The early 230SL model produces more HP per liter than a 250 or a 280SL. I always wondered how this was done and looked into it a bit when I was building my engine. It came down to three main things as far as I could discern. Namely; ignition timing, valve timing, and exhaust flow.
1) Early 230SL's have vacuum advance distributors that have a very fast advance curve that is almost straight up when viewed on a graph. No other distributor advances as quickly as this unit during the time period they were produced - I was fairly sure this had some effect on performance.
2) 230SL's also have fairly hot cam timing. The intake valve opens around 12 degrees BTDC and only the 280SL euro cam comes close to this timing.
3) Probably the greatest performance attribute of the 230SL is the tuned header system. Driving two similar 230SL's, one with, and one without header pipes, it becomes clear which is the better performer. It's not hard to tell which one has the headers as it produces quite a bit more noise coming out of the engine bay.
One can imagine that the marketing department at MB had some big shoes to fill after the last 300SL rolled off the line. The new 230SL would have been regarded or disregarded based on many different bench marks and one of those items would have been sound. The engine sounds powerful even if it isn't. The new car had to at least look the part and I think it did it well enough. Once the buying public became used to the new model, the factory began to tone the performance down a bit in the interests of making mas produced parts at a greater profit. Header pipes are not cheap to make relative to cast iron manifolds.
When I built my engine, I looked at all of these things, and more, to compile the best that was on offer over the entire range of the 113 build. I used a 250SE engine, 230SL header pipes, 22mm sway bar from an early car, 230SL cam advanced to 12.5 degrees, dual point distributor from a 300SE with VA throttle body, matched and ported cylinder head, balanced engine, 280SL clutch and pressure plate, oil cooler from a 250S, visco fan clutch, 280SL ribbed oil pan with windage tray, 5 speed ZF trans, 82 liter fuel tank, red ignition coil, 280SL seats, and limited slip 3.92 rear axel. There are no modern parts in my car and everything is vintage from the building period '63 to '71.
My car will go 10 KM on one liter of fuel and it will do 200 KMPH. It's as fast as most 280SL's but uses a lot less fuel.