straight swap?

[cygnusx1]

Actually Bast is correct. The factory dump valve(pop off valve) is AFTER the throttle body. That does NOTHING to vent boost pressure when you snap the throttle closed to shift. You do need a BOV before the throttle plate once you get up over stock boost (7psi).

 

Protect your turbo, get a BOV before the TB. And dont forget to either eliminate the POP OFF VALVE or tighten the preload with washers.

[Afshin]

Actually, I am correct

There is a hose going out of the j pipe that is before the throttle plate and bypasses the throttle plate and dumps the surge pressure into the intake manifold. If you look, there is a valve at the end of the hose on the intake that is spring loaded and opens at about 3 psi. Now when the throttle plate is open manifold pressure is same as j pipe pressure (eg 7 psi thru out), so the valve remains closed. When the throttle plate closes, then manifold pressure goes from positive to vacuum while j pipe pressure goes up, so the differential pressure across the valve greatly exceeds 3 psi, opening the valve and releasing the pressure into the manifold. The nissan engineers where not that ignorant, there needed to be a pressure relief system to avoid compressor surge. However, hardly anyone seems to be aware of this alternate system.

[cygnusx1]

Huh! Air going around the throttle plate like that simulates an opening throttle. Not a good thing when the driver has their foot off the gas and is expecting deceleration. You are saying that that system feeds air to the engine regardless of the fact that the throttle plate is shut? That is scary.

[Afshin]

It's only a small amount air for a brief period to relieve the pressure surge. It has been working fine for the thousands of l28et's they have made, I don't see how that would be scary when it's proven to work. It also has the benefit of letting out the pressure very quickly since it's going from positive pressure in the j pipe to around 20 psi of vacuum in the manifold, so the air actually gets sucked out.

[Guest zspeed260]

I HAVE A LOT TO LEARN..lol..there is so much involved with the turbo ...geez...well im glad there are people here that do understand this stuff..

[mobythevan]

20 psi of vacuum

 

actually did you mean 20 inches of vacuum

 

just messing with you

[Afshin]

Oh sure, nit pick on poor me on a Sunday morning before having had my cup of coffee.

Potato vs potaatoe; inches vs cm; pressure vs vacuum, coming vs going... all the same anyway, right?

[cygnusx1]

Hmm, like they say, you learn something new everyday. In that case, I have two BOV's because I still have all the stock stuff hooked up, including that hose from the J-pipe to the intake manifold. I wonder if that is what is keeping the RPM's up between shifts

 

Thanks.

[mobythevan]

I removed that line and blocked the intake when I installed my DSM BOV. My setup seems to work great. That original bypass is fairly small diameter and I can't imagine it could move much volume of air during the bypass. Like they say, maybe it works up to 10-12psi.

[fl327]

I run an HKS SSQV with the valve tightened a little bit, it works sure enough, but its even better when you are doing doughnuts and modulating the throttle, everytime you throttle on to get the car to spin you are instantly rewarded with a PSHHHHHHHHH..

Im in Heaven.. 8)

[cygnusx1]

I can't imagine it could move much volume of air during the bypass. Like they say, maybe it works up to 10-12psi.

 

That's right. It probably was enough to relieve surge at 7psi but even with the large pressure differential to vacuum, the small diameter hose limits flowrate. No doubt the Nissan engineers calculated all of this to work at 7psi with the stock T3. I am going to plug off that hose as well and give it a try. BTW I am using 1.25" return hose for the TurboXS BOV (Bypass Valve) and it works well. There is NO sound from the bypass valve. It is stealthy but I miss the "pfffffft" between shifts.

[Afshin]

The diameter of the hose is indeed the limiting factor as pointed out. However, it flows more than one would think because of theres is positive pressure on th j pipe side and significant vacuum on the other end which increases flow velocity. I'm aslo running a DSM BOV as boost is set at 17psi, but I would not worry about it until the car is intercooled and boost is turned up above at least 10 psi.

[Guest bastaad525]

what is the actual name of the bypass valve that is stock on ZXT's? Is it dump valve?

 

it wasnt the pop off valve that I was talking about though, it's another valve but i never knew what it was actually called.

