BMW document year 1984 states following: “Bypass valve with less spring force (65 N instead of 200 N) so that it opens in low load range with 0.2 bar already.” Year 1981 document says: “In order to prevent critical conditions even for hard continuous load, the boost pressure was limited to a maximum of 0.6 bar.”
Wastegate valve face diameter is 35mm and diaphgram opening in base about 65mm. As we know spring forces (65N and 200N), surface area is calculated to know pressure needed to overcome spring:
Area for valve is (0,0175m*0,0175m)*pii = 0,00096 m2
Area for diaphgram is (0,0325m*0,0325m)*pii = 0,00331 m2
- Exhaust pressure P = F/A = 65N/0,00096 m2 = 67708 Pascals = 0,68 bar / 10 psi
- Diaphgram pressure P = 65N/0,00331 m2 = 19 637 Pascals = 0,20 bar
- Exhaust pressure P = F/A = 200N/0,00096 m3 = 208333 Pascals = 2,08 bar
- Diaphgram pressure P = 200N/0,00331 m2 = 60422 Pascals = 0,60 bar / 9 psi
Note: 1 bar equals to 100000 Pascals
Note2: Motronic WG solenoid controls charge pressure under diaphgram. This is achieved with pulse width modulation via negative pulses. Active solenoid pulses at fixed 12Hz rate, higher voltage signal ups duty cycle and makes closing time longer. At duty cycle of 50% half of the time solenoid is closed and diaphgram sees half of the manifold pressure. Wastegate valve opening pressure is then at about 0.4bar. No voltage on solenoid makes it left open, full manifold pressure is under diaphgram and wastegate opens already at about 0.2bar.
Note3: Disconnecting Motronic WG line to diaphgram results to similar boost level increase as using L-jetronic WG line connected (10 psi vs. 9 psi).