Anyone bypass their precats by either an aftermarket y-pipe by modifying the stock y-pipe? If so, any problems like the check "engine light going on"? I want to go for it but all I hear from people is that the "check engine light" is going to go on. Is the power gain nocticable?

 

Tons of people have y-pipes. CEL is not an issue. Do yourself a favor and read threw the FAQ thread at the top of the posts. It helps with ?s like this.

 

There's no point trying to find gains by trying to modify the oem y-pipe. You'll understand why by looking at motorvate.ca y-pipe analysis. Two problems you won't be able to deal with with your "pre-cat bypass" approach are:
* oem's T-collector
* squished cross-section in oem y-pipe where O2 sensor is.