How do pipes affect performance?
 

It may sound simple but I just can't wrap my head around it. I'm thinking about changing the pipes on my bike but I've heard that different pipes will change the performance of the bike. I want to go with short straight pipes for a sweeper look but would super short pipes negatively affect the engine etc?

As a very general rule of thumb, increasing exhaust flow will increase horsepower at higher RPMs at the expense of torque at lower RPMs.

Exhaust geometry on bikes is tricky. While greater flow can be helpful, you also need to be mindful of the scavenging effect. Basically, exhaust pulses help to pull spent charge out of the cylinder, increasing performance. If you change the exhaust you can alter and harm the overall effect, thus decreasing performance. I'm not sure on the specifics, since I've never done any real mods on bikes.

The effect also exists for cars, but it tends to be less of a concern, since the numbers are bigger in general.

Sweet god, someone else that actually understands exhaust mechanics. Or yeah what he said.

I found these old pages from Nissan guru Mike Kojima. What he writes is focused more on small displacement four cylinder engines, but the principle shouldn't be that much different when it comes to motorcycle engines.

http://www.nissanperformancemag.com/november02/nerds/
http://www.nissanperformancemag.com/december02/nerds/

A lot of people throw time and money at something with no real way to know the results of their tinkering.

The only way to truly know how you have changed performance is to have a benchmark DYNO test on the stock machine and subsequent runs to see the changes.

In my experience changing exhaust alone will not have a positive effect on performance, but along with changes in jets, airbox, port polishing, etc. some gain can usually be had. Only you can decided it it's worth the investment in time and money.