Guys, I came across a nice article on exhaust modification... After reading this I understand why people start complaining after undergoing some mods which goes wrong, and spikes up fuel consumption or sloppy performance etc.... Its simple science... My conclusion - I should stick to OEM exhaust. Hope this helps... Wear helmet & Ride Safe

The Article is below: -

The automobile plays an important role in the transport system of India. With increase in population and living standards, the vehicle population also increasing day by day. There is steep increase in the number of two wheelers during the last few years. All this are increasing exhaust pollution and particularly in metros as density of these vehicles is very high. As we know, more the emission, more the amount of fuel we have to burn in engine.

Fuel consumption is directly depends on working conditions of an engine. Particularly for two wheelers, an extensive analysis of energy usage and pollution shows alternative power systems are still a long way behind the conventional ones. Moreover many bikers uses modification in their exhaust to get so called exhaust tune and sporty look.

Consider a normal bike converted into dirt bike and exhaust has been modified to get sound like dirt bike. I saw many FZs, FAZERs and other bikes with modified exhaust like R1. Then I decided to observe the effects of this modification and I had performed one experiment.

Engine does a process called scavenging in between completion of exhaust stroke and starting of suction stroke. This process is for removing the burned gases from the cylinder with the help of fresh charge.
The scavenging efficiency of engine is directly affected by speed of the engine, exhaust valve opening, exhaust port dimensions, exhaust pipe location and position, dimensions of catalytic converter, muffler design. All these parameters finally affect the back pressure of exhaust. Back pressure in the exhaust is required to remove the gases from exhaust pipe.
After completion of exhaust stroke, Pressure in the exhaust pipe becomes more than the atmospheric pressure. Because of this, gases flow automatically from pipe to the atmosphere with high velocity and produces tune that we listen from exhaust.

Now to get the smooth (Nearly Laminar) flow of exhaust gases, designer keep the value of back pressure within limit by introducing slant in muffler (You can observe this slant in each bike above 125 cc), muffler design, path travel by gases etc. according to the capacity of engine. Now at the time of modification, we are influencing following parameters: Experimental SetupResults

So guys, here you have the results of modification done in exhaust. I did the modification only with opening angle of exhaust; many other factors are yet remaining to be examined. Many other factors also contribute to these effects.
You can judge the results what if all the factors contribute together. This is what generally happens when we modify our exhaust with local dealers or garages. We blindly are increasing pollution and fuel consumption of our bikes. Many calculators are available on net for calculating correct required back pressure for your bike engine. You have to insert some technical specifications and it will provide you the correct back pressure.

I personally suggest you not to modify your exhaust ever. Because it is optimum design provided by the manufacturer and designed by their best designers. Even then if you are so passionate about noisy bikes (there is vast difference in tune and noise), first you get the calculated back pressure and then get the correct dealer for correct exhaust.