So I've had a problem with boost taper in my 05 evo. Set it at 20, tapers to 17. Started out by pressure testing everything, everything was fine, no leaks. Moved on to the wastegate. I unplugged the line running to the wastegate after a track day, since I still had race gas in I figured it was fairly safe. At 5000 rpm I floored it. Boost quickly climbed to 22psi and them tapered to 19 at redline. So the max the car could hold would be 19 regardless of boost controller. I then decided to make a block off plate for the BOV. I have the Buschur pipes with the 1G BOV (crushed). Took the car out again. Viola, boost shot up to 25 psi at 5000 and tapered to 22 psi at redline. The BOV was slightly opening and causing 3 psi of drop at redline. I then performed the 1G BOV fix decribed by Dejon Tool, without the external valve. Boost still would hold 22psi at redline, but you could tell it wasn't opening quite as quick as before. I've ordered the Dejon kit and will report when it comes.

 

Interesting, nice find...