I was wondering if the HKS exhaust is emissions legal in the State of California. Will it past the smog check?

 

An Exhaust does not have anything to do with a smog test. All its function is to cut down on the sound. Same with the resonator.

If anything it may not pass state inspection cause it may be over the limit of 80dBs

Dixit

 

so what can I do when it comes to emissions test? Put back the stock muffler? I heard changing the y-pipe might reduce the sound, is that true?

 

Like I said, the exhaust will not affect the emmissions test at all.
Not one bit. Now the the aftermarket Ypipe will affect it for sure. Because of the fact that it removed one of the factory built in pre-cats. But like 100s of posts have mentioned that if you warm up the car even with an aftermarket Ypipe, you will pass the Emmision 100% fine. The pre-cat is supposed to help reduce the pollution when the car is cold.

And an aftermarket Ypipe will INCREASE the sound, not decrease.

Dixit

 

Y-pipe + HKS = LOUD!

GC

 

Well just a y-pipe with stock exhaust won't increase that much right? As compared to a cat-back or a muffler. It'll sound deep at WOT right

 

Y-Pipe and stock exhaust is quite acceptable. The intake that I have is the largest increase in sound and the y-pipe was minimal additional sound. With my stock exhaust, the cruising sound is, essentially, stock. WOT is noticably different, but not too loud.

 

Ypipe + Custom Muffler + No Cat = Hella Loud!!!!