aftermarket exhausts: Explain please

[Goin Ape Nutz]

Ok... so I need confirmation of my belief on after market exhausts, or confirmation that I am an idiot and should STFU.

My basic understand of exhausts comes from cars. The smoother and easier the flow of the exhaust out, the less back pressure, the cleaner the air/fuel mixture, the smoother the airflow in, which all results in better engine performance and increased response and power.

Now I know the above listed algorithm isn't exactly correct, so anyone who wants to correct it please feel free to do so sans flames. If you can tell me more about it, please do... When I put a after market yoshi on my vtr250, I did so thinking that I was going to get a small increase in hp/trq from the smoother, free flowing yuck going out... even had to adjust my carbs to compensate fuel/air mixture (or did i?... I did anyhow).

Ok. Feel free to correct or affirm my beliefs.

[ktravelet]

depends what you want. Less back pressure, higher flowing will give you more HP, the opposite will give you more torque.

[Jody@Sportbike Solutions]

What you're saying is basically correct, but there plenty of other variables to account for. Even the best exhaust systems work at optimum efficiency within a relatively narrow RPM range... everything else being a compromise. Factors such as primary & secondary pipe diameter, primary & secondary pipe length, collector size/style, exit pipe size, etc all combine to determine where this powerband lies, how broad it runs, and how much power increase, if any, you'll see.

In the most general terms, smaller diameter pipes and/or longer primaries will keep up exhaust momentum and increase scavenging to give better low-mid rpm range response and torque, at the expense of high-rpm horsepower. Larger diameter pipes and/or shorter primaries offer high-rpm power gains, where the volume of exhaust gasses are great, and the time in which to expel them short.

Within those parameters, designs with large radiused curves and reduced turbulence certainly increase performance.

Still to consider are the effect of intake design, cam profiles, etc. All of which should be taken into account when choosing/designing an exhaust.

[Goin Ape Nutz]

^^ thank you

[Tuffguy #2]

Bikes even have smaller HP gains then cars since they are already pretty well tuned out of the factory. Most cars are tuned for mpg instead of performace. I think most bikes are already pretty well performance tuned right from the get-go.