According to calculated ID differences with the stock MAF housing, this should flow 10% more air. Velocity will be lower and reduce G/Sec readings proportionately, but use of 4 bar FPR seems to bring everything back into line nicely. Driveability, low end grunt and tip in power all seem to improve nicely. It is a mod you can definitely feel. All Fuel issues are now in line with the FPR. No cels, and within 4-6% of 0 adaptation levels! Write-up here: http://forums.audiworld.com/tt/msgs/1064906.phtml
A 4 bar FPR Bosch # 0 280 160 575 is necessary for this mod (2001 Passat 20v)
After reading this article http://www.autospeed.com/A_0629/page1.html I decided to remove the MAF screen and vane straightener as it appeared to be a significant restrictor of intake efficiency. The before/after graphs speak for themselves. This might not work as well with the OEM airbox as that's where the flow straightener comes into play. CELs have been reported with the stock box (air turbulence across the MAF element).