Dirt Rider did a baseline test on a completely stock 2021 KLX300.


23.4 HP

15.4 TQ



Before the tuner... the next test will be with the same exhaust and an FMF Megabomb header just to see what the header alone will make.

 

Below are my dyno test results with a totally stock airbox and snorkel. The slip on exhaust was the only mod. I didn't dare take off the snorkel without a tuner as it would severely lean out the A/F ratio. I have a tuner on order and will do a dyno test with everything opened up when I install it..

By itself, the exhaust made 7% more horsepower and 7.5% more torque.

25.05 HP

16.52 TQ

 

I've run the crap out of my 300 with aftermarket can, with DB killer, no snorkel, and a Twin Air filter. It's showing no signs of running lean. Little to no popping, and header is a nice golden brown color. I'd rather keep the fuel range, than whatever minor gain a piggyback patch tuner will yield, but I'm riding remote off road areas.

 

Yeah, that's a mystery to me as I have zero experience with the bike having only ridden it 1000 miles. Plus the fact that there is absolutely NO information on the fuel management system.

Your experience is worth noting. Perhaps the stock O2 sensor feedback loop system does have enough range to compensate for changes. With the warm weather and Summer blend gas, I'm getting superb gas mileage,averaging 86 mpg and the engine is just broken in. .

 

For the type of riding PHM14 does, his header color is absolute proof that his setup is not running too lean. His DB killer is likely not allowing the slipon to flow any/much better than the stock slipon..

The O2 sensors are used ONLY during closed loop fueling operations - this is lower speeds, lower throttle, ops. As soon as the throttle is "nailed" or anytime it is past 1/2 - 3/4 , the O2's are NOT controlling fueling - as such is open loop fueling governed only via internal tables, temp sensor input, etc.. Sooo, open loop ops will go lean when/if the airflow thu-out the engine is enhanced.

 

Thanks for the useful information.

Hey, you want to see brown?
t's past brown and headed into purple. And I ride just normally like a regular transportation vehicle between 4,000 to 5,000 rpms, which should be closed loop.

 

I have a 2019, so EFI, but if I buy a slip-on and the EJK together than can use a pre-EFI exhaust? My understanding is that the only difference between a pre-EFI pipe and an EFI pipe is that the pre-EFI part would not have an O2 sensor bung. But if I'm gonna run an EJK with the O2 sensor bypass anyway then I don't even need the bung (or a home for the O2 sensor at all), right?

 

As far as I know, you can...
I believe it's designed to take the place of the O2 loop system, because it comes with an O2 sensor bypass plug.

 

Most single cylinder dual sport and supermoto bikes have a loop in their headpipes like this in order to make them longer. Long pipes lengthen the round trip time of reflected exhaust pulses so they affect lower rpms. This produces more torque at low rpms.



KLX bikes lack this loop making their headpipes uncharacteristically short.


Chambered headpipes can compensate for this by adding exhaust volume which makes them behave like a longer pipe.

So what if the exhaust pipe were to be lengthened from the OTHER side? I took a piece of scrap pipe and cut a hole for the O2 sensor just to see how much difference it would make on my KLX. I had done exactly the same thing on my DRZ so I already know the concept works.

(DRZ muffler)



Extending the pipe into the muffler made my DRZ's already long exhaust pipe a total of FOUR feet long.



It was to the point that the engine pulled strong at low rpms without lugging regardless of what gear you were in. I took a piece of scrap pipe, tapered one end, flared the other end, ground out a slot for the O2 sensor, and tapped it into place with a mallot.






My bike has the stock 14/37 SM road gearing for lower engine rpms at higher cruising speeds. 6th gear only becomes useful at 40 mph (4,000 rpm) and higher because of the stock torque curve. Under 4,000 rpm in 6th and the engine lugs and it's never good to lug an engine. You can tell when an enging lugs at low rpms because it doesn't run smooth and "bucks".

I took a test ride yesterday after installing the pipe and immediately noticed the difference. I could now ride at 35 mph (3,500 rpms) in 6th gear with NO lugging, NO "bucking". The engine runs totally smooth. The exhaust still sounds exactly the same because the exhaust impluses are still being dissipated within the total volume of the muffler regardless of where they enter.

So now I'm going to make up a nicer part and run it.

 

I'm trying to confirm which year (part numbers) for the KLX 300R exhaust and muffler that will fit my 2018 KLX 250 (which is now a 300 motor)
The frames look so different, it doesn't look like the 2021 300R muffler would bolt-up to the 2018 frame but I can't tell from these diagrams.
Can anyone confirm which year 300R exhaust & muffler will fit correctly on a 2018 250 frame?
Thanks in advance!
Dave

2021 KLX300R Muffler



2021 KLX 300R frame vs. 2018 KLX 250 frame