The Subaru Twin Turbo system is a bit of a black art and few people really understand what’s going on, but there are a few little tweaks and mods that can be done to make it all a bit better.
One of the more common mods for the BE/BH Legacy is the Hose 10 Mod. Long story short, the standard vacuum layout has a vacuum line that vents the factory BOV (Blow Off Valve) line into the pre-turbo intake pipe. Its an odd setup, as in theory the BOV isn’t seeing as much boost or vacuum as it should.
The Hose 10 mod basically blocks off the vacuum line leading to the pre-turbo intake pipe, and runs the BOV hose straight to the intake manifold.
In most BE/BH Legacy, after the mod is performed, there will be slightly more boost, and quicker boost response. Its well documented and has almost no negative issues.
The S401 is special though. It has different turbos, more boost, less VOD, a retuned ECU and a late revision D intake and vacuum layout. The basic layout of the BOV vacuum lines is the same though, so the Hose 10 Mod can be performed, although, I havent heard of anyone actually doing it on an S401. Until now.
Starting off with the standard vacuum layout, the blue Tee is what we want to block off, and the other end of the hose going into the Tee.
I used a couple of small vacuum caps, and just removed the hose.
Easily done, takes only a minute or two.
The results? Not good. Well, nothing actually.
It drives the same, no more boost and the boost is no more responsive than usual. It added so little that I didn’t see any point in leaving it, so reverted it back to stock. Now I know; I guess STI got the S401 as good as it can be.