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Blow-off valves are important to preserve your turbo.  Under full throttle and full boost, when you let off the gas to shift, the throttle body butterfly slams shuts.   When the throttle body closes this creates tremendous pressures in the intercooler, 30 and 40lbs aren't unusual at this point.  This is because the turbo is still spinning full tilt, sucking air from the air filter, and cramming it into the intercooler, with nowhere to go, if you don't invest in a blow-off valve.  Without a blow-off valve this event will result in a chirping of the induction system.  That chirp is the sudden harsh slow down of the turbo, as the highly compress air burps backwards through the intake dramatically reducing the inertia of the turbine, suddenly slowing the turbine down to a crawl.  This sudden slow down reduces the spool time as you reapply the gas. Worse of all, burp chirping is very rough on turbo chargers.  This reverse exit of the pressure, causing the chirp,  will shorten the life by up to 50%.  This chirping noise is actually your hardware screaming for mercy.  In severe case we've even seen intake turbine shafts snap off from the sudden stop of the highly compressed air burping backwards, twisting the turbine shaft...... which is not good!
It's good to have a blow-off valve.  The benefits are:
1.  Longer lasting turbos. Relieving this pressure is gentler to the turbine.
2.  The turbine isn't suddenly slowed down from the "chirp burp", which improves spooltime.
3.  It's quieter, less annoying.
4.  People that hear your burp chirps may think you can't afford a blow-off valve.

The myths of why you don't need a blow-off valve:
1.  If you don't relieve the pressure, the intercooler stays pressurized during deceleration, waiting for the throttle to open. 
2.  The chirp sounds cool, it tells everyone you have a turbocharger. Can't argue this, but the price is high with the addition wear and tear on the turbine blades.

How do they work?   Blow-off valves are triggered by a vacuum  line that runs from your intake manifold the blow-off valve.  When a high vacuum occurs in the intake manifold (during deceleration, or shifting), that vacuum signal triggers the blow-off valve to open, allowing pressure out of the intercooler, preventing damage to the turbo caused from burp chirps.

Blow-off valves are ideally installed between the intercooler and the throttle body valve.  This provides the most effective location for fast reaction pressure relief.  It's important to have a very large port blow-off valve setup so it can dump the air pressure buildup as quick as possible.  At Outback we sell the best Tial blow-off valves for $250 (anodized)  and $200 (aluminum silver), which includes the mounting flange to weld onto your intake tube.

So when you hear those fast cars chirping with every shift, now you know what's going on.  They don't have a blow-off valve, resulting in over stressing the turbo.  The turbos on those cars will fail sooner than if it had a good blow-off valve installed.  Yeah, the chirp may sound cool, but now you know they cut corners on their motor and will pay the price sooner than later. The better sound is a whoosh sound from a blow-off valve, kinda like air brakes of a truck being released.







Myth #1 above has never been proven to be a benefit.  We often call this one "mechanic voodoo spin".  Logically speaking, when the pressure burps out the turbine intake, no pressure is saved in the intercooler, the inside of the intercooler goes to near 1 atmosphere of pressure, until the turbo spools back up.  This is something engine builders will tell you which sounds good, but in reality harms the turbo.  Be suspicious of anyone who says blow-off valves are a waste of money, that's just not true.  Why do you think they are installed in all production street cars with turbos?

As you can tell the reasons given by engine builders to NOT USE a blow-off valve are not in your best interest.  The best part they don't tell you is..... without a blow-off valve you'll end up buying more turbos because the turbo will fail sooner, increasing your builder's pocketbook.