I did some research and found few things.
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In any case, the tip of the exhaust should exit at a point where there is a low pressure region to minimise back pressure. It's a cheap form of weak turbocharging.
from
http://www.billzilla.org/engexhaust.htm
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I would like to increase the horsepower of my car's engine. What's the best way to do this? ::
Let exhaust exit more easily - If air resistance or back-pressure makes it hard for exhaust to exit a cylinder, it robs the engine of power. If the exhaust pipe is too small or the muffler has a lot of air resistance then this can cause back-pressure. High-performance exhaust systems use headers, big tail pipes and free-flowing mufflers to eliminate back-pressure in the exhaust system.
from
http://auto.howstuffworks.com/question395.htm
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How do exhaust headers work to improve engine performance? :: The idea behind an exhaust header is to eliminate the manifold's
back pressure. Instead of a common manifold that all of the cylinders share,
each cylinder gets its own exhaust pipe. These pipes come together in a larger pipe called the collector. The individual pipes are cut and bent so that each one is the same length as the others. By making them the same length, it guarantees that each cylinder's exhaust gases arrive in the collector spaced out equally so there is
no back pressure generated by the cylinders sharing the collector.
from
http://auto.howstuffworks.com/question172.htm
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ok, iam not so good in technical stuff. so, if i add exhaust to 1&4 and 2&3 cylinders then i can gain some acceraltion with no bad effect on engine
correct me
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still technically dumb