Before I get flamed this is only what has happened to my car, and im not saying for one minute that everyones car is the same.
My valver has been running a de cat and Scorpion stainless cat back, along with a K&N filter element in the air box, hooked up to a cold air feed taking air from the gap next to the front fogs using 60mm ally ducting. Hitting the age of 25 I decided a loud exhaust maybe wasnt for me anymore so ive just put the car back to standard. I got the original cat out of the garage, and bought a new Bosal standard cat back system. I also disconnected the cold air feed and re-routed it back to the old pipe that takes its air from inside the wheel arch. Apart from the lowering, the car is now as standard as when it left the showroom 11 years ago. I took the car out for a drive, and im not joking when I say this, but it was like someone had dropped a willy engine in. The difference in power was incredible. I now have low down torque, and more power from 4-7k than ever before. Even flooring it at !15mph, the car picks up speed eagerly right up to !36. With the old s/s and decat I could never get past !20. I have power in every gear, and the delivery is so smooth right up the rev range. Also my problem with lumpy idling has gone (due to the lambda sensor?).
Now I know enough about cars to understand back pressure and fuel mixture, and I think maybe the decat and zorst were messing up back pressure, the cold air feed might have been getting too much air into the engine, and the ecu couldnt compensate so it was running lean. Whatever was happening, it was slowing me down, id say by about 10bhp.
So, a word of warning to anyone expecting a few bhp from a decat and zorst, maybe this could actually slow you down! Im sure there are exhausts that could free up a little extra power, but as most people buy for noise or aesthetics then from a performance perspective spend your money on a thorough service
