I see the white “T” and assume that is it. That vacuum line goes to the Fuel Rail Pressure Sensor, FRPS for short.
That is not a good location to install boost gauge. If there is a vacuum leak it can cause the FRPS to send incorrect info to ECM and damage engine. I am not familiar with the Saleen unit, but on my Edelbrock, I was getting a 20% higher reading from the FRPS line than actual boost because it read pressure before intercooler. And there is always a drop I. Boost after the intercooler because of temperatures difference and air flow restrictions. Why you are reading lower, I don’t know. You could have a bad boost sensor. The first one I had from Areoforce did not work correctly. Here is the warning from Edelbrock about tapping into the FRPS line.
CAUTION: Never cut into the vacuum lines leading to the fuel rail pressure sensor and bypass actuator, on the driver's side of the manifold, for the purpose of tapping in a boost gauge. Interruption of the vacuum signal to the fuel rail pressure sensor can affect the fuel pressure reading to the PCM, which can result in engine failure! Furthermore, this port reads pressure before the intercooler, and therefore is before the inherent intercooler pressure drop. Readings from this port will always be approx. 20% higher then what the engine actually sees.
