Modifications - Blow-off Valves

The purpose of the a compressor by-pass valve is to relieve pressure in the intake tract when boost pressure exceeds a preset amount.  This is a saftey feature to avoid a dangerous overboost condition.  A blow-off valve functions during the transition from boost to vacuum.  It is designed to do the same thing - release pressure in the intake tract.  The benefits are two-fold.  When the throttle plate close while under boost the compressed air in the intake tract has nowhere to go.  Usually it would entire the combustion chamber.  When the throttle plate closes there is a sudden surge of static pressurized air.  This air introduces a sudden resistance for the turbocharger impeller which is spinning at a very high rate casuing it to abruptly slow down.  This sudden pressure places a large strain on the CHRA (center housing rotating assembly) and the compressor wheel.  Relieving this pressure increases the longevity of the turbo.  The other benfit is increased turbo spool-up.   This is because the compressor can stay spinning in a range closer to optimal between shifts and therefore take less time to get back into boosting range.  Although this does not increase horsepower it can shave up to 0.5 seconds off of a 1320' time.

OEM Options

Some have had good success with the first generation DSM (Talon/Eclipse/Laser) factory by-pass valves.

Blitz

The Blitz is excellent quality most noted for the volume of the sound emitted when it actuates.  For boost levels much above 17 psi, it is recommended that the optional stuffer spring be used as leakage may occur.

HKS

The new style HKS is constructed from metal and should easily stand up to most boost levels.  Earlier versions were plastic and a few people have experienced failure over time.

The Apex'i Twin Chamber blow off valve is also a high quality unit.  The twin chamber design in theory allows effective operation under both low and high boost situations.  Note that the 1994+ models cannot use the Apex'i without some modification as it interferes with the 6 speed shifter linkage.