You'll almost never see an adjustable pressure regulator on the CIS Mercedes cars. They have a system pressure, a control pressure, and a delivery pressure. Simply raising or lowering the system pressure isn't necessarily going to yield beneficial changes in either the control or delivery pressures. Having said that, I'm not going to say that there isn't any possibility of tuning with CIS pressures. I am going to say that there is a complex relationship between the three that needs to be understood before any meaningful tuning can begin.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff