A friend of mine said he did it on a few of his cars and noticed better sound and a slight increase in power. I live in Indonesia so do not need to care about emissions. Wondering if someone has done such mod and would care to comment.

 

I would bet the gain in flow is nothing compared with the muffler. I would bet the flow through the cat converter is plenty, the muffler is the big stopper. I would tend to think your friend is having self-fulfilling prophesy - he thinks it's gained power, but hasn't really. Street rodders are actually putting them on in some cases. Probably a legal issue for them in "sniffer" areas. I'd want to see dyno gains or consistent quarter or eighth mile times as proof.

The real power will come through proper jetting and a free flowing head pipe and muffler, but not in huge quantities there. The big gain is big bore.

 

thanks for your comments. sounds like this is not a worthwhile mod. talking about big bore,
I got the efi bike with the stock muffler. Would the big bore work using the stock header and muffler? I recently heard a friends 09 klx 250 with the power bomb and Q4 muffler and this bike was extremely loud, way too loud for my taste.

 

KLX300, I believe has no cat, as it's an off-road bike not required to pass emissions. It's the quietest and still lets the bike breathe better.
On the plus side, it lists the 250 on the muffler, so it appears stock...for where that might matter, like some places in Canada.
If you want, you can remove the straw at the end of the stock muffler(a member here even put an adjustable outlet on his), then drill and tap some holes to accept the e-bay cnc'd pipe end for $35. It will breathe better, still be pretty quiet and if you don't like it...250 mufflers are cheap.

 

Haven't tried it yet. I am scared that something might go wrong with my bike.

 

I have a 331, EFI. I tried to refit my stock muffler for laughs while I was cleaning up my FMF. Huge overheating issues on the stock muffler. Felt so hot after a short ride I believe my plastics would have melted.
Not sure why the excessive heat, maybe too small but plug? maybe new richer settings creating too much burn in the CAT?
I would hesitate a guess that you would need to open up the muffler and remove the cat to run with the BB kit. But I am no expert.

 

Remove the cat, it'll really purr...


HEYOOOOOO
(Rimshot)

 

ohhhh snap!!!!

 

ive taken out cats on scooters
they are very free flowing looking not much to them

the biggest thing in the stock mufflers are the chambers inside
and tubing going through different chambers for sound

if you can totally gut the can, then at least it would be more free flowing and change the exhaust outlet size
but that will be alot of cutting and welding

ive done this to scooter mufflers but actually ended up just as quiet as stock
just no walls or tubing inside keeping the same outlet size
no rejetting needed either

maybe because the oem mufflers are double walled and thick they reduce alot of the noise already which also add to the extra weight they have


I would just buy some inexpensive muffler, like the dg $100 something on ebay

jet kit $45

then you could sell your stock on ebay $100? get some money back
and have all the flow you wanted

 

like I said I've listened to the FMF Q4 on another KLX and even at idle speed this thing was way too loud for my taste so an after market muffler is not an option for me.
Sounds like removing the cat is a lot of hassle and won't do much for sound nor power so I better leave as is.
thanks for all the comments!