Basics: Blow-Off Valves (BOV)

Following a lengthy conversation I recently had with one of my friends about blow-off valves (BOVs) and their cult-following popularity, I figured that may be a good place to start.

So why do Porsches, Audis and VWs (VAG cars for simplicity's sake) run diverter valves? Why not blow-off valves? Can they run blow-off valves if they wanted?

Definitely good questions, and the root of all debate for recirculating or vent-to-atmosphere (VTA) valves, but not so readily obvious without a little background about mass air flow (MAF) meters and air-fuel ratios (AFR). VAG cars run diverter valves because as far back as I can remember they all use hot-wire MAF-based engine management versus MAP-based management typical of Hondas or even converting to MAP via HKS VPC or AEM EMS or another stand-alone.

The problem with VTA BOVs on flow-based (MAF) versus pressure-based (MAP) cars is overly-rich AF ratios during purging. As you may or may not know, the VAG hot-wire MAF measures a given quantity of air entering the engine via electrical resistance through an exposed circuit in the MAF housing. This explains it more in detail:

http://en.wikipedia.org/wiki/Mass_flow_sensor

So what happens is the air passes through the MAF before entering the compressor half of the turbo, and the ECU adjusts load of the fuel injectors accordingly. Since the MAF measures the total volume of air right before entering a closed system, the ECU cannot account for the air that is lost if you run a VTA BOV post-MAF. So that compressed charge air that is lost when VTA skews the AFR so that there are more parts fuel than air, hence the rich AFR.

The rich AFR conditions during purging can cause drivability issues, mainly instances of stalling (particularly after accelerating quickly in low gear and then pushing in the clutch) as well as soot on the rear bumper above the exhaust tip and also something called "cylinder wash" or "bore wash," when the additional fuel in the AF mixture actually dilutes the oil lubricating the piston to cylinder wall contact. Bad news, since gasoline is a horrible lubricant.

So basically, VAG cars negate all those effects entirely by recirculating the diverter valve purged air back into the intake piping, eliminating any potential for overly rich AFR conditions. It's definitely possible to run a VTA BOV on VAG cars and a lot of people do it (just like they do on Subarus and Mitsus), but the car won't run as well as it would with a recirculating set-up. Ways to solve it are by converting to MAP (HKS VPC, AEM etc), running a blow-through MAF (where the BOV sits BEFORE the MAF on the charge piping) or using a hybrid BOV that vents partially to atmosphere and partially back to the intake.