When under boost, when you close the throttle suddenly, during shifts etc., the high speed flow will create a pressure pulse at the throttle which will bounce backwards to the turbo and create a reverse flow shock which makes a crunching sound and can physically dammage the turbo compressor impeller wheel. This also stops or greatly slows down the turbo, which makes it harder to get back on boost after shifting gears.

The solution is to create a valve to dump the prsssure in the turbo pipes when the throttle is closed. There two things that you can do with the released air. You can dump it back into the intake, or dump it to atmoshpere.

With an engine management system that uses a air mass meter, the flow needs to be vented back into the intake downstream of the air mass meter, otherwise the enigne will not run properly at that point in time and may throw a CEL (check engine light).

If you dump to atmoshpere, you can make a loud noise and frighten people. The dump valves can discharge at faster rates, but can be slower at intiating flow. These valves cannot be vacumm activiated, as they may then be open at idle allowing unfiltered air to enter the engine. A design could be vacuum assisted and still be closed at idle. I will not go into that.

Bypass valves can be open at idle without concern about unfilterd air entering the engine. So they are vacuum activated and are open at idle. The flow rates are not as good, but they open sooner and stay open. If you open a bypass valve to atmosphere, you are asking for big trouble. They can be discharged into a one way valve or an air filter. Part of the problem is the resistance of the long return hose.

So each valve has is merits. The bosch OEM diaphragm bypass valves can make tooting noises with modified boost pressures, and they diaphragms do fail. After market bypass valves can have the same problems. If you get a replacement unit, get a spring and piston design. They are quiet and nothing seems to go wrong.

You will notice that many of the posts about dump valves seem to centered around making noise, not performance.

A replacement bypass valve is a simple install. Dump valves typically involve cutting and welding.

• dump, hooter, bypass valves = Greek to me, Brian G., Thu, 12 Jul 2001 09:07:37
  • dump-&-bypass valves 101 (long), Dean, Thu, 12 Jul 2001 10:39:07 <-- Viewing This Message
    • How do you tell if OEM diaphram bypass is bad?, Richard P., Thu, 12 Jul 2001 15:48:18
      • quick check, dave n, Thu, 12 Jul 2001 21:38:25
        • i'm going there. THX for the tip! nm, chrystobal, Fri, 13 Jul 2001 01:45:18
    • can you dump to air with the stock bosch BOV? is it safe? nm, chris., Thu, 12 Jul 2001 11:52:23
      • I would not try it. You could get unfiltered air, jj, Thu, 12 Jul 2001 13:58:36
    • Dean is AWESOME!!!, Brian G., Thu, 12 Jul 2001 11:06:58
      • yeah i know, i love reading his stuff. nm, chrystobal, Thu, 12 Jul 2001 21:02:49
    • Re: dump-&-bypass valves 101 (long), Bill Learning Curve, Thu, 12 Jul 2001 11:01:32
      • I'm not Dean, but... your whoosh sound is normal., jj, Thu, 12 Jul 2001 11:17:15
        • Thanks JJ, is the BPC also known as the APC solenoid..., Bill Cannastra, Thu, 12 Jul 2001 11:26:17
          • Re: Thanks JJ, is the BPC also known as the APC solenoid..., Dean, Thu, 12 Jul 2001 14:25:24
          • The BCP and APC valves provide the same function i.e., jj, Thu, 12 Jul 2001 13:17:36

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