Last week I did a little experimenting. I plugged the return line on the greddy s BOV and let the BOV dump to atmosphere. I did this to see what would happen to A/F and power if I temporarily ran BOV to atmosphere while I conduct IC testing. One of the results was you could hear the BOV leak during part throttle spool up. Not a small leak but a very audible one. I couldn't hear any leak at WOT and the A/F seemed to not change at all. Power felt good. Drivabitilty on the other hand was complete crap. Stalling all over the place, very rich during light cruise, etc. I was thinking that at part throttle the pressure in upper IC pipe was greater than in the intake manni. Meaning there was no boost on top of BOV to hold the valve on its seat. This valve has the small inner spring removed, and is set to open only 4-5mm or so. It is very easy to push open with thumb. So I decide to try MR BOV which kills your thumb trying to open it. MR installed recirculating. Instantly I noticed the car felt slower. It also stalled more often and ran a good bit richer. I made this swap right after an alternator change. So I was thinking the ECU has to relearn some fuel trims and I shouldn't jump to conclusions. So I drive it for a few days. Power never seems to return. Today I swap back in the S BOV. Well my butt dyno was not lying. The car is a good bit faster with the S in place. A/F is most of the problem I am sure. A/F with S = 10.8, A/F with MR = 10.3. G-tech g's in 4th gear: MR =.52G, S =.62G. This is with 1.8 bar at redline on both valves. Slight spike to 1.9bar.

Now obviously form the A/F change we can easily deduce one of the valves is leaking under WOT. Question is which one? When you have a boost leak and a MAF it is very easy to see the A/F go rich. This is because metered air in not making to the engine so there is access fuel. But does this same rule apply when recirculating?

 

In my testing, the stock 05 VIII plastic DV started to open up at 8-10psi. This made venting to atmospehere imposible at part throttle, because the DV was opening up so early, with very little boost. The spring tension is just too loose. This cause super rich part throttle boosting and misfires and stalling.

Now, i run a GFB dual vent BOV. As you probably know, you can control how much recirculates and how much VTAs. I run full VTA with this BOV, because it does not open up easily in part throttle, because i spent lots of time getting the spring tension perfect for part throttle boosting. If i were to allow the BOV to open up too early, my part throttle, in boost AFR's would be insanly rich. But becaue i have set the spring pressure up perfectly, after lots of testing...i don't have a single problem with AFR's in part throttle or WOT. By doing this, i am able to keep the metered air in the system longer, which keeps AFR's in check. Of course, you have to walk the fine line of a tight BOV spring but yet not allow compressor surge to happen. It takes time to get perfect, and it takes the right BOV with LOTS of datalogging.

As for your question, if you are recirculating, it does not matter when the dv is opening up, the air is still not being let out of the charge system, which will keep AFRs in check. IF you have a boost leak, no matter if you are recirc or VTA, you will have excessively rich AFR's. especially in boost.


CJ

 

I am thinking this statement cant be correct. I do not have a boost leak. And There is a definate A/F change between the two valves. This means that one of the valves leaks more than the other. The leak will be recirculated. Yes, there is no loss of metered air in the sytem. But one is allowing more air to the engine rather than back to the front of the MAF. This Only leaves one conclusion to choose from.

a/f is richer for the valve that leaks more under boost
or
a/f is richer for valve that leaks less under boost.

 

My statement is obsolutly correct. You see, the air is recirculated from the DV AFTER the MAF, note before it. Therefore, if the DV is openeing up, the air being let back into the intake of the turbo is already metered for by the MAF. It is not going to be let out of the system, no matter what. When the throttle plate snaps closed, the engine in going to be in vacume. What happens in front of the throttle plate makes no difference when the throttle plate is closed, so long as the air does not leave the system for WOT. For part throttle, it's a different story, as the throttle plate might not be closing all the way. You are letting off the throttle just enough to stop the turbo from building more boost. So, if the air is being let out of the system, it cause a rich condition because that air was metered by the MAF, but in any other case of recirculation, it will be recirculated back into the intake AFTER the MAF sensor. It would only matter if the DV is stuck open, and leaking the charge back into the intake constatnly. Which, in this case would not be happening. If that were the case, the boost would not be able to get into the engine as well, and would cause a somewhat rich condition. Kinda like a boost leak. Both BOV/DV's you speak of would not be opening that soon.

On my car and other customers cars, i have not seen a rich condition from just switching recirculated BOV/DV's, no matter how easy they opened up. I'm not sure why you are seeing this condition. Unless you have a malfunctioning DV that is stuck open or opening at the first sign of boost. Even then, i really doubt it would cause a big difference in AFR's as long as it is actually recirculated.

