Hello all, I have my KLX300 torn down to the frame for a full rebuild. While I'm at it I am very likely going to install Billy Blue's 351 (or 331, I'm in Vegas temps get seriously hot here. Have to ask his opinion of the 351 in the extreme heat) and his 33/34 pumper carb.

I'd like to keep the bike as quiet as possible. I'm not into loud bikes. From my research the stock exhaust is about at quiet as it gets but, I'm sure it's pretty restrictive, especially with a big bore kit and heavy compared to most aftermarket cans. I hate to go as far as a big bore kit and carb then choke it all down with the stock exhaust.

From my research here on the forum it looks like the only other really quiet option is the FMF Q4. Is the FMF with a larger head pipe worth the expense or, is is nearly as heavy and choked down as the stock muffler?

Also, I'm assuming that going to a larger diameter head pipe will sacrifice some low end torque at the benefit of top end power? If so, I'd just assume keep the stock head pipe - If that is true.

Thanks all for any advice.

 

Since you are starting with the KLX300 muffler, odds are you have better flow than the KLX250 mufflers. A fair number of riders who want to keep their bikes quiet will try to get the 300 pipe.

I would do some measurements before going off any direction. Measure the head pipe ID and measure the tail pipe ID so you know what you are starting with. The 250 has a really squeezed down exhaust, restricting the flow. Supposedly the 300 is bigger, but never having measured one I don't have any idea.

Now I will tell you I did some calculations with an on line exhaust calculator and learned the ideal straight tubing size for the 250 was 1-3/8"ID. Odds are the 300 would be within a small fraction of the same. This was proven out to me when I picked up an aftermarket KLX300 head pipe. It was 1-3/8"ID. I used similar information building an exhaust for an SR500 and it was proven accurate by the dimensions used by flat track pipes.

Summary - find out what you have then do some research on aftermarket sizes.

Larry Roeseler did a fair amount of work with the 300 back in the 90s as Stroker Performance. One of the people there continued on with the work as Four Stroke Works doing some of the hop up work for KLXs among others. They sell a head pipe and a tail pipe/can. Just another possibility.

 

The 300 header is same-as the 250 header except that its' bell (the area where the midpipe from the silencer slips in.) flares out to a larger Dia. - as the 300 midpipe IS a larger Dia. than the 250s'. Point is - making a 351 push exhaust thru it can't be optimal. Even a 250 loses power in the upper mid to top end with its' (same size as KLX300) stock header.

A very good solution would be the get the FSW header (and its' heat shield) , use your stock 300 silencer with the end-cap removed (or an ebay "biggy" endcap).

Alternatively, you can install a complete performance 250 system - KLX250 Megabomb+Q4 for instance.

My '07 KLX300R is running a KLX250S Powerbomb+Q4. It's very quiet to my ears..

 

I'm running 351/pumper/stock exhaust. I like it, not into big noise.
It will go up any hill I need to go.

 

As to the noise of the q4 question, here a vid I put together comparing the stock can to the Q4 on the 250 engine with stock header.

https://youtu.be/EywT9uzMWhs

I've got a 331 now and would say it's about the same loudness (checked with the db meter) but with a little deeper tone.

Tbh, I'm considering to going to a complete 300 exhaust setup. The Q4 is just a bit too loud for me. I don't mind the loss in any uper-end power. I'm mostly down in the lower RPM's any way. That, and police here are starting to crackdown on (loud) aftermarket exhausts and ticketing high. Even ticketing for lack of heat shields.

 

The key information - the thing you need to know - is that the Powerbomb or Megabomb header significantly softens the noise emitted out of a PC4 or a Q4. Outrecordings vid is a good reminder, to those of us that know the difference, that the stock header creates a far louder exhaust note.

I haven't used my stock header for so long that I've forgotten just how loud it makes ANY slipon sound..

 

That's a good point. I've read that they do soften the sound. Would love to know by how much. I considered getting the Megabomb for that reason but there would be a hit in torque possibly?

 

EXCELLENT info in this thread thank everyone!

Outrecording great video you put together! Awesome resource for people trying to choose between stock and Q4.

 

By extrapolating from all the dyno data we have, it seems that with stock cam timings, there would be a loss of TRQ in the midrange ( Perhaps below 6.5K rpm) compared to the stock header - but with a greatly enhanced powerband in the upper-mid to top end.

Check your riding style - specifically how much time do you spend below 6K.. I rarely need biggy throttle response and acceleration @ 4-6k. So big increases above would serve me better.

The data shows that (Megabomb+PC4, MCM, lidless with lidless carb setup) produces about the same power levels below 6.5k as (Stock Header, Stock cam timings, loud slipon, lidless with lidless carb setup). Above this, the former "runs off and hides" from the latter.