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There is very much to 4 stroke exhaust performance. I'm going to make this lengthy, but accurate. The obvious primary functions of the exhaust system is to direct hot, diry gases away from the combustion chamber and to baffle noise made during the combustion process. In addition, and one of the most important aspects of the exhausts functions is to encourage a fresh, cool, fuel and air mixture into the cylinder area using a process call scavenging. Scavenging, is quite difficult to explain unless one somewhat understands thermodynamics, however, I will try to put it into laymans terms. For every gas pulse resulting from combustion, there is a wave created (for lack of a better term) This wave is the exhaust gases. After the combustion process the exhaust valve opens, creating a pressure difference, allowing the exhaust gases to exit the combustion chamber. As this wave rushes out through the exhaust manifold, there is a negative pressure behind it, which pulls in the fresh AIR FUEL MIXTURE from the intake manifold. This happens when Intake and Exhaust valves are open simultaneously during the OVERLAP CAM EVENT. This where the exhaust system comes into play. The exhaust systems are designed with 2 things in mind. 1) Where the bike is designed to performed the best, and 2) Governmental Regulation. Keep in mind, that a sportbike is designed to live it's life at high RPMS, and it's with that in mind that the exhaust systems are designed. To keep this simple I will keep the physics brief. However, it must be stated that exhaust systems and Intake manifold systems are designed using a simple physical law called the venturi effect. Basically it states that as (air) velocity increases, the (barometric) pressure decreases. For example, air traveling through a tube shaped like an hour glass will speed up as it reaches the middle, and slow down as it travels away from it.

Basically, if the exhaust pipes are too big in diameter, the exhaust gases do not travel fast enough and the pulses will overcome each other travel back into the combustion chamber. Obviously, exhaust gases will force some of the air/fuel mixture to stay in the intake manifold resulting in a lower volumetric effieciency for the combustion. Basicially, less air/fuel, less power.

Like wise, pipes too small in diameter do not flow ENOUGH to remove the gases away from the exhaust manifold and gases end up staying in the combustion chamber producing the same effect as stated before.

For reference this is simply put. Larger pipes in diameter, will flow more gas and will improve TOP END PERFORMANCE. With more RPMS, the combustion event happens quicker and exhaust gas velocity is sped up enough to overcome the large pipes and even take advantage of their larger flow characteristics. This is why Nascars have virtually no exhaust system. They spend their lives at the TOP END of their RPM range.

Pipes smaller in diameter will have much better LOW END PERFORMANCE. The pipes are smaller in diameter which increases the speed of the exhaust gases via the VENTURI EFFECT, which in turn increases the SCAVENGING effect I explained earlier.

So basically it boils down to building an exhaust system which fits your bike's purpose. You can't have the best of both worlds. Build pipes with more top end performance characteristics, and you sacrafice low end performance. Likewise when opposite.

So when it comes down our pipes on our sportbikes, you must keep in mind that these bikes are designed to be on the track where HIGH RPMS are sustained. However, because these are street bikes, they must give them so amount of street ability. So stock pipes are designed to give a tolerable mix of low end and top end performance. They also must conform to Governmental Regulations, which is where many aftermarket pipe companies take advantage because they do not have to conform to these laws. This is why aftermarket pipes MAY see gains in performance. But peak numbers mena nothing. One must check the dyno graphs and find out exactly where these HP gains are being made. Yes, they might make +6 HP, but you might only see it at 12500 RPMS, and how many of you actually ride that high in the RPM range? Think about it.

This is really an overview, there are many concepts I have left out (blowdown, reversion, sonic and thermal scavenging) for the sake of simplictiy. However, this should give you a basic idea that 4 stroke exhaust is essential and just as complex and important as two stroke exhaust.

Let me know if you have any other questions.
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