I have a 2009 and 2012 KLX and have stock exhaust. As we all know it is pretty plugged up with the stock pipe, but it is quiet. I am looking for a way to obtain a bit more power and have the bike breathe better but.... keep the bike quiet. Has anyone had luck modifying the stock pipe at all? If not what is the quietest pipe I can get? Q4?

 

My experience is that there is no real performance benefit to removing the endcap and/or gutting a stock exhaust. There really isn't a significant weight savings either. You'll gain some decibels but that's about it.
Stay away from Two Brothers if your mandate is to keep the volume down.

 

Q4, or possibly a stock KLX300 system if you can find a used one on ebay.

 

Wildcard modified the tip....he cut out the "straw" and opened it to the internal diameter IIRC. He's got an exhaust mod thread, search his posts.
You'd need to turn out the A/F screw if you do that.
FWIW, I put a used 300 header and a PowercoreIV on, for 2 days...it was loud. Ended up taking it off and taking it apart to see, if maybe, it needed repacking. Still haven't repacked it or ordered the insert to make it more quiet. Hopefully that helps. My butt dyno said it gave good low end power and lots of noise, that's about it.
I did like being able to rev the motor to get someone's attention, it was louder than the horn and very light, in comparison to the stock.

 

Get more gains from getting more air in. Try a UNI filter and snorkle removal. Not much use letting more out if its not getting in. Also Marcelino cam mods in the sticky threads wake it up.

 

The OEM muffler is relatively heavy and restrictive. You open up the airbox a bit...a no-brainer...and install a quality aftermarket muffler with the appropriate jetting...a Dynojet kit prefereably...and that's all most people need to make a much more pleasing power response. Great gobs of added horsepower and torque?...No. A fatter powerband and better response?...Yes.

 

Modding my stock muffler helped a LOT.

However as soon as i have the extra disposable income i'm going to toss that heavy, ugly, still restrictive POS and install a full FMF or Muzzy exhaust (header and muffler).

 

I've had stock, then and HMF Performance (loud and raspy) and FMF Q4 (quieter than the HMF, but louder than stock). I think the HMF provided the best performance, but I'll take the Q4 over the noise of the HMF.

 

The stock muffler is where most of the restriction to flow exists, as I did my mods one at a time and the muffler replacment was the single biggest gain I got from stock. Much more than jetting, and I go between stock and a performance ac and can tell almost no difference.

 

A huge thanks to all of you for the responses. Much appreciated.