I have a 2003 Mazda protege speed that's standard and came stock with a turbo on a 2.0 liter engine my question is it has a recirculating blow off valve and I was wondering if swapping that for an external blow off valve would hurt anything?
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Depends on wether it's maf and uf so measured wherw

 

No. When the throttle closes, although pressure is trapped in the intake, the fueling stops during decel, the BOV vents to the atmosphere, and the O2 helps to recover fueling. It might momentarily run a tad rich, but you won't even notice it because correction is fast. The O2 correction will help it get back to stoich, or whatever the programmed air/fuel parameter is at that particular RPM with the Mazda ECU...