The Blow Off Valve (BOV) and the Diverter or Bypass Valve

Dec 16, 2003 | By: Matyas Varga - SRO

Blow Off Valve - HKS Super Sequential



Ever wonder why rally cars make that sound you hear when you open a
bottle of pop? Well, maybe not exactly like that but more or less
similar to the sound of air being released (air, not gas!). You usually
hear this when you let off the throttle, sorta sounds like a woosh!. In
effect, what you are hearing is the sound of built up boost pressure
being released from the intake system. The reason for this is that the
turbocharger will keep spinning even after you let off the gas. So as
you close the throttle plate, allot of pressure builds up in the intake
system. This becomes problematic in that this excess pressure can cause
the turbines to seize. Ultimately, this would destroy the turbo unit.
For this reason, we incorporate BOV's, bypass or diverter valves. These
mechanism work because on the other side of the throttle plate, vacuum
gets built up in the intake manifold. Blow off valves, diverter and
bypass valves all work by detecting this vacuum. Having done so, they
use this vacuum to mechanically open a valve in order to relieve
unnecessary boost from the other side of the throttle plate.

Forge Diverter/Dump Valve




Now let us differentiate BOV's, diverter and bypass valves. First, a
blow off valve (seen top right) is common to high performance
applications in that it provides the least bit of compromise. A BOV
essentially releases this pressure straight out into the atmosphere.
Quite often you will find that these units take on particular shapes,
making them resemble musical instruments. I guess some people out there
really like to flaunt their gadgets. Just wait ti'll you see how much
they impress the ol' 5-0. Aside from this legal dilemma, the second
problem you encounter with BOV's is that the mass air flow sensor will
cause the engine management system to "think" that the air will go into
the engine and in turn will release an appropriate amount of fuel for
it. However, a BOV system will vent this air out before it gets to the
injectors causing your mixture to run lean. In turn, this will result
in unburned fuel to escape into the exhaust system. This is why you
often see flames and hear those "pop's" in rally cars. It is unburned
fuel exploding in the exhaust system. Doesn't take a genius to figure
out that this is not the smartest application for the day to day car.
For this reason, reasonable tuners will usually employ a bypass or a
diverter valve (see left). These units essentially redirect this
pressure back behind the compressor causing the net flow of air to
remain constant. This in turn slows the turbine down gradually and
allows the air flow sensor to work appropriately.