CJ

 

94AWDcoupe,
I don't think you have the concept yet.

https://www.evolutionm.net/forums/sh...ad.php?t=24611

 

it's pretty obvious to me that the MR DV is leaking more @ WOT. When you did these tests, did you keep both VTA? I'm a little confused by your orignial post.

CJ

 

I appreciate your time to explain things. But you have yet to explain why I am seeing two very distinct a/f ratois here. Just because you have never seen this on cars you have played with, doesn't mean its not happening on this car. I assure you it is. The power loss is real and a/f change is solely from swapping valves.

And it has to be that one of the valves is leaking under boost. No other logical explaination.

The question remains by looking at the a/f change, which valve is leaking? If you say a/f goes slightly richer when bov sticks open, then why wouldn't it go richer when bov leaks? Stuck open is a leak. And a leak is a leak. Same result. One is just more severe than the other.

What I am looking for is a technical explaination. Something like this:

BOV is leaking compressed air into the intake pipe before mass, this lowers the intake pipe vacuum. MAF counts go down. Engine leans out as a result.

or this

BOV is leaking comressed air into intake pipe before mass, this makes it easier for turbo to spool faster, MAF counts go up. Engine richens as a result.

I guess I need to take a closer look at MAF counts and boost between the two to figure it out myself.

 

""BOV is leaking comressed air into intake pipe before mass""

HUH?

 

it really doesn't matter on our cars if the recirc point is pre or post MAF, if the valve is leaking, you're going to lose metered air since where the valve is located is post MAF.

 

Are you talking about?

 

I think the one that leaks more runs leaner due to the leakage back into the intake pipe (pre-turbo) disrupting the velocity and reducing the MAF HZ, hence leaning out the AFR.

You should NEVER do the single spring mod to the GReddy Type-s. I have been preaching this for a long time. Especially VTA. That is where you really F-up your AFR, stall, have a bad idle and part throttle cruis problems due to a weak spring.

 

Man I dont know whats going on exactly but the car is stupid fast with the S back on the car. The highest g force I saw in 4th with the MR on the car was .52G. I just took the car for a spin with S and saw .66G in 4th. This is a major difference. Like the difference you would see between 3rd and 4th gear. I would like to beleive the S is leaking more than the MR. That would mean I would gain power with the MR retuned to 10.8. But having to lean a car out to go faster is not usually how it works. If you put cams in your car and the fuel curve leans out, that is a good thing. means you are flowing more air. Adding more fuel gives you more power.

 

I have tested greddy valves on the dyno on two occasions. both where aem powered. placed a rubber glove on the BOV with a tie wrap. Then you have shift to second without lifting throttle. Then at full throttle in third you only have to watch to see if the glove blows up or not. I tested an r and and s this way and they both hold fine. The r was tested at 33psi with street spring on softest setting. The s was tested at 29psi with the small spring removed. both valves passed the glove test.

There are three things that hold valve closed. First is the spring pressure. Second is boost on top of spring pressure. Boost at manifold should be the same as boost in the IC pipe. So you are only left with spring pressure to hold valve on seat. That would be true if you didn't know about the third thing that holds the valve on its seat. SUCTION.

The suction I am reffering to is this. ever seen how a sand blaster works? Take a steel hose and blow 150psi through it. Now attatch T fitting inline. at the leg of the t attatch a hose and place the end of the hose in a bag of sand. You now have a sand blaster. The hose will have suction and suck sand into the 150psi airflow. But does this work at lower pressures? One day I had this brilliant idea. I was going to make a valve in my exhaust that relieved pressure when the pressure in the pipes got to high. So I drilled a hole 1 inch diameter in my 3" exhaust. I then build a flapper that was held to the pipe with a spring. I then started the car and took it for a drive. I found two things about flapper that made me look stupid. One, the flapper flapped like crazy at idle. very noisy. no way you could live with that. Second at wide open throttle the flap didnt open. It got sucked closed. psi inside exhaust pipes is around 3-7 psi. More psi than that you should be looking for the next larger pipe.

So there is suction at 150psi and suction at 5psi. Anyone want to guess if there is suction at 25psi? I am quite certain the answer is yes.

So while that whole 16 page thread about BOV testing was nice, he completely left out the real world testing. real world testing includes suction on the valve.

 

The suction you mention is called the venturi effect. You didn’t mention the other thing that holds the valve closed and that is the differential between the surface area on top and bottom of the valve. Five PSI is equal to ten PSI if the surface area is doubled. Example 1 PSI on top of a thumbtack equals 100 PSI on the bottom.

 

because he was mentioning leaking air before mass(which i'm assuming he means mass airflow sensor)and after mass. it doesn't really matter before or after, a leak is still unmetered air escaping.