From the vehicle code found on the PA DMV website. There could be more but I gotta get back to work! Here's the website:

http://www.dmv.state.pa.us/vehicle_code/index.shtml

§ 4523. Exhaust systems, mufflers and noise control.
(a) Compliance with established sound levels.--Every motor vehicle operated on a highway shall be
constructed, equipped, maintained and operated so as not to exceed the sound level for the vehicle as
prescribed in regulations promulgated by the department. The test procedures and instrumentation to be
utilized shall also be established by regulation.
(b) Compliance with exhaust requirements.--In addition to any requirements established under
sections 4531 (relating to emission control systems) and 4532 (relating to smoke control for diesel-powered
motor vehicles), every motor vehicle shall be constructed, equipped, maintained and operated so as to
prevent engine exhaust gases from penetrating and collecting in any part of the vehicle occupied by the
driver or passengers.
(c) Mufflers and related equipment.--Every motor vehicle shall be equipped with a muffler or other
effective noise suppressing system in good working order and in constant operation and no muffler or
exhaust system shall be equipped with a cutout, bypass or similar device.
(d) Unauthorized modification of equipment.--No person shall modify the exhaust system of a motor
vehicle in a manner which will amplify or increase the noise emitted by the motor of the vehicle above the
maximum levels permitted under subsection (a) or violate the provisions of subsection (b). Headers and
side exhausts are permitted provided the vehicle meets all the requirements of this section.
(e) Fire equipment and racing vehicles.--This section does not apply to fire equipment or to racing
vehicles being operated in an organized racing or competitive event conducted under a permit issued by
local authorities.


§ 4531. Emission control systems.
(a) Compliance with established maximum levels.--No vehicle manufactured in compliance with
the requirements of the Clean Air Act (77 Stat. 392, 42 U.S.C. § 1857), or any amendments or supplements
thereto, shall have emissions exceeding the maximum permissible levels prescribed by law.
(b) Limitation on alteration of system.--No person shall disable, change or alter the emission
control system of a vehicle. Original emission control components or replacements in kind shall be present
and functioning on all vehicles. A subject vehicle may be equipped with any added components which are
designed to improve emissions. It is unlawful for a subject vehicle that is not in compliance with the
preceding requirements to be operated under its own power until a reinspection at an official emission
inspection station establishes its full compliance, provided that it shall be lawful for the vehicle to be
operated under its own power by the vehicle owner while en route to the official emission inspection station
for a reinspection.
(c) Limitation on sale and operation of vehicles with altered systems.--No person shall knowingly
sell or operate a vehicle whose emission control system has been disabled, changed or altered from its
original design specifications, except for in-kind replacement of system components and added components
which are designed to improve emissions.