' Reversion - the myth of back pressure '
as ' The Misunderstanding of Back Pressure ' and ' Reversion - the Myth of Back Pressure '. They might not be the bedtime reading of choice for everyone , but for serious Harley-Davidson workshops and bike
builders worldwide they have achieved almost cultlike status . " Armed with such a deep understanding of what works and , just as importantly , how and why it works , we have been able to metallurgically sculpt every inch of our systems , both externally and internally , to resist faults , corrosion and overheating with the highest quality metals , materials and coatings . " Our 3-step exhaust systems are designed to work with the exhaust inertia pulse and give a broad powerband on 80 ci - 131 ci EVO and Twin Cam
Myth - " The Misunderstanding of Back Pressure "
Back pressure fights against performance . Back pressure holds back power . Back pressure is necessary in 2-stroke engines , but not in 4-strokes ? Not so much . The reason some folks think that back pressure is good , is because when they put a baffle in ( which creates back pressure ) the bike runs stronger . But it is not doing so because of the back pressure , it is running stronger despite the back pressure because it is reducing reversion . It is wrongly assumed that the back pressure that baffles create increases power . It is the reduction in reversion from baffles that increase power . The baffle is reducing power because of the back pressure , but it is increasing power by reversion reduction . In most cases the power gained by reduction of reversion is more than the power lost to back pressure , so there is a net gain of more power , despite back pressure , not because of it . What would be best is if there could be a reduction of reversion without an increase in back pressure .
Reversion
A slow flowing port typically allows excessive amounts of exhaust gases to back up in the port and re-enter the combustion chamber ( reversion ). The exhaust gases dilute the intake charge and ruin carburetion and throttle response . An antireversionary ( AR ) flange is often installed in a header pipe where the pipe intersects with the exhaust port . This feature helps when the exhaust port is inefficient and slow flowing . An AR flange shrouds the port , thereby catching much of the back-flowing exhaust , which improves performance , particularly at low rpm . Some exhaust
Standard baffle
ports are designed with a mismatch at the bottom of the exhaust header pipe . The mismatch functions in the same fashion as an AR flange by reducing exhaust backflow . The mismatch creates a ' ledge ' at the bottom of the port that stops reversionary exhaust gases from backing into the combustion chamber .
Header Pipe Diameter
Header diameter ( the inside diameter ) is typically the most important factor in exhaust system design because it sets the torque curve . Increasing diameter improves top-end power at the expense of low-end torque . Changing pipe length will move the torque curve either up or down the rpm scale . A shorter pipe favours top-end horsepower while a longer pipe caters toward low-end torque . Three-step headers of specific tailored design will overcome many of the issues matching mufflers , pipe diameter and pipe length , however , does not meet current EPA regulations .
Pro Performance catalytic convertor
Header pipe diameter is determined by engine displacement ( bore and stroke ), compression ratio , valve diameter , camshaft specifications ( lift , duration and timing ), the critical rpm band , and has a major effect on exhaust gas velocity . If pipe diameter is too small , back pressure increases . Back pressure is defined as flow resistance created in the exhaust system . The higher the back pressure , the higher the engine ' s pumping losses will be , since the piston must physically force the residual gases out of the cylinder during the exhaust cycle . The internal combustion engine is , after all , basically just an air pump . Elevated back pressure also reduces low-lift exhaust flow during the period called " blowdown ." An effective blowdown period will efficiently use expanding exhaust gases to expel combustion residue from the cylinder . The blowdown period begins at exhaust valve opening and ends when cylinder pressure and exhaust system pressure are equalized . Camshaft timing has a major effect on blowdown . By using
32 AFTERMARKET MOTO DESIGN - OCTOBER 2021