight ratio is fairly close in numbers for these two cars. So, the 280 has the HP to move the car but is lacking slightly in torque; the real  number that gets you there quickly.
This is why a 280SL  four speed is your best choice  for the 3.27 and an auto 230SL is your poorest choice, in my humble opinion.

[naniroma0]

Gernold in NH (I think) had a solution with a switch in parallel to the full throttle/downshift switch to make it start in first but you would have to manually switch it back every time to allow it to upshift further. Being a fully mechanical car (almost no electric controls) it's not easy to modify it.

Radu

Hello,
Where can I look for this mechanism to start the gearbox in 1st?

[rgafitanu@gmail.com]

Hello,
It is not a mechanism but a switch. This switch bypasses the kick down switch under the accelerator pedal that you mount on the dash. You should be using a momentary switch that you hold down for as long as you need the 1st gear. I found that starting with pedal to the floor engaging the kick down switch is easy and now I got used to it.
Gernold's website in NH is www.sltechw113.com
Radu

[naniroma0]

Hello,
I have the 3.27 to install, and it will be necessary to place the swich on the board
Thank you Radu

[nirmal]

I had a 4.08 axle in my 280sl automatic.

I switched over to a 3.27 axle about 2 years back. The car cruises like a dream as the engine is lower as is the sound level.

I manually start it on a lower gear and move it up as it accelerates.

The best choice would be a 3.69 axle for the automatic.

Nirmal
1969 280sl auto

[ctaylor738]

I have a 280SL automatic converted to the 3.27, and it's a mixed blessing.  Great on the highway, but slow as a snail in stop and go driving.  I would only convert if you plan to do a lot of long distance driving.

You will also have to remove the circulation tube, and plug the holes.  In some cars, this has caused the gear oil to leak out the vent.

I have Gernold's magic button installed, and it works great.  It drops the transmission one gear when pushed, after which the trans will upshift depending on the throttle position.  It doesn't need to be held down.  It's nice because you can get a boost without flooring the throttle.

And, I sold my 4.08 to Dr. Dan. I believe it's now in his Red Rocket.

[Peter]

where can the circulation tube be found exactly?

[Peter]

I understand that  the  “Gernold's magic button”  is a same as the kick down button but then manually operated, is that correct?  Any special adjustments?

[ctaylor738]

No adjustments needed for the magic button.  On my car, it's installed on the front of the center console and I can easily reach it with my right hand.

IIRC, the tube runs from side-to-side across the top of the "pumpkin" and it hits the frame on the 113.

[Peter]

I did the change from 4,08 to 3,27 and have now several month experience.

I must say, a good improvement without losing much acceleration. Please note that I have a 280SL. For 250/230 I don’t know if the acceleration loss is manageable. 

Frank and Leon van Dijk, classic Mercedes Benz specialist near Amsterdam is doing this change several times a year. The experience is that it is important that the engine is “healthy”.

[Benz Dr.]

I have a 280SL automatic converted to the 3.27, and it's a mixed blessing.  Great on the highway, but slow as a snail in stop and go driving.  I would only convert if you plan to do a lot of long distance driving.

You will also have to remove the circulation tube, and plug the holes.  In some cars, this has caused the gear oil to leak out the vent.

I have Gernold's magic button installed, and it works great.  It drops the transmission one gear when pushed, after which the trans will up shift depending on the throttle position.  It doesn't need to be held down.  It's nice because you can get a boost without flooring the throttle.

And, I sold my 4.08 to Dr. Dan. I believe it's now in his Red Rocket.

You did. It's a 3.92 and it works very well especially coming out of a corner when the LSD kicks in.