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Trick the AAC to be a Blow Off Valve?

650 Views 19 Replies 6 Participants Last post by  83na280zx
Is it possible to do this? Can you trick the Auxiliary Air Control Valve to do something like this? or is it pretty much useless. Will I need it for emissions, etc? In that case, do I even need the Vacuum control box for emissions control?
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If you have an AAC, you already have a BOV right next to it venting excess turbo pressure to the #4 intake runner when you drop-throttle under boost?
ok, then why does vacuum still build up between shifts?

"ok, then why does vacuum still build up between shifts?"

Uh, if you foot is off the throttle, and the engine speed is high, you WILL get vacuum in the manifold.

Are you sure you are reading your boost gauge correctly?
Am I mis informed?

"Are you sure you are reading your boost gauge correctly?"

I believe I am, but I'll pay more attention when I drive it later today.

"Uh, if you foot is off the throttle, and the engine speed is high, you WILL get vacuum in the manifold."

I had the understanding that a Blow Off Valve vents the vacuum pressure to keep the turbo spooled. When I shift, the turbo drops pressure to like -22 vacuum (i think it's in Hg). I'm not good enough to have my foot to the floor whilst shifting. Could you explain this to me, or direct me to a thread that does?
..vacuum pressure? do you mean vacuum or pressure....they are opposites...
....boost (pressure) is recirculated by the blow-off valve...(usually when it sees vacuum in the intake manifold)....the blow-off valve helps the turbo to "keep spun up" when the throttle plate closes and there is no where for the boost to go......cuts down the stress on the turbo too.... or thats how i always understood it.... .......s
yeah, the BOV just makes it so the turbo will spool faster once you get back on the throttle. if you have no BOV, the pressure reading on the gauge will spike immediately once you let off the throttle. then, since the pressure has no place to go, it will kinda 'go back' towards the turbo, and keep it from spinning as freely as it would have, essentially, stopping it. then, it takes much longer to spool when you step on the throttle again. if its reading 22 in/hg off throttle, thats good.

O.K., depending on your GAUGE, "Vacuum" MAY be read as "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
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Yes, much clearer thanks.

BUT, another question: Does this mean that a BOV on the intake pipe between the turbo and TB isn't necessary?
....if you are you talking about the vacuum control valve (on the manifold's #4 intake runner )...i would get rid of it...if you already have one somewhere else (like between the turbo and TB )..... ........s
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.
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Re: uh uh uh...

...yes i see...
i had a lot of trouble with my t3/t4 running 12+ psi ,intercooled (didnt do it w/o the ic) w/o using an aftermarket blow off...(and the vcv was still intact)...
....but only after adding the 420cc injs and jwt thing.....was it just because the engine was running (accellerating) so much faster? ....s
De schmaydee,
With the VCV intach your becomes useless. The VCV is activated by vacuum so as soon as you close the throttle you create vacuum in the intake which will such the valve open. This will vent the so-call excess boost into the intake. This will happen very quick and therefore your BOV will not see any boost. This is just from my understanding. Someone else may have an argument to that.

Exellent explaination..


WTF is that supposed to represent?

Make it clear, it didn't make any sense to me.

I had no "troubles" running 15 through the stock VCV and no I/C...
Re: huh?

...... running 12-15 psi w/my i.c. hooked up the the turbo would make the "goose gaggling" noise when i shifted.
......if i ran w/the stock j-pipe the noise did not occur.
...i had to use my hks bypass valve when i had the ic hooked...that got rid of the "goose" noise....the hks would work....i have it actuated by vacuum.......
being the dumbass i am/was i didnt know about the vcv and ran it also...seemed to work pretty well once i got it (the hks) adjusted.... .....s

Post Edited (Nov 10, 5:20pm)

So then "if i ran w/the stock j-pipe the noise did not occur." Which is exactly what I said: if you add captive volume to the system, then you have to releive more.

For almost any boost pressure (I tested to 21...) with the stock J-Pipe, the stock valve does fine, though it did start to get noisy, and had some signs of pulsation on drop throttle (I can draw a graph easier than I can explain it any better...) that indicated there was beginning to be an initial flow restriction over 17#. I surged the compressor on max pressure at 21psi, so that's where my testing stopped.

But yeah, add volume, address the problem.

With stock volume, as I'm supposing Jade has, there is absolutely no reason to spend $$$ on an aftermarket BOV.
I get that noise too..

Tony D, you suggested that I don't need a BOV on a stock system. Well if adding the air-to-water ic on that system with 14 psig I get the flutter from the turbo, what would you suggest then? Many a times I have posted that I don't have a stock car... especially if i've run 13.9s on 205/60/15 tires. I've had pictures posted on my www.cardomain.com/id/midnightzxt website. I'm sure you've seen it, but if not, check it out.

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.
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Tony D, I apologize if you feel that I flamed you. It was not my intent. I know you have a lot of testing done and such on the Zs and I respect that, and I by no means what to start any bad relationships. You obviously post much more than most people on the board, and thus the comment I main was greatly exagerated. I apparently hit a button that I did not want to press.

On the topic of BOVs and bypass valves. I have a very very very basic knowledge of them, and have never done any testing of my own. Of course you will have to spoon feed it to me, because compared to you, I know nothing. You don't know that I'm a 20 year old that started truly modding cars when I was 18. I'm still a beginner and I apologize for not knowing what you think I should. Sorry.

Re: uh uh uh...

wow, a lot of info crammed in there, so i have a greddy type s bov, do i need to plug the VCV off in order for my BOV to work correctly? or will running a vacuum line to the aftermarket BOV get it working correctly? i haven't installed it yet, im waiting on a weld-on flange to put onto my J pipe
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