The exhaust system is one of the unsung and certainly misunderstood components on the motorcycle. Most of us only think of its function as expelling noxious gases, the by-products of combusting fuel from the engine and away from the rider.
That is certainly the main function of the exhaust pipe, as it was since the earliest days of the internal combustion engine. However, the understanding of gas kinetics which is in itself based on fluid mechanics and thermodynamics, has made the exhaust pipe a crucial element in influencing engine performance, fuel savings, and character.
Surf the (exhaust) waves
As the exhaust valve(s) open, the hot burned gases rush out of the combustion chamber into the exhaust port and exhaust header. And the pressure of those gases is so great that the gases travel at the speed of sound, called a negative pressure wave.
Since it travels into the pipe at such velocity, it creates a low pressure region behind it, allowing it to drag fresh fuel and air mixture into the cylinder. This way, the combustion is filled much faster than just relying on the piston’s down stroke to create a partial vacuum, especially at higher RPMs. (This is where “valve overlap” timing plays an important part.) This effect is called “scavenging.”
BUT! This scavenging must stop at some stage, otherwise unburnt fuel will also be sucked out of the combustion chamber, causing the engine to lose power (and also make it fail emissions tests).
Thus, exhaust engineers figured out a way, in the form of the muffler's reverse tapered end cap to reflect the exhaust’s wave back towards exhaust port to stop the unburned gases from leaking out until the exhaust valve(s) close and seal off the combustion chamber. This positive wave is also known as back pressure.
Now you know why using a straight pipe makes the engine lose power, especially at any throttle opening other than fully open!
The trick is to time the back pressure must be timed correctly. Remember that the gasses travel at the speed of sound and the exhaust valves will open and close 50 times per second at only 6,000 RPM!
So, the pipe’s length and diameter must be carefully designed, hence why some exhausts coil around like intestines especially for Vee engines as the rear cylinders are nearer to the end of the exhaust, as all cylinders must have equal pipe lengths.
But pipe length and diameter only solves one problem
However, there is a caveat. One can only design the pipe’s diameter and length to of be of benefit at a certain RPM range. This is because the exhaust’s pressure wave is different at different RPMs. For example, a two-stroke’s expansion chamber gives the engine its character.
In other words, you can design the exhaust for better low-down torque but you lose top end maximum horsepower. Conversely, a pipe designed to provide higher peak power will compromise the engine’s low end torque. Fuel injection and cam profiling and timing cannot fix this because the exhaust is made to fit the fuel injection, cam profile, and timing in the first place!
Enter the exhaust valve
This is not to be confused the exhaust valves in the cylinder head. Instead, it is a separate valve that usually sits in the downpipe just before the muffler.
It was Yamaha who created the powervalve for the two-stroke engine called the Yamaha Power Valve System (YPVS), and it was also them created such a system for the four-stroke engine. Called Exhaust Ultimate Powervalve (EXUP), it made its debut on the 1990 FZR1000.
It consists of a butterfly valve that sits in the pipe and actuated by a cable. Its job is to reflect pressure waves back to the combustion chamber and go to fully open state at higher RPMs. So, you have good low RPM torque for acceleration coupled with high maximum power output for a higher top speed. Additionally, the engine response is also smoother. This technology is now utilised by all superbikes (apart from the Ducati Panigale).
Should I decat?
The big ugly box under the engine contains the catalytic converter. Inside is a honeycomb structure made usually from platinum. Nitrogen oxide from the combustion process reacts with the platinum, turning into water vapour and carbon dioxide.
Granted, removing the cat will let the bike save some weight and the engine to breathe freely but you’ll be ejecting lots of pollution into the environment. Personally, we’ll keep it on.
Does a louder pipe equal more power?
A loud pipe usually means a less restricted exhaust gas flow which could mean better engine power. The simple answer is not any longer, as fuel-injected motorcycles still inject the same amount of fuel at a certain RPM as programmed into the ECU. This programming relies on oxygen sensors along the pipe changing the pipe along will not result in extra horses.
Instead, you need to retune the ECU to fully reap the benefits of a new exhaust, just as rejetting the carburettor in the old days.
So, why bother changing the exhaust at all?
There are still benefits from changing the exhaust system to a an aftermarket item.
One, the bike will save so much weight. Less weight means better acceleration, better braking, and better handling.
Two, the engine will have better performance, provided that you retune the ECU.
Three, the bike sounds better. But please don’t use an exhaust so loud it’ll wake up the dead!