Re: Blow-off Valve/Turbo Bypass ??? A bit long. . . - Saab NG900 & OG9-3 Bulletin Board - Saabnet.com
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Re: Blow-off Valve/Turbo Bypass ??? A bit long. . .
Posted by Dean (more from Dean) on Mon, 11 Jun 2001 09:12:55
In Reply to: Blow-off Valve/Turbo Bypass ??? A bit long. . ., Turbo Newbie, Mon, 11 Jun 2001 01:15:24
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There is only that one valve to vent turbo pressure. (ignoring the wastegate...)
As you make mods to improve turbo spool up rates, that valve will start to make embarassing sounds. The only solution is to install a metal piston type bypass valve. TurboXS or Forge will do nicely, with 1" or 25mm fittings.
Bypass valves open with manifold vacuum, and thus cannot be vented directly to atmoshpere. So the discharge is routed back to the intake and filtered air.
Bypass valves or dump valves release pressures during shifts and other times when the throttle is abruply closed. Without these, the pressure and high velocities will create a reverse surge that bounces back from the throttle body which will create reverse flow and shock waves in the turbo compressor. This can cause physical damage to the compressor wheel. This also abruptly slows down the turbo, which increases turbo spin up recovery after the gear shift. So venting this pressure keeps the turbo rpm's up throught the shifts.
To get loud discharge sounds you can get a dump valve. No danger of unfiltered air entering the intake with those. Yes they make noise, but are not neccessary more effective. With systems that have air mass meters, they often cannot be used because the spilled air will create mixture problems.
Many folks have fitted piston type valves with the objective of stopping the tooting sounds, not making more noise.
If you want to hear the turbo spin up, the K&N, air box mods, discharge silencer removal will do it.
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