I am waiting for a 2013 STI and have been looking into upping the performance, but before I go down that road I would like to thoroughly understand the components in a forced induction vehicle.

I've done a bit of reading but I'm still confused as to the difference between a blow-off valve and a wastegate. These two components seem to do the same job ie. prevent too much boost/pressure from being created. From what I understand, the BOV releases air if the PSI gets past a certain point and the wastegate prevents exhaust gases from spooling the turbine past a certain pressure. Am I missing something? I'm sure I am. Could someone please clarify for me?

  • Possible duplicate: You'll find several questions and answers discussing these components. You might find this question in particular to be informative: mechanics.stackexchange.com/q/2731/57
    – Bob Cross
    Commented Feb 3, 2013 at 21:15
  • 1
    Clarifying the original intent: if you are looking to up the performance of a forced induction vehicle, neither a wastegate or a blow-off valve will help. Neither is a power producer. They're just necessary plumbing.
    – Bob Cross
    Commented Feb 4, 2013 at 13:16
  • A blow-off valve does increase throttle response which is a performance improvement. Though I will grant that most people are under the impression that it adds power. Commented Jan 12, 2015 at 13:41

2 Answers 2


The waste gate is designed (as you'll see in Bob's excellent answer to his linked question) to avoid spinning the turbo up unless needed. This saves fuel and wear.

The blow off valve or dump valve is designed to stop a compression wave passing back into the turbo when you close the throttle, as this wave could stall the turbo completely, which can put high stress on the turbo, as well as making it take a long time to spin back up again (turbo lag)

  • 3
    Turbo Esprit models without BOVs are well known for high turbo failures. Adding a BOV is the typical first modification, just to avoid the the need for repeated turbo replacement. :-) Commented Feb 4, 2013 at 12:56
  • A wastegate is also required to not overboost with the turbo, when no other valves are used to release the pressure.
    – Elias
    Commented Jan 9, 2015 at 14:19

You've understood how they work, now you must think about "when" they work to understand why they're there.

A turbo utilizes the exhaust gas to spin a turbine which which is mechanically coupled to another impeller on the intake side which forces air into the engine. As the engine produces more exhaust, the turbo spins faster and produces more boost until the set limit is attained. This limit is controlled by the waste gate , which "sees" the intake pressure and opens up when the limit is reached to vent off or bypass the exhaust gas around the turbo (vented to the catalytic converter or atmosphere), thus preventing it from spinning faster and creating more boost. It doesn't vent out the intake air.

Hence , the waste gate controls the maximum boost pressure (limited by how much your engine can take/ turbo rev limit etc).
Changing how much your waste gate "sees" can allow you to increase this limit (increase maximum boost). Look up - ball and spring valves, bleed valves, boost controllers.
A blow off valve, on the other hand, lies on the intake side and is meant to open up when the throttle is closed (by sensing engine vacuum). This vents out the pressurized intake air to the atmosphere. This is required because as you rev the engine up , the turbo spools up as well. The instant the throttle is let go, the turbo continues to create boost which now has no where to go. This causes the turbo to quickly spool down which creates a lot of impulsive stress on the turbo. Consequently , the turbo (now having spooled down) will take time to spool up when the throttle is re-applied.

Hence the blow-off valve protects your turbo by allow it to continue to spin (rotational inertia) and vent out the un-required air, when the throttle is closed. A consequence of which is reduced turbo lag eg. between gear shifts etc. Installing high flow blow off valves may improve turbo spool performance to a perceivable extent.

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