2008 Mitsubishi Evo GSR: Blowoff Valve Walkaround

You already know that t he BOV is a valve that, at times, allows boost to vent back to the intake. Primarily, this is to prevent compressor surge, which is an unstable aerodynamic condition experienced by the turbo's compressor.

A word on compressor surge: it rapidly loads and unloads the turbo's rotating group in the axial direction. Prolonged surge will pound the crap out of the turbo's thrust bearing.

The valve remains closed (i.e boost is not allowed to vent back to the intake) during wide-open throttle operation for obvious reasons. 

Visually , the flow situation in the intake during WOT looks like this:

When you then lift the throttle, like for a gearchange, manifold vacuum is applied to the BOV's top chamber, yanking the valve fully open (this generates the loud "psshh" sound you hear from some modified turbocharged cars). At the same time, the amount of flow through the main intake pipe drops substantially since you're off the throttle.

Many people don't realize, though, that the BOV is open during certain part-throttle conditions. This is intentional--it increases the amount of flow that the compressor "sees" during part-throttle conditions, which makes for much better transient boost response. Mash the throttle from a roll and boost rises much quicker than it would otherwise.

What this means is, at times like when you're just accelerating moderately from a stoplight, the BOV will be continuously venting moderately low-pressure air. Compare this to a gearchange, when the BOV vents a big blast of high pressure air all at once. 

During this part-throttle venting, there isn't much flow coming through the main intake pipe. This means that right at the point where the two flow streams merge, disruptions can happen. And some aftermarket intakes place the MAF right near this merge point.

Flow disruptions can freak out the MAF (mass air flow sensor). When the MAF is freaked out, your engine loses track of how much flow is entering via the intake.

Without any idea of what's going on, the engine may just go into limp-home mode. Project Evo experienced this a few times after the cams and boost pill were installed.

Turns out this combination of hardware gave our engine enough ability to generate ample boost at part-throttle. This extra flow vented by the BOV was enough to make the MAF go crazy at times, and we'd have to pull over and cycle the key to clear the fault.

It was an elusive issue--sometimes it would go weeks without a hiccup. One week it happened three times. This seeming randomness made it hard to pin down what was going on.

But a solution is now in place. More on that later.

Jason Kavanagh, Engineering Editor @ 20,870 miles.