In addition to the "performance" aspect, there's also a service/functional aspect to using one of Greazzer's overflow valves. On the later vehicles (60x engines) the overflow valve has a fairly weak spring and a plastic element inside of it. When (not if) the plastic element warps or breaks, the fuel pressure in the fuel rack drops at low RPMs and the "check valve" function of the overflow valve is lost. The result is (or can be) rough idle, poor low-RPM power, or rough starts from cold when sat.

I had the latter issue with my SDL. A new overflow valve from the dealer with tax (or ordered online with postage) is roughly what Greazzer is charging for the older style valve that gives a positive seal and can be serviced in the future when it gums up or gets spring fatigue. Since I can't seem to find one of the valves to save my life, the choice to pay the dealer or Greazzer was a no-brainer. My car is much happier in the mornings, no more craptastic cold starts.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]