Trick the AAC to be a Blow Off Valve?

HERE WE GO AGAIN!

O.K., depending on your GAUGE, "Vacuum" MAY be read as "Pressure"

because VACUUM IS PRESSURE!

There are two scales to read pressure
1) PSIA (Absolute)

2) PSIG (Gauge)

The difference between the scales is one uses atmospheric pressure as a given, and only reads the ammount above atmospheric (Gauge Pressure)

Absolute is actually your REAL pressure, and with an absolute bgauge, you will realize that you will only get confused if you are already lost, so I will stop with that line of thought...

What your Boost Gauge is alled is really a "Compound Gauge" that is, it has TWO DIFFERENT SCALES ON IT. On reading GAUGE PRESSURE (Boost) and the other reading VACUUM (a pressure BELOW atmospheric, but above a total vacuum of "0")

So when you are on boost, you read 10PSI(G), and when you drop throttle you read 22"Hg (which when converted is roughly -10PSI(G), so you actually are showing the engine inlet with a 20PSI(G) difference in inlet condition---understand how turbos make power now???)

So reading -22"Hg is actually a VACUUM condition in the manifold, and has ABSOLUTELY NOTHING TO DO WITH WHAT THE TURBO IS SEEING!

To read what the turbo sees, you would have to have a sensing line in the turbo wastegate line, and I assure you, when you drop throttle, the stock BOV dumps your pressure from the turbine outlet right into the plenum/runner junction on #4 cylinder! You would simply show "Boost" or "Nothing"

The engine manifold (plenum) behind the throttle plate, however is something totally different. After you dropthrottle and close that plate, you will still have valves opening, pistons pumping, etc etc etc, and it will quickly draw down the observed pressure in the intake manifold to a stasis point equitable to the flow available from the pressure source. This is what you are reading.

If you had a SINGLE SCALE on your gauge, reading in a compound gauge action, you would see numbers from -15 to somewhere around say 45psi(G), so when in boost you would read 10psi(G), and off boost, cruising or idling, you would read closer to -10psi(G) or thereabouts, with occasional spikes on drop throttle to high vacuum-equivalent readings of -15psi(G), then settling to around -10psi(G)

On the other hand, if you had a gauge that read ABSOLUTE PRESSURE (PsiA) your gauge would start at zero, and go to 60psi(A).
using the same numbers, at idle you would see 5psi(A), and under boost you would see 25psi(A)...

This will have Jack throwing fits, I almost am...

Anyway, what you are reading while really a pressure in absolute terms, is really not a pressure when you call it vacuum.

You are reading your gauge wrong, only because you didn't understand that vacuum is not read the same way as pressure. Even though you do have pressure in your manifold, it's below atmospheric, and therefore not considered pressure for this application...

Clear as mud now?

Did I make any headway in you understanding or grasping what is up on this confusing subject? LOL

uh uh uh...

That was not his question!

The "Vacuum Control Valve" in Nissan Speak is INDEED a blowoff valve! It's function is to prevent pressure stalls, and to MINIMIZE high-vacuum excursions on drop-throttle keeping oil consumption to a minimum.

The answer to the question he asked is a qualified "YES" since you already have something performing that function, you do NOT need to add another device that will do the same thing, only with less desirable results...

With an BOV venting to Atmosphere, you ONLY have a 10psig differential at 10psig upon drop-throttle.

With the BOV (Vaccum Control Valve) venting to number four runner/plenum junction you have a 25psi differential upon drop throttle---with a higher differential pressure across a given orifice, you will get MORE FLOW across it with a higher differential.
Therefore the smallish-looking "Vaccum Control Valve" actually VENTS OFF THE BOOST faster through it's 1/2" orifice than a valve 1 1/2 times it's orifice size!

Basic Engineering Theory is not a strongpoint here, so I don't expect to win an argument through logic, but suffice to say unless you SERIOUSLY pump up the boost, or add some serious captive volume (I/C and piping) there is ABSOLUTELY no reason to install another BOV onto a stock ZXT, or even one modified with say 15psi ad a smallish intercooler.

Above that, you will have to deal with exhausting the volume faster, and have a much increased volume, but you should STILL take into account the oil-consumption argument when considering where to vent any upgrade BOV at a later date.

Well...

I'm sorry if I am not up on your car mods. People flame me continually because "they don't read all the posts" apparently inferring that I do.... So how shall we handle this? Badly, I surmise...
Your signature says"1982 280zx Turbo: 13.948 sec at 100.02 mph and getting better" which is possible without an intercooler... So how am I to know that you have one? Read all your posts?
How many times has it been said "vague questions give vague answers".

What I have posted (as you so lovingly stated "Many a times I have posted that...") many times over is the general rules of thumb and specific engineering guidelines that you should follow when considering wether you need on or not. You actually should try to figure it out for yourself from those guidelines so you will remember it---that is how you learn things.

How many times will I have to repeat the SAME THING?

O.K., so you want it spoon fed, here it is:

1) The stock ZXT has a BOV already installed in the junction of the #4 Intake Runner/Plenum Junction.

2) The Stock BOV (some call it vacuum control valve, same freakin' difference...) will work fine up to 17psi ON THE STOCK CAPTIVE VOLUME.

3) If you add captive volume, you MAY or MAY NOT need an aftermarket BOV. The deciding factor in this will be how the turbocharger is reacting under drop-throttle conditions at full boost

Now go back, READ MY POSTS, and see how many times point 1,2,&3 were said OVER AND OVER AND OVER.

The italicised portion is the SPOONER portion that you obviously can not comprehend, even though it was implicitly stated in my last response to DeSch. in the post entitled "AHA!"

You state your boost gauge goes from + to - on drop throttle, so you have proper BOV action from the stock unit, if you are concerned about "noises" I'd suggest yoyu INSTRUMENT the turbo piping before the throttle body and see what the pressure does when you drop throttle. The stock VCV is a differential pressure check valve, and if pressure drops to below 3psia TOTAL DIFFERENTIAL between manifold and turbo piping it will close. If the compressor is still spinning such that air is still overfeeding it will "shSHshSHsh" with a typical hystersis curve of decreasing amplitude. This is not necessarily slowing the turbine excessively, but if it's a big problem in your application, you may well toss the BOV and install a correct manifold referenced Compressor Bypass Valve to keep the turbine in frewheel in drop-throttle AND cruise conditions.

But you asked about a BOV, not a bypass valve---they are different.