Even with the emissions system intact, there is still backpressure behind the DPF and behind the turbine. That matters because pressure after the turbine affects pressure before it. Drop a pound after the turbine and, at full power, turbine inlet drive pressure can fall by three or four pounds. That reduction unloads the piston on the exhaust stroke, cuts pumping loss, and gives the engine back power it was wasting just trying to exhale. That is the point of the Banks Monster Exhaust System. As a DPF-back exhaust system, it is built to reduce restriction after the emissions equipment and improve exhaust scavenging at the tip. The result is better engine breathing and measurable horsepower gains, without removing or bypassing the factory emissions hardware.
The discussion begins with exhaust back pressure downstream of the emissions equipment. Although a 5-inch exhaust system is already relatively low restriction, there is still measurable pressure behind the aftertreatment system.
That remaining pressure matters because reducing restriction downstream can influence pressure throughout the exhaust system, including conditions upstream of the emissions equipment and turbocharger.
The key point is that reducing pressure behind the emissions equipment produces a corresponding reduction ahead of it. According to the explanation, removing 1 PSI downstream can result in approximately a 1 PSI reduction upstream of the aftertreatment components.
This means improvements made toward the rear of the exhaust system can affect the pressure environment experienced by the turbocharger and engine.
The discussion then moves to the turbocharger's expansion ratio, which describes the relationship between pressure entering and leaving the turbine.
According to the video, reducing turbine-outlet pressure by 1 PSI at full power can produce a substantially larger reduction at the turbine inlet. The stated result is approximately 3 to 4 PSI less turbine-inlet pressure for every 1 PSI removed from the outlet.
The pressure entering the turbine is referred to as drive pressure. Because of the expansion ratio across the turbine, lowering downstream back pressure can produce a larger reduction in drive pressure.
The argument is that reducing restriction after the turbo does more than lower tailpipe pressure. It can also reduce the exhaust pressure the engine must overcome before and through the turbine.
Lower exhaust pressure reduces the load on the piston during the exhaust stroke. As the piston moves upward to expel combustion gases, pressure in the exhaust system resists that movement.
Reducing that pressure decreases pumping work, allowing less engine output to be consumed simply pushing exhaust gases out of the cylinders.
The design objective was not simply to increase exhaust diameter. Banks says the system and exhaust tip were engineered to promote scavenging and help exhaust gases leave the system more effectively.
In this context, improved scavenging is intended to further reduce resistance and improve the overall evacuation of exhaust gases.
Banks says the exhaust system was heavily instrumented during development to measure pressure changes and determine how those changes propagated through the exhaust system, turbocharger, and engine.
Based on that testing, Banks reports a double-digit horsepower gain from the Monster Exhaust. The gain is attributed to reduced downstream back pressure, lower turbine drive pressure, and decreased pumping work during the exhaust stroke.
