Used to change out the spring pressure on the Tial 50mm BOV to affect the BOV response tuning to reduce comprssor surge (different spring levels affect amount of vacuum required on each engine to open the BOV on throttle release. Engine Vacuum Reading Color Use Spring - 22 and -23 in/Hg Pink -12 Psi (-24 in/Hg ) -18 and -21 in/Hg Un-painted -11 Psi (-22 in/Hg ) - 14 and -17 in/Hg White -9 Psi (-18 in/Hg ) -10 and -13 in/Hg Black -7 Psi (-14 in/Hg )