The Great Blow Off Valve Debate:
First of all to understand the functionality and benefits of a blow
off valve you have to have a complete understanding of how a turbocharger
works. If you don't understand the basic mechanics of a turbocharger you
can forget about trying to understand the benefits of a blow off valve.
If you do understand the mechanics of a turbocharger, read on...
An Explanation of Compressor Surge:
The turbine in a turbocharger in its ideal mode is spinning at a very
high rate and therefore generating lots of pressurized air that is fed
into the OPEN (wide open throttle) intake manifold which translates into
more power, but what happens when you take your foot off the gas when shifting
between gears? The turbine in the turbo is still spinning at a high rate
but now you've taken your foot off the gas and the intake manifold is CLOSED.
Now the turbine spinning at a high rate and is trying to push out all this
compressed air but since the manifold is closed, that compressed air has
nowhere to go. What happens next is that the turbine slows down because
of this pressure build-up between the compressor and the intake manifold.
Since the pressurized air can't go forward into the engine it heads the
wrong way back towards the turbo where it slows the turbine down (compressor
surge). After you shift gears and you step on the gas again you want to
have the turbine spinning fast so that its producing boost but since you
took your foot off the gas and caused compressor surge, it slowed down,
so you have to wait for the turbine to "spool up" (spin fast) again. This
delay waiting for turbines to spool up is what causes some of the performance
degradation that were trying to avoid.
The Blow off Valve:
What if we could figure out a way of eliminating compressor surge? Then
the turbine in our turbo would have a much easier time keeping its speed
up and we wouldn't get the lag in power waiting for the turbine to spool
up between shifts. Enter the Blow off valve. At that moment in time between
shifts when compressor surge is slowing the turbine down we'll just poke
a hole in the tube running between the turbo and the intake manifold and
let all that air leak out somewhere. The turbine can keep spinning fast
& we'll have lots of boost when we step on the gas again. We'll also
want to plug up that hole we made because we have a purpose for that pressurized
air now, where as when we let off the gas we did not. The blow off valve
is a gizmo that's constantly opening and closing giving you that cool sneeze
sound when it "pokes" that hole in the intake manifold tube to keep the
turbine spinning fast. We've eliminated compressor surge but we did confuse
the engine just a bit since the ECU already told the injectors to issue
fuel to be used for the air that we've now thrown out. Other than that
sounds like a good idea right? so what if were running a bit rich for a
moment, no big deal. Before you place your order read on...
The Re-circulating Valve... a better idea:
How about instead of poking a hole in the turbo to intake manifold tube
and just letting the air leak out wherever it wants to, we could direct
that pressurized air somewhere else where we could hold on to it until
we need it again? The intake side of the compressor housing (between the
intercooler and the turbo) would be a good place wouldn't it? that way
we wouldn't slow the turbine down and we would have that pressurized air
that would have otherwise been thrown out, ready in a really good place
when we need it. On top of that eve got still got the correct air/fuel
ratio going since we're not throwing and air out, just moving it from one
place to another within the intake system. Instead of having to suck in
air for the turbo to pressurize, we've got some pressurized air with the
correct fuel amount already waiting. Now we've got all the benefits of
a blow off valve and then some. That's a re-circulating valve. Let's all
order re-circulating valves then and skip the blow off valves completely.
They're probably gonna cost a bit more than the blow off valves since they
work a little better right? Now the big surprise... Re-circulating valves
were engineered into the design of your 90+ twin turbo z on the drawing
board... you've already got them...
In conclusion: You could always disconnect your stock re-circulating
valves (which you have to do to install blow off valves) and spend 500-600
bucks for blow off valves and what will you get? better performance? No,
basically you do get the cool sneeze sound though you aren't going to be
any faster than you were before (but you might sound like you are). If
by chance you are running boost levels that exceed the airflow capacity
of the re-circulating valves (~600+ hp?) then you've probably spent enough
money and have enough experience and knowledge to know where the benefit
reverses. For the rest of you who weren't in the know, I hope I've helped
you save some time and money...
zxttsteve
