The previous owner of my Evo threw on a HKS BOV that vents to atmosphere. I was told that it's tuned to do so, but I want to bring it back to recirculating. Even if it's tuned, is it safe to do this?

 

If it's still using the factory MAF, I'd recirculate it. There is no real way to tune for the extra fuel dumped between shifts. Now if the car is on Speed Density, that is a different story and can run vented or recirculated with no negative effects.

 

Our true tuner gurus will have more valid things to say than me, but sounds like a re-tune!

 

Right! It's on MAF.. wondering if I would need to retune for something like this.. (Planning to retune after a turboback, stock evo 9 turbo swap, and FMIC..) Okay to wait?

 

Hi y'all. For anyone also in my situation, I tried to hook up a recirculating BOV to replace my VTA one and it caused idle issues and eventually caused the engine to die out several times. My guess is that the ECU believes it has enough air when it doesn't. Replaced the VTA bov back and it's back to normal. If you're car is tuned to VTA, leave it until you can tune it for recirculating again!

I plan to replace the bov to recirculate when I delete the EGR and retune it. Healthy engines are key!

 

You experienced Idle issues when you swapped to recirculating from VTA? Sounds like your VTA bov is vacuum leaking and the ECU has learned (fuel trims)/or has been tuned for the leak (IACV settings). Swapping between the two shouldn't cause idle problems if there is no leak. Someone correct me if I'm wrong.

 

You can't normally 'tune' around maf+atmospheric bypass

The maf and computer has no way of knowing when the bypass is open.
The only thing you can usually do is program a fuel cut when throttle position goes below a certain threshold. The Apexi SAFC-II has this feature, which allows a user to run atmospheric bypass with fuel cut (its actually a maf voltage clamp) during throttle transitions.

Depends on the factory ecu in question but most that I've seen (all of them really) do not have any special feature or function to allow users to switch back and forth between open and recirculation

Okay, that said, why did your car run poorly when you recirc'd the maf...
A maf sensor is incredible sensitive to perturbations in airflow. The shape, direction and pattern of the air as it runs through the maf is all highly relevant to maf sensor performance.
For example if you run the engine without a bypass(bov) most turbos at idle produce more air than the engine wants, so the compressor wheel speeding up gradually begins to surge as air is forced backwards through the housing. If the maf is anywhere nearby it will buffet the maf-hotwire (air leaving backwards through the compressor causes air that was going towards the compressor to 'stall' or even reverse direction through the maf)
this of course causes a massive a/f ratio disruption because the maf is no longer able to meter air going into the engine. The surge condition at the compressor causes it to slow down and eventually its moving slow enough again that air begins to once more flow properly through it and towards the engine, the a/f returns to being stable, and as the compressor wheel speeds up the process repeats over and over... forever.
This is an example of a dynamic system which is 'unstable' it will never find a 'resting' spot

In order to properly use and recirculate the maf there are general rules:
1. no bends in the pipe near a maf, say at least 8" to 12" of straight pipe on each side is ideal or at least close minimium, some can do with less it depends on the other items in the list
2. keep the maf reasonable distance from the compressor to keep the compressor wheel perturbation from buffeting the maf hot-wire (this is more important when bypass performance is sketchy or not used properly)
3. make sure the recirculation re-entry port is far enough downstream from the maf sensor (close to the compressor inlet) to avoid buffeting the maf-hotwire (just like air from a surging compressor, this is the same air quantity but instead of flowing backwards through the compressor it flows through the bypass, thus allowing the compressor to continue speeding up without surging which improves economy and response slightly by conserving kinetic energy originally imparted to air molecules by the compressor)
4. absolutely pressure test everything, every pipe for leak, a maf sensor will only work 100% when there are 100% leak free plumbing

couple points to re-emphasize,
Alot of people don't realize that one bypass is not the same as a next bypass. There are varying level of quality and control among units, and even design differences which need accounting for.
For example HKS SSQV style bypass is not a true recirculation bypass because it does not allow idle-recirculation to maintain compressor wheel speed, and may cause compressor surging during idle.
It behaves as if there was "no bypass at all" during low speed/idle operation where engine vacuum can't pull it open.
In comparison, a bypass such as Greddy Type-S "push type" of bypass can be adjusted to hang open at idle properly, thus recirculating air at idle, like a factory bypass unit from almost any car (nissan, toyota, mitsubishi, etc..)
Besides that, there are differences with installation and use of these bypass units. a bypass should be installed as close to the compressor as possible. Some manufacturers are starting to put the bypass directly on the compressor cover in fact (BorgWarner). The closer to the compressor, the hotter and faster the air will be moving when it exits the bypass, which helps open the (push type) bypass and conserves maximum kinetic energy.
Furthermore, a large enough vacuum line, dedicated to the bypass, should be used, because the large air cavity inside the bypass (or wastegate type units) requires not only a strong vacuum signal, but also a large enough hose to evacuate and replenish those air molecules as quickly as possible.
to summarize, The two most common mistakes I see with bypass units are:
1. wrong bypass location chosen
2. poor vacuum hose size/adaption

As for the maf, try to think of it as an absolutely perfect device for measuring air and therefore tuning the engine with (it is ideal), however, it must ALWAYS have a very nice, clean, straight flow of air through it, that does not speed up or slow down suddenly, that does not reverse direction, or become turbulent, EVER. So while the maf is amazing at doing it's job, it is often impeded by mods because so few people understand how important it is to keep the air flowing through a maf consistently nice and smooth, and what that entails

 

What a write up! Thank you for all the information! I hope people beyond me can use it. I'll shed some more light on my experiences so we can work out the problem and solutions. I have a Greddy Type RS that had a ripped diaphragm which I replaced. It looks like this. For those endowed with Greddy Type-RS BOV knowledge, this has a single spring mod but something peculiar as well. The washer to compress the internal spring which controls how "hard" or "soft" the sound is is quite large, definitely not the original piece. That being said, I replaced my HKS SSQV BOV (Venting to atmosphere) with the Greddy and Recirculated it. I should mention that I'm using Injen's Intake system and hooked up the couplers accordingly. The Blow off valve was tuned to blow off "hard" which gave me a flutter sound so I made it softer and now it blows off cleanly even at very little boost or no boost. Another quirk of the previous owner was to plug off the bottom hose connection on the intake pipe. I hooked up this connection to the Grimspeed 3-Port Boost controller. After trying to replace the BOV, the vacuum line was too small so I got a bigger one.




This is a more detailed story of my experiences and I'm still happy to answer any questions. That said, there are many places of failure and my set up now is just disconnecting the Greddy from the Intake pipe and venting it to atmosphere. I haven't gotten the chance to give your write up another read, but it's worth my bookmark for sure! I'll see if I want to experiment a little further and see what else could be causing problems. At the moment, I'm idling just fine! (Not with the A/C on though, another story :P)

 

Hey y'all another update. Replaced the Greddy with the HKS and bought a fitting to recirculate it. Car is running fine! That Greddy BOV was causing the problem one way or another. Service light would blink on high RPMs on the Greddy but it doesn't now on the HKS.