The EXUP is a butterfly in the collector pipe, controlled by the ECM, and
opens at high rpm, and restricts flow at lower rpm. Many people remove it,
and as a result, go slower. I don't think Yamaha would have used it if it
didn't produce torque gains at low rpm. It allows the use of a high flow
muffler, while still having the restriction needed at low rpm.
Jon
STL MO
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