Fiesta ST - OEM Airbox Limitation

If you are running an aftermarket turbo (not including hybrids) on your Fiesta ST, in conjunction with the stock airbox, then you are going to want to read this.

I recently had (2) separate instances with customers where they were complaining that their car was falling on it’s face at around 6K RPM. Both cars running the S280 turbo. After viewing logs and their maps, I could not figure out what would be causing this cut. I was about to contact Cobb, to ask if there were any software glitches to be aware of, or if they had any ideas. Until one of my customers (you know who you are 😊) was able to figure out the issue! He was running a RAMAIR induction hose, going to the stock airbox with drop in filter replacement. He was able to determine that at high RPM, the induction hose was actually collapsing from vacuum. I immediately asked him to drill a few holes in the bottom of the airbox, and the issue went away completely. In fact, not only did his issue disappear, but he picked up some decent power gains too!