Zetec Engine Hacks

Underdrive Pulley Installation
The Underdrive Pulley, or UDP, uses high-school physics to reduce your accessories' drag on your engine, therefore allowing more power to the wheels.
Adjustable Cam Gears
Note: These instructions help with a Timing Belt installation. Installing a pair of manually adjustable cam gears will allow you to fine tune the valve timing. When used properly, the fuel economy, torque, and horsepower curves can be manipulated to the driver's needs. Note: Expensive Equipment (a Chassis or Hub Dyno) is needed for proper tuning of adjustable cam gears after installation.
Cat-Back Exhaust System Installation
I'm writing this Cat Back install up AFTER doing one myself and making quite a mess. This is not how I did it, but it's how I'm going to do the next ones!
Camshafts - Removal/Installation
Installing a set of aftermarket camshafts can improve your engine's performance by opening your engines valves farther, keeping them open longer.
Porting the stock Throttle Body
How to bore out your stock TB for imporved throttle response.
Zetec Stumble Fix
On a lot of Foci, the DPFE and EGR act sort of strange when cruising, and can give the feel of throttle "flutter" or like you're driving into gusty winds when you are not. Here's a highly unofficial Fix from FocusHacks.com!