Originally Posted by BillGrissom

On first read, this sounded like the ALDA was a factor. On more thought, it seems you proved the ALDA was not limiting boost until ~13 psig. To clarify for others, the ALDA limits fuel to avoid over-boost. The waste gate does also, but by bypassing exhaust around the turbo. By removing the turbo's "pressure line", mach4 disabled the bypass. The higher 13 psi limit was then due to the ALDA becoming active. Thus, the lower 9 psi limit must have been from the bypass opening early (weak spring).