i have a 3in straight pipe on my zetec (installing turbo in a few weeks) and it is super loud and VERY raspy. then i put the svt cat on and its still pretty loud. but then again i am running 3 ins all the way back with no muffler.
I would say be careful of an exhaust system that's TOO free-flowing -- I've had my 2003 ZX3 (non-SVT) for about a month, and it came equipped with the stock ZX3 exhaust manifold and flex pipe, followed by what I call a "******* cat-back": It had what looked like a 2-1/2" straight pipe back from the flex pipe, and a TINY little "muffler" (smaller than the stock ZX3 lunchbox, if you can believe it) at the back end.
Needless to say, this setup was PLENTY loud -- too loud for me, in fact. Not too loud to notice that I could hear what I believed to be valve clatter at higher RPMs, though. Nothing severe enough to worry about, and I'm not really familiar enough with Zetec engines to know if that's common, so I wasn't really worried about it.
Thankfully, I managed to locate a stock OEM SVT exhaust system (nice Ford parts, all stainless, low miles, still shiny, and embarrassingly cheap). I got the SVT exhaust installed yesterday and the transformation is amazing. The exhaust sounds great, and most surprising of all, now that things are MUCH quieter I can't detect ANY
valve clatter at any RPM now. The engine also seems CONSIDERABLY more responsive across the entire RPM range.
Even more interesting, I have logged about 120 miles on the car since having the new exhaust installed and based on the movement of the fuel gauge it seems
that I'm getting significantly better fuel economy on this tank of gas (and I've been doing lots of heel-and-toe shifting while enjoying the new exhaust note). I know the needle on the gas gauge alone is a poor tool for tracking fuel economy, but I typically reset my trip odometer every time I fill up, then do the "miles traveled divided by fuel required for the next fill-up" formula to calculate my mileage between fill-ups, so I'm pretty much aware at any given time of roughly what kind of fuel economy I'm getting based on the miles traveled since my last fill-up.
Anyway, my point is, I believe that the "******* cat-back" setup wasn't providing proper back pressure, thus leading to valve clatter (or perhaps detonation that I was interpreting as valve clatter?), occasionally sluggish throttle response, and poorer fuel economy.
Prior to installing the SVT exhaust, I had been experiencing fuel economy numbers ranging from 27 - 30 mpg, slightly lower than the 30 - 32 mpg I was enjoying with my previous car, a 1998 Saturn SL2 (equipped with a DOHC 2-liter 4-cylinder engine, not all that different than a Zetec). I plan to continue tracking my fuel economy numbers over the next few weeks and I will report back, but I think the bottom line is that my new yet "more restrictive" stock SVT exhaust system is actually MORE efficient overall than a more "open" system.
In the meantime, I would heartily recommend taking into account proper back-pressure considerations when opting for exhaust solutions for a normally-aspirated Zetec.