CBV vs BOV, about avoiding 'spiking rich'.

[Boxman]

When I built my engine, I kept hearing the following argument of CBV vs BOV: in our cars, since fuel-metering is done through air-mass, the CBV is classically labelled the winner, because - contrasting a BOV - it doesn't dump "already metered air" to the atmosphere, which would cause a rich condition since fuel has been added for this air. But some log-results and thought experiments have made me reconsider this point;

The recycling of air is an ideal case, and now that I'm running high pressures I'm starting to notice the shortcomings of the CBV setup as well. Just for the record, I've added the valve in the post-intercooler piping and recycle it into the intake tube right before the turbo. The realistic case seems to be that all air is not actually recycled. Since the whole charge system is pressurized, a lot of air needs to find a way back, while the volume of tubing after the CBV is very limited, causing air to flow back through the MAF into the airbox, essentially being metered twice (the MAF isn't directional), causing a rich condition anyway. I can see this in my logs - shortly after lifting a pretty big spike occurs in the air mass.

Apart from that, I guess it's not like the metered air makes it into the engine anyway, since you shut the throttle valve and the idle valve is not going to let significant volumes of air through. The engine will still run 'rich' as the metered air doesn't actually make it into the block.

With that said, I think a BOV might even be better as it dumps the air that wouldn't make it into the engine to begin with, but also avoids double metering.

Therefore I wondered what your approaches are, how you setup your blow-off systems and how you guys think we can tackle this 'problem'.

 

[venderbroeck]

As much as I 'hate' the idiotic sounding (and stupidly overpriced) bov :p, it could maybe be done with a correction using the boost sensor. On throttle close use a 1d lookup table to correct the metered air mass, with 0 correction once you reach ambient. You could calculate the correction, or just determine it emperically with logging. Not sure if the amount of air lost would be very consistent, but I imagine it wouldn't have to be super precise.

Or you could ignore the maf altogether on throttle close, and use a map with replacement values referenced to rpm only, until a certain rpm threshold is reached. 

 

 

[Boxman]

Sounds like an interesting approach. Willing to try it out, although I'd have to install a MAP sensor (currently running without it, except my analog one) first.

In the meantime (one never stops doing research :D) I read that the flow reversion near the MAF could also be caused by an improperly positioned connector in the intake tube. Angling the backfeed towards the turbo might give improvements, so I'm going to experiment with a small insert that directs CBV flow towards the turbine. Right now it's a simple 90 degree T-piece.

[Calvin Sonnik]

A possible solution might be a combination of both.
Using the BOV to release the air mass that is already passed the turbo, then closing when the air pressure post turbo reaches closer to ambient and stays closed even with the full manifold vacuum. Then the CBV does the rest.
This could be accomplished using a 'tight' adjustable BOV.
you may also try increasing CBV flow by enlarging the ports in the compressor housing.

Just some thoughts,
Calvin
 

[Boxman]

Experimented further with this today; first made a custom interconnect that directs the flow from the CBV towards the turbo inlet, to counter flow reversal. Looked like this, but spoiler: didn't help. I think I don't actually have flow reversal but something else entirely, more on that below. This thing replaces the interconnect that hooks up the CBV line into the silicone T-piece in my custom-made intake tube. The new piece goes all-the-way through and inside the intake tube, and directs the CBV air downwards towards the turbine intake.

Didn't help one bit.

After some pulls I still had the same behavior as can be seen in this log. The purple line is the air mass when I lift. You can see it blips pretty high, resulting in a large inector duty cycle (yellow) and hence rich condition. I think it is specific to my setup with the long CBV return line. Since I read everywhere the BOV/Recirc valves are best placed as close to the throttle body as possible, I did so and added a pretty long return line (the yellow hose) to my intake tube. Whenever I lift the throttle, this 'empty' line (since it is part of the intake vacuum) suddenly becomes (momentarily) part of the charge circuit and needs to be filled with the pressurized air until the air rejoins in the intake tube. Since the turbine is still spinning, this air to fill this yellow return line is probably drawn in through the inlet still.

So that 'peak' in air mass is - I think -  the momentary air mass required to fill the yellow line between CBV and inlet tube the moment I lift the throttle.

You see two other peaks there, which result from me tightening my CBV spring too stiff, making it open-and-close several times, giving me several times the same effect. Loosened the spring now.

If this all is true, it should be fixed by placing the CBV as close to the intake tube as possible, so the yellow tube is already pressurized when I lift throttle, thus no 'new' air required. Alternatively, I could cut the pressure circuit short already in the hot-side of my intercooler tubing. Will try either one of these options out tomorrow.

Or it may still be flow reversal regardless of my thingy, but then there is no way to avoid that except for an actual valve/flap after the MAF.

[Boxman]

Figured out a big part of the issue, and it's really really silly;

The vacuum hose to my CBV was too long and thin, impeding vacuum flow when lifting throttle and making the CVB 'slow' - as a result, it simply would not open in certain cases, resulting in flow reversal through the compressor.

Used (a lot) thicker vacuum hose, car drive-ability changed drastically.

[Rowdy850R]

i have a few ? i bought a basket case lets start there. Previous owner has set up with a CBV on a Garrett 75 trim and a Tial QR recirc BOV ever see a car use both? i get MAF spikes on tuner pro as well..thoughts appreciated.