J
Jack Klarich
Guest
Exhaust Facts
Let's start with the basics.
Without getting into a lesson in physics there are three factors that effect the performance and tuning of an exhaust system.
Sonic pulse, thermal pulse, and exhaust gas velocity.
Sonic pulse.
Think of this as a shock wave.
This pulse travels through the exhaust and at the end of the pipe actually turns around and heads back toward the exhaust valve.
The wave will also pull exhaust gas back along its path and if it reaches the open exhaust valve the gases will dilute the air/fuel mixture in the combustion chamber.
Under the right (or wrong) conditions this wave will even push some of the air/fuel mixture back through the intake valve into the intake, a common occurrence with short drag pipes.
Thermal pulse.
Similar to sonic pulse, this is a wave created by the hot gas exiting the exhaust valve.
The thermal pulse also reverses direction at the end of the pipe but unfortunately travels at a different rate, thus reaching the combustion chamber at a different time.
Exhaust gas velocity.
This is the rate at which gas travels through the exhaust system.
The optimal velocity is 300 feet per minute.
For the most part all stock Harley engines require a 1-3/4" diameter pipe to maintain 300 fpm.
Bigger is not always better and a larger pipe will actually slow this velocity thus restricting flow.
Reversion.
What is it?
It is simply the exhaust pulse flowing backwards momentarily during the valve overlap phase of the camshaft at low cycling rates.
During the overlap phase the piston is pushing out the last of the exhaust gases and prior to reaching top dead center (T.D.C) the intake valve and the exhaust valve is still closing.
At this point in the engine cycle both valves are in the open position.
At high cycling rates the inertia of the incoming intake charge and the outgoing exhaust pulses keep the exhaust flowing in the proper direction.
But at low cycling rates, as the piston is pushing out the last of the exhaust gases the intake valve opens and some of the spent exhaust charge is pushed into the intake manifold.
As the piston reaches T.D.C. and begins the intake stroke the exhaust valve is still not completely closed.
As a result, the piston pulls from the intake and exhaust valves simultaneously causing the exhaust gases to flow in a reverse direction.
Let's start with the basics.
Without getting into a lesson in physics there are three factors that effect the performance and tuning of an exhaust system.
Sonic pulse, thermal pulse, and exhaust gas velocity.
Sonic pulse.
Think of this as a shock wave.
This pulse travels through the exhaust and at the end of the pipe actually turns around and heads back toward the exhaust valve.
The wave will also pull exhaust gas back along its path and if it reaches the open exhaust valve the gases will dilute the air/fuel mixture in the combustion chamber.
Under the right (or wrong) conditions this wave will even push some of the air/fuel mixture back through the intake valve into the intake, a common occurrence with short drag pipes.
Thermal pulse.
Similar to sonic pulse, this is a wave created by the hot gas exiting the exhaust valve.
The thermal pulse also reverses direction at the end of the pipe but unfortunately travels at a different rate, thus reaching the combustion chamber at a different time.
Exhaust gas velocity.
This is the rate at which gas travels through the exhaust system.
The optimal velocity is 300 feet per minute.
For the most part all stock Harley engines require a 1-3/4" diameter pipe to maintain 300 fpm.
Bigger is not always better and a larger pipe will actually slow this velocity thus restricting flow.
Reversion.
What is it?
It is simply the exhaust pulse flowing backwards momentarily during the valve overlap phase of the camshaft at low cycling rates.
During the overlap phase the piston is pushing out the last of the exhaust gases and prior to reaching top dead center (T.D.C) the intake valve and the exhaust valve is still closing.
At this point in the engine cycle both valves are in the open position.
At high cycling rates the inertia of the incoming intake charge and the outgoing exhaust pulses keep the exhaust flowing in the proper direction.
But at low cycling rates, as the piston is pushing out the last of the exhaust gases the intake valve opens and some of the spent exhaust charge is pushed into the intake manifold.
As the piston reaches T.D.C. and begins the intake stroke the exhaust valve is still not completely closed.
As a result, the piston pulls from the intake and exhaust valves simultaneously causing the exhaust gases to flow in a reverse direction.