@billhon
Changing the 4.09 to a 3.27-3.69 rear axle is fairly well traveled modification, and there is a LOT of information on the forums.
I am starting this project on my 4-speed, having assembled the components including a 3.27 rear-end. This modification has proved successful over many years for 4-speed and automatic transmission w113s - thought the automatic can gain from a forced first gear switch in some case (different topic). I note a recent post of the SL market place values wherein it was stated that the 3.27 rear axle adds a decent amount of value to the vehicle. The following is what I have generally gathered about this topic:
1. Determine what you ratio have currently (just to be sure) - its is stamped on the left side of the differential casting forward of the driveshafts.
2. The most common modification seems to be a swap for a 3.27 axle from a sedan, as these still have some availability. This is generally a full rear end swap, end extensively discussed on the forums - I suggest you search "3.27" - let us know if you have no luck in the search.
3. Some members have swapped the ring and pinion in the differential with brand new items, but that takes a lot of experience to get right, and probably challenging to get aligned perfectly, even for a good mechanic not specifically specialized .
4. The rear-end swap is an opportunity to refurbish the axle, new boot, seals, etc. whilst it is out.
5. ** The rear end swap from a sedan comes with some key items to be addressed:
a) Use the same w113 compensator spring (that is straightforward).
b) Use the w113 parking brake cables (that is straightforward).
c) The 3.27 diff has a different vent tube location (on top on main housing rather, than on driveshaft side as in w113), there are implication from this, see below.
d) There is a rigid oil U-pipe across the top/front of the sedan 3.27 differential (most common) that interferes with the w113 bodywork, likely there for additional cooling from the higher torque V8 sedan. The consensus is that it not really needed for the lighter w113.
Options to address this include:
- i) Simply remove the rigid pipe and plug the openings - however this possibly/probably introduces very serious oil splash issues from the vent pipe 5.(c) in a significant proportion of installations (but note some members have had not problems with this approach after many years of use);
- ii) Remove the rigid U-pipe and plug the openings, but then relocate the top oil vent in 5.(c) via modest machining to the w113 location (it has been suggested this is perhaps best suited to a more comprehensive teardown due to the risk of shavings in the diff). This addresses the issue of 5.d(i): @Dr Benz developed this solution, and it is probably the cleanest non-invasive approach overall
[see
https://www.sl113.org/forums/index.php?topic=9633.25] ;
- iii) Replace the rigid U-pipe with a flexible hose (suitable rubber or braided steel have been used) that then sits cleanly away from the bodywork. This is a straightforward and effective solution, assuming you can get the hose fabricated somewhere;
- iv) Retain the rigid U-pipe, but cut off its middle section and replace with flexible hose - this was proposed, but not sure if it actually proven in practice - easier than getting hose fabricated in 5.d(iii);
- v) Modify/bend/mold the (heavy gauge) rigid pipe so that is clears the w113 bodywork - this is a trial and error approach it seems, and likely requires a lot of lift the new axle in and out. It is not clear to me (yet) whether this is a viable approach. (A 3D model of a successfully modified rigid pipe would be great of course!);
- vi) modify the bodywork of the w113 to make space for the rigid hose. Mechanically this is the lowest risk, but does involve "permanent" changes to your w113. Many very experienced members have done this, and @stickandrudderman posted that this is his preferred approach.
To be honest, I have not finalized on which approach I will take with respect the rigid tube. It will likely be 5.d(iii) as I am not planning to disassemble enough to be comfortable with 5.d(ii) - but maybe others will advise me differently.
