alexeiwoody wrote: Reading posts here that an atmospheric venting bov causes the car to run rich and that loses power?? Running rich means you have more parts of fuel to parts of air which would result in more power. Also a bov doesn't quite work like that.

Running rich or lean depends on tune, not on any bolt on. Haha sorry I couldn't hold back! Goodluck mate
Sorry, but this is just bad advice. Pure and simple bad advice. You're wrong on how fuel burns (and how and why it doesn't), and you're wrong on how a BOV operates. But thanks for coming.
To start with - fuel requires air to burn. That's an undisputable fact. Then, there is a theoretical ideal ration of air and fuel, at which point you get the most complete burn. This is known as a stoichiometric mixture. Generally in automotive applications, that's considered to be 14.7:1 (it can vary for various reasons that we won't go into here). That's theoretical though, and doesn't take into account a whole pile of other issues (such as timing, oxtane level, etc). Under boost, most tuners aim for something much safer (betwen 10.5 and 11.

- this is a richer mixture than the ideal stoichiometric mixture, and helps prevent knocking and leaning-out, which can lead to engine problems. The result is that you don't quite get a complete burn, but you do burn most of the fuel well, and you do so under safe conditions for the engine.
Running rich or lean depends on tune only when the tune has accurate information on parameters such as fuel flow and how much air the engine is breathing. The problem with an atmospheric venting BOV is that the amount of air taken into the engine's intake system has been measured (relatively accurately) by the MAF, and it's the MAF readings that the ECU uses to determine how much fuel should be injected into the engine (depending on whether or not the engine is running closed or open loop, an ECU may also use an O2 sensor in the exhaust system to determine how completely the engine is burning the fuel it is injecting, though one thing to note however is that this measured post combustion and can only be used to make adjustments after the fact).
Now, to make things really simple - let's say you have a single cylinder engine, and that your MAF measured 147 grams of air entering the intake system for a single combustion event. To achieve an ideal (stoichiometric) mixture, the ECU would inject 10 grams of fuel into the cylinder for this single combustion event, apply spark at the relevant time, and in theory, you'd get perfect combustion, with a relatively clean exhaust emissions, and a given amount of power).
(Now please, if anyone disagrees with this, say so now).
Now we introduce a turbocharger. The turbo compresses the air, so that the same mass of air takes less volume. Let's assume we're running 14.7psi of boost (one bar), that means we have two atmosphere's of pressure in intake systems after the turbo, and let's say, again for simplicity, that this means we now have 294 grams of air in the intake system for this single combustion event (and the MAF has measured this 294g of air - it has to come in via the MAF, it doesn't appear from nowhere).
The ECU will, at this point, inject 20 grams of fuel for this single combustion event. And given that fuel has a specific power (the amount of energy released when you burn a single gram of it), we'll get double the power, because we've burned double the fuel.
So now we know how turbos work.
But what happens if we have an atmospheric venting BOV?
As we know, as the engine burns one charge of air and fuel, it continues to operate - the engine sucks air in, the turbo compresses it (based on exhaust energy from the previous combustion event turning the turbo), ready for the next combustion event.
So, again to simplify - the engine draws in 294g of air past the MAF. The air gets compressed by the turbo - and then the throttle snaps shut for a brief second. The BOV does its job, venting air from the intake system to the atmosphere - and let's say, for the simplicity of the argument here, that exactly half the air gets vented to atmosphere.
This leaves the engine in an odd position - it has measured 294 grams of air, but only 147 grams of air are actually present.
So the air enters the combustion chamber, and the ECU, thinking it has 294 grams of air present, injects 20 grams of fuel, applies spark and...
Only half the fuel burns (there isn't enough air to burn the rest). This has two major impacts.
Firstly, you only get half the power.
Secondly, you get unburned fuel in your exhaust, or more correctly, a mix of unburned fuel and partially burned fuel (which means we technically burn slightly more than half, and get slightly more than half the power, but let's keep that aspect of the argument simple.
And what does this do? Produces dirty exhaust. Soot, to be precise.
You'll note that the ECU tune had NOTHING to do with any of this. Regardless of the AFR the ECU is aiming for, regardless of the timing of the spark, the simple facts of physics can't be overcome - there's less air in the intake than the ECU expects there to be, and thus it injects more fuel than required, and you get a rich mixture, an incomplete burn, less power, and dirty exhaust.
You could, in theory I suspect, tune the ECU to know that after a sudden closure of the throttle that the BOV may have operated, to adjust fuel mixes for a period of time after the throttle opened again and thus to inject less fuel (to counter the fact that there's less air in the intake). But this would be a guess (given no BOV I've ever come across has a feed back to the ECU to indicate for how long it vented, or any method to measure how much air it vented), and could actually be dangerous, as it could result in lean burn (should the BOV not have functioned as expected) which can cause heat and detonation/knock issues), which can lead to engine damage.