For the folks out there driving the 1.8t models, there often
seems to be much confusion about Blow-Off Valves and Diverter
Valves.
The main function of diverter valves and blow-off valves is the same:
to vent excess pressure from the space between the turbo and
the throttle plate when the engine goes from a high-boost state
into a low-boost state (e.g. shifting gears). When this happens,
the throttle plate closes, yet the turbo is still spinning at
high rpms building pressure between itself and the now closed
throttle plate. This pressure will cause the turbo to slow down,
decreasing boost and causing turbo lag when the throttle plate is
again opened. The result is a marked loss of power in the new gear
until the turbo has a chance to spin up again. (not to mention
shorter life of the turbo itself).
So the solution to this is to place a valve that will vent the
pressure from this area during abrupt throttle changes. A diverter
valve vents the air to the other side of the turbo while a blow-off
valve vents to the atmosphere.
Let's get one thing straight before we progress further:
there is absolutely nothing wrong with the stock diverter valve if it is functioning
correctly (i.e. not leaking boost).
Now let's figure out why people would want to use an aftermarket
BOV or DV. The reasons boils down to engine sound and genuine need.
For anyone running a high-boost aftermarket chip, the stock diverter
valve should be replaced with a higher quality diverter valve, or possibly
a blow-off valve. This is because the stock DV will be more prone to
full or partial failure due to such high boost levels.
Many people opt for a blow-off valve because of its dramatic sound.
This is fine as long as air-fuel metering has been taken into account
(BOV's throw off metering done by the Mass Airflow Sensor). If a BOV
is simply thrown on without regard to the MAF effect, the car may run
too rich after venting and have bad habits like stalling at stoplights.
My personal opinion is that 95% of modified VW's out there are just fine
with a quality aftermarket diverter valve and that using a BOV in an
application where it is not truly warranted is just asking for trouble.
THE MANUFACTURERS:
