Split Port Injection is what SPI stands for. The Ztec puts out more power and is the type that SVT built off of in the SVT Focus. Zetec has dual over head cams. Im sure someone will give us a lot more info, thats all i got.
The SPI, while fairly low-tech is a decent motor, and has a cult-like following by some people. It is just your basic SOHC Inline four. The one cool thing about the SPI is that it has a set of valves that reduce the runner area to increase low end air velocity, and therefore torque.
The Zetec, as used in the standard Focus, is just your basic DOHC cam on bucket type design. Nothing fancy, no variable valve timing or variable runner length... But a good little engine.
Both are fairly choked off for emmisions/CAFE purposes, and respond well to basic bolt-ons.
it is true that they are both restricted by emissions not to mention just about everything else intake exhaust tb and a hole lot more the focus of america is your basic american car restrictive for once i would just like to see all the foriegn cars come over here and not get changed things would get real interesting with the nissan silvia, skyline, honda's type r cars and spoon cars bmw's real rides just to name a few that would be a blast