[cygnusx1]

Afshin, thanks. I finally got around to plugging up the factory "BOV" in the intake manifold and I love it. I can now hear my TurboXS BOV venting anger and the RPM's don't "hang-time" between shifts anymore. Grrreat! I can shift quickly again; like the old days of N/A motors. I am surprised that nobody mentioned that valve when I posted about a year ago. Just after the swap I was asking why the RPMs don't drop between shifts...btw is that your real name? just kidding dude

 

Much GRATITUDE for that peice of info!

[Tony D]

The stock Dump Valve is discussed in factory documents as a "Vacuum Control Valve" and the diuscussion to this point is exactly correct. It has a 3psi check valve function. It does NOT leak boost because the DIFFERENTIAL between the sides of the valve must be at least 3psi for it to open. As it does open due to the high differential of the drop-throttle condition it actually performs a DUAL purpose:

1) it vents (sucks) the excess pressure from the J-Pipe

2) it prevents a stratospheric spike in manifold vacuum, keeping the valve seals from seiving oil into the intake. When I went to 15psi, I simply added a second valve and another tap on the J-Pipe. Works, cheap.

 

I like the oil consumption aspect of the valve's operation. I think I will keep it regardless. I am using an 80 ZX N/A manifold, but drilling and tapping it was no biggie. Matter of fact, if I were to do it again, I would NOT put it on the top of the manifold, but UNDERNEATH, out of sight, out of mind. When was the last time you saw one of the stock BOV's fail???

 

Hell most people don't even know they're there , much less know the symptoms if it would fail!

 

On of the FIRST things I did to mine was get rid of the "emergency relief valve" which will keep you from going over 10psi anyway.

[cygnusx1]

Oil control by controlling vacuum is nice but by venting the j-pipe into the manifold, engine braking suffers and so does RPM matching between upshifts. I would rather have a more responsive motor both in +revs and -revs. I'll let the aftermarket BOV take care of the job of venting boost back into the turbo inlet. The stock pop off valve is holding 14psi with the spring stretched to 2" free length and a 1/4" washer on it. Side effect may be more oil consumption. I'll keep my eyes on it. Besides, all N/A motors see stratospheric spikes in vacuum with little or no negative consequence.

[Afshin]

btw is that your real name

 

So "vacuum control valve" glad you remembered Tony, I was trying all night to recall the silly name and I couldn't. I also did not think about/realize the secondary benefit related to manifold vacuum spike and oil consumption... Good to know. (So far I have no oil consumption with my supra BOV , even after a full day at the track, however my valve seals are relatively new)

[Tony D]

With good valve seals, not venting to the manifold is not an issue. But with a vehicle with 180K+ miles on it, the seals DO start seiving a LOT more oil when it's not used! Turbo or N/A...

In my personal experience, when I updated to a 60mm throttle valve on my N/A L28 EFI setup, the oil consumption went THROUGH THE ROOF! from less than a 1/2 quart in between oil changes to that much in 1000 miles (so a 3X increase).

All due, in my opinion to the removal of the BCDD under the stock Throttle Valve...

 

The N/A engines have something to limit vacuum spikes, too! The BCDD!

 

And when they go bad, they have a high idle that won't go away. I don't sense any "overspeed" when I lift the throttle, nor when I reapply. I will have to see that when I observe more closely in the future.

 

Anyway, both N/A and Turbo engines had devices to limit total manifold vacuum excursions. This is why so many people think they need to "rebuild" their engines at 180K---they slap on a big TB, run the heck out of it, and start realizing they have a plume on decel, start using more oil, etc etc etc. Compression is still fine, but those repeated spikes to 30"Hg don't do any favors to older valve seals! I was so disturbed by the increase in the oil consumption after the TB swap, I even asked others if they had noticed it...

 

Being it was the ONLY variable changed on that car at the time, there is little else it could have been. When I drive it hard, it sucks oil, and on the freeway at cruise, there is nary a problem.

[cygnusx1]

I definately see more vacuum now with the VCV plugged. I see about 22-23 inches on decel. Maybe my VCV was NG anyhow because it did hold the revs up between shifts. Hmmm.