When you are doing downpipe, like if you're upgrading from k20 to k24 and you were running 2.0 on k20. Now when you'll jump to 2.25 you'll get a balanced top speed and instant acceleration along with cat hooked and car will have smoot looking sound with no petrol smell coming out of tail pipe.
If you run more wider pipe like 2.50-3.0 with no cat you'll get good instant acceleration and a dropped top speed and due to removal of cat you'll get a loud car, once you'll drop your feet in gears its going to make you hear pops n bangs.. And also strong petrol fumes coming out...
And once you do too big or wide downpipe, concept of overkill comes in action...
So do share your requirements and if got time do visit me at G13/4 or at haj complex road.