Just had a thought/question.

Using a boost control device like a Reliaboost or anything else that ultimately vents to directly to atmosphere is this potentially something that could act like a vacuum leak and throw off the fuel metering? I realize it's a very minor amount that is vented but could it potentially have an effect or am I just worrying about nothing? It just seems strange the lengths a lot of us go to to stop the vacuum leaks only to put on a part that essentially does just that.

As I understand it, (and correct me if I'm wrong please) on any MAF type system that once the air is metered, anything downstream that leaks air out or into the intake system would have to throw it off to some degree. The DME is expecting a given mass of air and supplying fuel accordingly and the bleed hole is causing some of that air mass to be lost. Wouldn't this cause a richer air/fuel ratio? I guess that's better than a lean mixture in terms of the potential for damaging your engine but I was just wondering if anyone has any insight on the this.

I realize it's probably not a big deal but after searching on it I couldn't really find anything one way or another on the concept.

 

A manual boost controller, or reliaboost will use some metered air to regulate boost via sending it from the IC pipe to the wastegate. If this is what you meant by "ultimately vents to directly to atmosphere", then yes, a little metered air is lost. BUT, you should not worry about this because the factory boost regulating system will use the same metered air and send it to the wastegate also. So by adding a MBC or reliaboost, you're not losing any more metered air than the factory system (cycling valve) does. I'm not sure about this, but the fuel maps might compensate for this loss, but since it is so small, I doubt it. Maybe someone that programs custom chips can answer this question.

 

The boost that an MBC see's is restricted not vented. The connection to the wastegate from the MBC is a dead end to a diaphragm. So there is no loss of metered air anywhere.
You may have some confusion between a MBC and a BOV.

 

I definitely mean an MBC, take the reliaboost for example, it has a small hole drilled in it to relieve pressure on the wastegate diaphram when the valve in it closes. Otherwise the wastegate would stick open without something to let that pressure off once you're off boost or the boost level is below the threshold that the check valve allows through.

I've got a TurboXS MBC which does vent to the atmosphere whereas some MBC's don't. If I have to I can put the vent end of the TurboXS so that it feeds back into the intake like the factory CV does. I'm going to run an LBE in front of it to delay the boost hitting until it spools up and then the TurboXS will limit the ultimate amount of pressure that the wastegate sees once it is enough to get through the LBE and also bleed the air off so that the wastegate doesn't stick open after the LBE shuts.

 

The hole on the ReliaBoost is down stream of the check valve.
Under vacuum it will not leak as the ball is seated against the seat.
Under Boost is can leak a bit but its such a small amount it wont matter.
Once you are off boost the ball seats again and this vent provides a way for the air to leave the line to the wastegate so it closes quickly, reducing a boost spike.
This benefit outways any slight boost leak. IMHO