1994-2002 [Subscribe to Daily Digest] |
The bypass valve (BPV) is located between the high pressure and low pressure (inlet) sides of the turbocharger. The valve diverts charged (pressurized) air from the high-pressure intake back to the low-pressure turbo inlet during closed throttle conditions. This keeps the turbo spooled (momentarily) off-throttle, therefore reducing lag between shifts. This also prevents damage to the turbo from having charged air forced back against it (a pressure wave).
An upgraded bypass valve (BPV) will not really make much of a difference on a typical streetcar. The stock Bosch valve is usually good enough for anyone except the racers. On an automatic transmission that is definitely true. However, even automatic transmission streetcars generate a pressure wave when the throttle is closed while under high boost, like abruptly removing your foot from the accelerator during hard acceleration.
The real deficiency of the Bosch Valve is that it cannot hold the higher boost pressures from an upgraded Engine Control Unit (ECU). The stock Saab 2.0L turbo engine with a 5-speed produces .79 bar (12 PSI) maximum and the automatic transmission version produces .63 bar (9 PSI) maximum. A reprogrammed ECU can generate at least 1.1 bar (16 PSI) even in an automatic transmission car and the stock Bosch bypass valve will leak some of the boost at full throttle.
Rob Mustard (aka REM in Seattle)
1998 Saab 900SET; 5-door; Automatic; 77,000 miles
UPGRADES
ENGINE (dyno tested at 193 hp and 215 lb-ft at the wheels; or approx. 230 hp and 250 lb-ft at the crankshaft)
-SpeedParts ‘Stage 2’ Engine Control Unit (ECU).
-Saab stainless steel sport exhaust with added center resonator.
-Forge MotorSport FMCL007P piston type boost pressure by-pass/diverter valve.
-3” inlet tube to factory airbox with ITG performance foam air filter.
-Smoothed/ported throttle body transition casting and removed turbocharger outlet tube ‘silencer’.
SUSPENSION
-205/55-WR16 Pirelli PZero Nero M+S tires.
-Swedish Auto Specialties 22mm (7/8”) rear anti-sway bar.
-Koni Sport adjustable front struts and rear shock absorbers.
-Abbott Racing steering rack brace and polyurethane tracking control arm bushings.
-MP Performance sub-frame brace.
LIGHTING
-European E-code headlights with Philips ‘VisionPlus’ halogen 60/55 watt H4 bulbs.
AUDIO
-Infinity Kappa speakers – 3.5” 2-way speakers in the dash, 6”x9” 3-way speakers in the rear parcel shelf, and 6.5” mid-bass speakers in the front doors.
-Dynamat ‘Super’ & Dynamat ‘Premium’ sound deadening material in doors and rear cargo area.
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