i have a 2000SI which has a 1.8 block and a 1.6 head. i used to have a turbo in there when it had all out 1.6 so i punched out the cat to create a straight pipeand create more flow. now that i have this set up it seems like the car has created a back preassure which you could feel when it hits around 2700 rpm and then starts running fine after a few seconds. dunno if this makes the car misfire or what not but have you guys had this problem and if you have then how did you fix it.

 

fix it by getting a new cat and not punching out a hole....
That doesnt create a straight pipe and more flow....

 

hehe, my friends used to do that. Get a steel pipe and beat it through the cat. Just spend like $40 and get a high flow cat