Aftermarket exhaust parts and the engine. However, the pipes do "ring" with the exhaust pulses and increase underhood noise. Headers can be ceramic coated to reduce the heat radiated in the engine compartment, and to increase the temperature of the pipes which increases the velocity of the exhaust gases. Terminology: Catalytic converters are necessary to reduce emissions but create back pressure due to the exhaust gases being forced through a catalyst, and therefore decrease high-end engine power. Many modern catalytic converters only produce 1-3 psi of back pressure, though this restriction worsens further with use. Hi-flow catalytic converters can replace the standard units
Exhaust mixer increasing thrust. The exhaust thrust from a jet engine is equal to exhaust mass flow times exhaust velocity, i.e., "Thrust = ṁv", while the energy to make that thrust is given by "Energy" = 1/2"mv". A mixer helps reduce the fastest exhaust velocities from the core of the engine, while making the average exhaust velocity faster, producing more thrust with the same energy. Mixing efficiency is defined in a number of ways, but simply, compares the actual thrust gain (due to mixing) to that of the ideal case. Annular mixers have a low pressure loss, but a low mixing efficiency.