If you want to run a straight pipe, there is some math to do to get the most power at certain RPMs, fatter pipe at higher rpm=more power, skinner pipe at lower RPM =more power but less up top.
big pipe
At low RPM, the big pipe will provide stagnant exhaust gasses, This gasses cool down, this also causes stagnant gasses to form, these give bad flow characterists to the exhaust and make the engine work harder to remove the exhaust.
At high RPM, the same pipe will have enough pressuer where there is not stagnant cooling gasses. It will also be hotter. Hotter gasses move faster, thus causeing the engine to work less and giving more power.
small pipe
At low RPM, the small pipe will not build stagnant exhaust gasses as it takes less to fill the pipe. The pipe will be hot, gasses flow fast, no problems, happy power in low rpm
At high RPM, the pipe will be hot but restrictive enough where the engine is almost choking.
OEM pipes are usualy between, thats why after market pipes, that are bigger, increase power. Your muffler plays a massive role in this though
Short pipes work great at high RPM, cold air can't reach the vavle to warp it - look at dragsters
Long pipes are harder to push gasses out of, you need to find a happy medium.. oem pipes found that - as a reference.
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