For anybody that's interested. I replaced my 95 SL500 in March this year with a 96 SL600. I was planning to do a bit of tuning on the 500 but when I drove the 600 I was completely smitten - so change of plans. Anyway I decided that I needed to hear the engine, so the first thing to go was the standard exhaust. I had a custom system made here in Johannesburg by a specialist company (TNT Performance Exhausts), but nothing too extreme as I still wanted to be able to drive on the highways in relative peace. Then I added a set of K&N air filters, and finally had the ECU upgraded using a pair of Unichips with custom mappings by RG Motorsport. I am a very happy person with the results. I think MB was very conservative with their SL600 mappings as they needed to differentiate between it and the SL60. My wheel HP went from 268 to 311 - equating to about 450hp at the flywheel at sea level. Before I get flamed for my calculations, we are at an altitude of around 5500ft here in Johannesburg, resulting in an approx 18% power loss for normally aspirated engines.
Anyway the results on the road are better than I hoped for. The car sounds great and feels so much faster. I not going to say it has been transformed because I thought it was brilliant before, but now it's just so much better.
Nice, what did you do with the exhaust specifically? Got any pics?
Also, I completely agree with you that the stock tune on the SL600 leaves a lot to be desired. The a/f ratios can be richened by almost a full point at WOT, and can be leaned out half a point or so during most of the rest of the range, without causing any problems. The timing is also very retarded, and you can advance it significantly with no issues as long as you use premium fuel.
Stuartknight gained 40hp just from an ECU tune, which sounds to be about what you gained as well. You have to remember, in German-market trim, the factory power rating for the M120 was 410hp, and the actual real-world number was closer to 440hp. And that's stock. Beginning in 1994, and for all US models from inception forward, they de-tuned it to 389hp to comply with emissions regulations.
So yeah, getting it back to where it was supposed to be is a good thing.