I just posted the same thing in another thread. If you've got a Roadster, the Nismo with Crawford Cats + Pop Charger is loud as ****! I'm going to put the stock muffler back on myself and see if that helps things. Any offers for a Nismo muffler (without Y & B pipes?)

 

Over the years I've been reading this site people have posted lots of dynos and such. The general idea is that the stock exhaust system (after the y-pipe) is pretty good. However, the stock y-pipe is quite bad. It is not mandrel bent, so its bends are significantly pinched/crushed. Lots of airflow problems happen there. Just look at the pics comparing the stock piece with any of the aftermarket pieces. The difference is night and day.

So, if you're on a budget and don't really want to change the sound of your exhaust, just installing a nice y-pipe would probably meet your needs.

 

So which Y-pipe would you recommend? which is the quietest?

 

Hmm... I don't think any of the y-pipes increase loudness much. They're pretty much just similar to the stock pipe, but with quality mandel bends instead of cheap crush bends. I'd bet you could go with any of the popular aftermarket y-pipes and be happy.

Some popular I've been seing around are:
Nismo
Fujitsubo
JIC
Gruppe-S

And a bunch of others. I think there is a good deal on the Gruppe-S pipe right now. $275 on their site. That's about the cheapest I've seen for a good y-pipe.