I put my Stillen Y-Pipe on my 4th gen today. It fits good and sounds as I expected (same under normal driving, a slight sucking sound at WOT). The rest of my exhaust is stock, and I'm only using a K&N panel. The car is acting funny. Before the Y-Pipe, when I let off the gas, the car slowed down immediately without braking. From what I understand about electronic fuel injection, this is because the ECU senses that I am running at a relatively high RPM and no throttle, so it runs the fuel mixture all the way lean, thereby pumping only air through the engine to conserve gas and slow the car down. My car is not slowing down well while it's in gear. It seems to keep pushing more when I give it no throttle. Could this in any way be due to the Y-pipe? Are my fuel injectors not going full lean when I give no throttle in gear? Or is the just a side effect of a lower backpressure exhaust system? I know this is not all in my head! Thanks in advance.

I just got mine the other day also, and I noticed the exact same thing...I was thinking it maybe has something to do with the back pressure??? I dunno.

I have an UDP and the car really slows down when lifting up the gas in gear. The y-pipe will balance this all out. I suppose my Wilwood brakes(not yet installed) will be used more however.