Jersey Posted June 19, 2003 Share Posted June 19, 2003 Hey all. Question: What does the VCV do and when does it open/close? It threads right into the 4th intake runner and the hose coming off the other end runs to a hard line that's t'd with the hardline behind the Air Regulator then continues straight to the J-pipe off of the T3. Couldn't find anything last night in the FSM about it. '82 ZXT motor. Thanks in advance. Quote Link to comment Share on other sites More sharing options...
SleeperZ Posted June 19, 2003 Share Posted June 19, 2003 I've wondered that myself. When I was using the stock turbo manifold, I never had it hooked up, and never gave it a second thought. Quote Link to comment Share on other sites More sharing options...
Jersey Posted June 19, 2003 Author Share Posted June 19, 2003 I put in all new pipes for the I.C install and replaced the stock J-pipe with a 90deg 2" piece and didn't blow an extra hole in it and weld the 3/4" nipple on like the stock J-pipe had. I just plugged that side of the "t" that went to the stock j-pipe. Just can't understand what that valve would do? Mix warm air from the J-pipe in with the intake air? Doesn't make sense. The valve opens when there's positive pressure (boost) and flow back to the J-pipe to get rammed back into the throttle body...and only from the 4rth intake runner? That doen't make sense either. Strange. Quote Link to comment Share on other sites More sharing options...
jeffp Posted June 19, 2003 Share Posted June 19, 2003 That part you are talking about controls the idle on a stock engine. If you had an intake off of the car you would see that the inlet/outlet which ever you like to look at it, are actually down into the main plenimum. The two relays with the vacuum diafram that are in front of the AFM on the bracket controls the operation of the part via vacuum. So basically it is the bypass air valve for the idle. If you look at the fitting on the manifold, you will see that it is like a PCV valve and it will only let air into the manifold and not the other way like under boost. When I installed my new system, I used the hole in the manifold for my bypass air valve. I did not rertain the one way valve in the intake. Quote Link to comment Share on other sites More sharing options...
Lockjaw Posted June 19, 2003 Share Posted June 19, 2003 You know, when I put a ZXT engine in my 240z, it had the annoying habit of the rpms hanging at 3k when I made a little boost and then shifted gears. Partial throttle sort of thing. After about 6 months of playing with it, I removed that valve, and plugged it with some brass fittings, and problem solved. Quote Link to comment Share on other sites More sharing options...
Jersey Posted June 19, 2003 Author Share Posted June 19, 2003 Thanks for the replies guys. Unfortunatly, i'm still not clear on how it works. Ok, scenerio - I'm boosting say 10psi...air is rushing out of the T3 and through the J-pipe. Right after the first bend of the J-pipe is a port, a 3/4" nipple with a hose attached to it going to a metal tee on top of the manifold. One side of the tee is from the Air Regulator. Dead opposite of that is the AIC Valve. Now the other two sides of the tee, which are opposite of each other are - one from that 3/4" J-pipe nipple and the opposite side goes to that VCV. Now under boost, some air must be pushing through that nipple so, why wouldn't that valve open and have a good amount of unmetered air rushing into the intake, if you say it lets air INTO the manifold? I trust what you're saying is right, but i'm just not fully getting it. If you can't explain it any easier, i understand, somtimes it takes a while to sink into my damn Italian head! ha. Thanks again. Quote Link to comment Share on other sites More sharing options...
stagefumer11 Posted June 19, 2003 Share Posted June 19, 2003 You know, when I put a ZXT engine in my 240z, it had the annoying habit of the rpms hanging at 3k when I made a little boost and then shifted gears. Partial throttle sort of thing. After about 6 months of playing with it, I removed that valve, and plugged it with some brass fittings, and problem solved. so your saying between Gear changes it would hold rpm for a while?. mine does that on boost. what does this valve look like ? Quote Link to comment Share on other sites More sharing options...
Len Posted June 20, 2003 Share Posted June 20, 2003 Don't need it unless you are running air conditioning. then its nice to up your idle to allow for added stress on engine caused by creature comforts. Len In Oklahoma you'd be thankful you got it!!!!!!!!!!! Quote Link to comment Share on other sites More sharing options...
Jersey Posted June 20, 2003 Author Share Posted June 20, 2003 Thanks guys for the responces. I know i've got a few of you confused on what i'm talking about so i'm trying to find a picture to post. Sorry for the confusion. Quote Link to comment Share on other sites More sharing options...
Sean73 Posted June 20, 2003 Share Posted June 20, 2003 On a stock 280ZX Turbo, the idle is controlled by the ECU, which tells the vacuum control modulator (thing attached to the AFM) to open/close the AAC valve (saucer thing on intake), to meter the correct amount of air to maintain a constant idle. Off idle the AAC should remain closed, because you don't need it when you're on the gas. If you eliminate the AAC on the intake, then you need some other way of controlling idle: 1) a screw on the throttle body, or 2) a bypass valve with adjustment screw (comes stock on some NA 280ZXs). This is what people do when they use the clean non-EGR 280Z intake, which doesn't have a slot for the ZXT AAC. The thing I don't understand, is why does the AAC and air regulator need to draw air from the J-Pipe? Since the devices are only relevant at idle, why not just get air from a hose after the AFM? Quote Link to comment Share on other sites More sharing options...
