Hello,
The engines in the sedans and SL's both have the same compression. They are designed to run on high test fuels. All the main componants including pistons, crankshaft, connecting rods, oil pan, cylinder head, cylinder block, intake manifold, exhaust manifolds, even injection pumps and much more are exactly the same. The 230-SL is the exception it's engine is unique to the SL. It does share some parts with some sedans but the cylinder head, intake and injection are completely unique to the 230-SL.
The different horsepower rating of the 280 "SL" engine over a 280SE sedan engine is probably due to a slightly "hotter" camshaft and the different distributor used in the SL.
The engine motor mount alluminum supports are different from sedans. The SL engine sits about an inch lower due to these supports.
Prior to 1969 most Mercedes automatics in sedans and SL's used the "K4C025"
When Mercedes upgraded to the newer, smoother automatic transmissions in the sedans in 1969, they did not upgrade in the SL's. The SL's kept the older automatic until the end of its production. The engine/transmission match plate is different on the early and late automatics. You can easily switch these engine/transmission match plates. You will have to switch the automatic flywheels on the later 280-SE sedan engines. The standard shift flywheels are the same.
There may be some minor plumbing and accessory differences but these engines are mostly the same in their series. Always think of the series (group)of sedan and SL engines in this way: M127 (230SL= unique), M129 (250SL=250SE), M130(early 280SL=early 280SE), M130(late 280SL=late 280SE).
Note the two different "early and late" 280 series engines. These are different animals with the heads and cylinder blocks being different and not interchangable. However they do share the same crankshaft, pistons, rods, flywheels etc.
The physical size and configuration of all these engines is basically the same so if you are not too picky almost any sedan or SL injected engine will work in your car.
Joe Alexander
Blacklick, Ohio