Shortening Yz450f Muffler

The length of the exhaust is critical to the tuning of the engine. In the most basic sense, a longer exhaust creates more low-end torque and a more broad power curve. A shorter exhaust system creates more top-end power and a slightly narrower power curve. Other factors come in to play as well including the diameter of the tubing, temperature and the velocity of the exhaust gasses.

If you shorten your exhaust on a four stroke, you'll give up bottom end for top end, but you may not get as much up top as you gave up down low. You will also change the fuel/air mixture, so it's probable that you'll have to re-jet the bike (or re-map if it's EFI).

If that's enough detail, stop reading and go on with your life.

The way this all works is that when the exhaust valves open, a positive dynamic pressure wave travels through the exhaust system. The temperature and static pressure within the exhaust system determine the speed of sound inside the system. That positive dynamic wave travels at the speed of sound in the system. When a positive pressure wave reaches an open end of a pipe (the silencer tip) a negative dynamic pressure wave is reflected back down the exhaust tube toward the exhaust-valves at the speed of sound. When that negative dynamic wave reaches the engine, it helps to pull out the last of the burned-up fuel/air from the last combustion cycle and also helps to pull fresh fuel/air mixture into the cylinder through the intake valves due to camshaft lobe overlap.

So the length of the exhaust system plus the length of the exhaust port is a very important measurement derived from the speed of sound (which varies with temperature and pressure) inside the exhaust system.

I had a Pro Circuit Ti-5 on my old '13 KX450f and it stuck out way too far and I was always nervous about dinging it up and messing it up so I called PC and they said that a lot of people actually cut it to shorten it up a little bit and I could cut off as much as 2 inches without hindering performance. It turned out great!

When or if you decide to do it make sure you make the cut on the exhaust tip end of the can instead of where it attaches to the mid pipe because you will mess up the mounting brackets. So you have to take the exhaust tip off and make your cut there. Also you have to cut the exact same amount off of the inner core that you did off of the can!

Good Luck!
Mike

On 2 strokes it's opposite, shorter for low end, longer for top end. Most 4 stroke mufflers are longer to meet sound requirements. FWIW.

I did this on my 2002 CRF450 (and many others).

Performance wise, I didn't notice much difference. There are so many variations of exhausts available it can't be much different that either one end of the spectrum or the other. You can probably get another can for $100 on eBay if you mangle yours so it seem it would be worth the risk.

On my Honda in 2002 I drilled the rivets from the back end of the can. I pulled the core and end cap out and carefully measured 2" and cut it with a cutoff wheel on a grinder. Then I carefully marked a 2" line around the aluminum can and did the same thing. I placed the core and end cap back into the can but didn't insert it the last 1/2" or so (13mm). then I transferred the holes for the rivets down onto the aluminum can. I drilled the new holes the same size as the holes in the end cap flange. Then after inserting the packing (2" shorter) I put a bead of red silicone sealant around the end cap joint and used a dead blow hammer to re-seat it in the can. Installed the new rivets and voila......works exhaust. Well, sorta.