Jersey Posted June 24, 2003 Author Share Posted June 24, 2003 Sean, i'm going to have tp disagree with your theory. I could be 100% wrong but, this is how I think it works. One side of the Air Regulator (front) "can" pull air from before the throttle plates. I say "can" because, when the Air Reg is cold, it opens and allows this air from before the throttle plate to go through the Air reg and to the AAC. If vacuum is applied to the AAC from the VCM because the ECU senses a cold condition and triggers the relay, the AAC lifts it off it's seat and allows the air coming from before the throttle plate to enter the intake, which causes the idle to rise. Cold condition, throttle plate closed, higher idle. When the little element inside the Air Regulator gets to a certain temp (60deg?) it closes off this port and doesn't allow the air before the trottle plate to supply the AAC. That's how i understood it anyway. And like i said, i could be completely wrong and am always willing to learn so dont take what i'm saying in a bad way. I've eliminated the VCM and AAC and my Z idles without a problem. Cold starts may be a different story but i'll worry about that then So, if by any chance i am right, the air coming out of the stock J-pipe nipple does not supply the AAC valve with warm/hot air. It's the air coming through the Air Regulator. What i was asking was what does the VCV - Vacuum Control Valve do, how does it work and does the stock J-pipe nipple have any effect on this valve? Wish the darn FSM would say something about how this valve works! Len - the AIC is the little hat shaped thing on the manifold that creates a higher idle up when using the air conditioning. Easy to get those two confused - AAC/AIC! I need a picture of a '82-'83 ZXT stock motor so i can post and point to what i'm taling about... Quote Link to comment Share on other sites More sharing options...
mobythevan Posted June 24, 2003 Share Posted June 24, 2003 I looked on mine and I know what you are talking about. The little piping from the j-pipe goes to several things and one of them is what appears to be a check valve on fourth intake runner. I am also curious what the purpose of this is. Quote Link to comment Share on other sites More sharing options...
Jersey Posted June 24, 2003 Author Share Posted June 24, 2003 Yes. you've got it. If you find out any info on it, would you please reply to this or send me an email? Thanks. Quote Link to comment Share on other sites More sharing options...
HowlerMonkey Posted July 18, 2011 Share Posted July 18, 2011 (edited) I know this is an old thread but none of the answers above is correct. This part is called the "vacuum control valve" but it's operation is different than it's name implies. In operation, it is a blow off valve which vents excess pressure between the turbo outlet and the throttle plate when the throttle is closed while under boost. Yes, it vents into the intake manifold runner. Because of the way this car manages conditions in which the injectors are on/off, shoving air from the J-pipe through this valve into the intake manifold has no ill effect because the injectors will be off during the condition in which this valve is open. If you are using a different ecu or injection scheme (or your tps is adjusted wrong), this valve's operation can cause it to hang rpms for a little bit because it's operation assumes the injectors are off when it is open. Another up side to this scheme is that all air metered is put through the engine rather than some metered air being vented to the atmosphere. It's actually a pretty brilliant scheme and likely responsible for stock 280zx turbos lasting longer than they would if the column of air would be allowed to "stack up" and spike in pressure when the throttle plate is closed. That said, it's capacity to flow is probably not enough when used alone with higher boost, added volume of pressurized air (intercooling system), or a larger turbo. If you're already running a blow off valve, removing this and/or plugging it should have zero effect Edited July 18, 2011 by HowlerMonkey Quote Link to comment Share on other sites More sharing options...
Tony D Posted July 18, 2011 Share Posted July 18, 2011 Howler Monkey has necroposted the truth... This is the "Factory BOV" and does the same job as an atmospheric valve, without the noise and with some added benefits. Because it vents to the plenum on drop throttle, it supplies much needed oxygen to help combust any HC that may have happened before injector shutoff. It also pumps pressure into the plenum preventing a massive vacuum spike which will increase oil consumption. Because all the METERED air from the AFM is actually going through the engine, this helps with preventing the drop-throttle rich condition those with improperly vented BOV's face as well. Remember in the Bosch AFC system, you can not simply exhaust air which has been metered by the AFM, it HAS to go through the engine somehow. Whether that is dumping into the turbo inlet, or into the plenum on drop-throttle it makes no difference. But the extra puff into the plenum helps with emissions, oil consumption, and some could argue egt... so that was where Nissan Engineers decided to vent it. It's capacity to flow is deceiving, because you are venting across a very high valve delta-p. This is not dumping 7 psi to atmosphere, no no no! It's dumping 7psi to a -11 to -14psi environment. This higher delta-p allows for a smaller orifice than would normally be required to dump to atmosphere. Quote Link to comment Share on other sites More sharing options...
HowlerMonkey Posted July 19, 2011 Share Posted July 19, 2011 What's really amazing is how well the factory "J-tube" and "vacuum control valve" work. Sure, the air is damn hot but there is only a fraction of the pressure drop you find as compared to the average intercooler setup. Quote Link to comment Share on other sites More sharing options...
Tony D Posted July 19, 2011 Share Posted July 19, 2011 Change to a more efficient turbocharger, and the heat drops considerably. 380ftlbs on a J-Pipe un-intercooled, 8.39psig @ 4500 rpms Quote Link to comment Share on other sites More sharing options...
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