Wis. Stat. § 42.05: Auto races.

(1) Except during the annual state fair and at other times between 8 a.m. and 10 p.m., every motor vehicle, as defined in s. 287.15 (1) (e), that is used at state fair park in racing competition or practice shall be equipped with a muffler which, at all times, shall be in good working condition sufficient to prevent excessive or unusual noise.

(2) It is unlawful to operate, or for the state fair park board to permit to be operated, at state fair park in racing competition or practice except during the period specified in sub. (1), any motor vehicle, as defined in s. 287.15 (1) (e), with the muffler or cutout open.

(3) Any person violating this section may be fined not more than $200 or imprisoned for not more than 6 months or both.