This is the factory OEM Mitsubishi boost controller solenoid located on the solenoid pack on the passenger side of the firewall. It is the driver-side-most solenoid with 2 vacuum hoses going to the top and the wiring plug pointing down. If you want to keep things stock, then this is your choice.
The "Free Boost Mod"
Long referred to as the "free boost mod", there is a simple trick to add a little boost to you VR4 or Twin Turbo. The lower of the two vacuum nipples on the boost solenoid has a small rubber insert in the nipple reducing the size of the opening. Simply remove that little rubber insert and presto! you'll have turned up your factory boost level! Follow the arrow in the image below.
Now this is reported to only be true for 1st generation VR4/TT cars, but the solenoid as it comes new from Mitsubishi has this little reducer in the vacuum nipple, so you can simply remove it prior to installation in your car.
NOTE: You are assuming any risk by doing this modification. There is NO RETURN OR EXCHANGE on this electronic part.
This fits 91-99 3000GT VR4 and 91-96 Stealth Twin Turbo.