Don't forget they have a different set up than we do in the U.S. They have stock equal length headers, while we have stock unequal length headers. Equal length headers provide more power than unequal. Also, think of sound waves and superposition. When the waves come out in phase (like in equal length), they do constructive interference. When they are out of phase (like in unequal length) at the right timing, they do that burble.
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