OXYGEN SENSOR ELIMINATOR KITS -Needed with EU5 EPA Territories.

THE MISUNDERSTANDING ABOUT BACK PRESSURE, UNDERSTANDING

 SCAVENGING & REVERSION

 

Explanation of how an exhaust system on a 4-stroke engine works and theory or belief that back pressure is required for an engine to run properly. Included further topics to explain the performance benefit of our Blow Performance Exhausts 3-Step Header design.

Please read the complete document. The sections listed below explain in detail what you should know about the topics.

DOCUMENT SECTIONS

1) Misunderstanding the Term Back Pressure in 4 Stroke Engines.

2) Two Stroke Engines – The requirement of Back Pressure.

3) The Ideal Exhaust System.

4) Harley-Davidson Performance Guide.

5) Blow Performance Exhausts 3-Step Header Design

6) Reversion

7) Exhaust System Elements

 

1. MISUNDERSTANDING THE TERM BACK PRESSURE ON 4 STROKE ENGINES

Back pressure is a term that misleads many into thinking it is a beneficial characteristic, that somehow their engine needs back pressure to operate correctly. The misunderstanding comes into play as we seek to increase exhaust gas velocity by restricting tubing diameter — restriction, i.e. back pressure may be a by-product or symptom, but is not the goal.

A restricted exhaust system is nothing but a hindrance. After all an engine is just an air pump, the more air and fuel we can force through it the more power it will produce, i.e., improves horsepower and torque.

Back pressure is NOT a good thing. Back pressure does NOT help produce power.

Back pressure confused with Delta P.

DELTA PRESSURE (aka delta P): Describes the pressure drop through a component and is the difference in pressure between two points.

 

EXPLANATION

When the engine is working to draw in air (intake), it must overcome the forces that resist air movement. These forces include things like gravity, air density, internal motor friction, resistance caused by the length and diameter of the tubing used, and the resistance caused by any medium that the air is drawn through such as filters or chemical sorbents or resistance expelling gases out. The sum of all these forces is called back pressure, and it is assured of how hard the engine must work. Any time the engine is working, it is always working against some level of back pressure. So, technically, back pressure is the resistance of air flow.

Back pressure is resistance, and resistance is BAD for performance, Delta pressure describes a pressure drop through a component or a difference in pressure. Delta Pressure is what is needed to basically get exhaust gases out of the cylinder and move through the exhaust tract for optimum performance. Now, for all the engineers out there, you understand that as a volume of air travel, its pressure is directly related to its traveling speed. Therefore, the faster the gases are moving, the more velocity or pressure it has. So, the higher the delta pressure (difference in pressure within the engine), the faster the gases will move through the exhaust tract.

Exhaust gases must travel at a certain speed and contain a certain velocity for optimum power production at any given RPM point. A certain delta pressure must be achieved to get the burnt air/fuel (exhaust gases) out of the cylinder on the exhaust stroke. By not having this certain Delta Pressure affects valve overlap (burnt air/fuel is sucked back into the cylinder on the intake stroke). It is virtually impossible to achieve this certain speed for each exact RPM point, so we must do some sacrificing to achieve the best power production.

A straight through exhaust (no cats, no muffs) enables exhaust gases to move much quicker than if there were a restrictive exhaust, each RPM point requires a certain amount of gas velocity for optimum power production. By altering the flow in an exhaust tract, you therefore basically affect your powerband, being too restrictive (stock setup) will just increase back pressure (resistance to air flow) and hurt performance.

BACK PRESSURE IS BAD and DELTA PRESSURE IS GOOD.

Production street legal exhaust systems are choked up to reduce sound or comply with EPA regulation. In an open, tuned system, sound waves and pressure waves travel back and forth in the system along with the gases. The sound waves have a large influence on the movement of gases and the length of the system is adjusted to take advantage of that. If you want to really understand how this stuff really works, get yourself a copy of "Scientific Design of Exhaust & Intake Systems" by Philip H. Smith and John C. Morrison.

 

2. TWO STROKE ENGINES-THE REQUIREMENT OF BACK PRESSURE

Exhaust back pressure is needed in a 2-stroke engine because there are no valves, camshaft etc. A two-stroke engines work by completing a power cycle in just two strokes of the piston (one crankshaft revolution). The cycle begins with the compression stroke, where the piston moves up, compressing the fuel-air mixture in the combustion chamber. As the piston reaches the top, the spark plug ignites the mixture, causing an explosion that drives the piston down in the power stroke. During this downward movement, the piston uncovers the exhaust port, allowing exhaust gases to escape, while simultaneously creating a vacuum that draws in a fresh fuel-air mixture through the intake port.

 

 

3. THE IDEAL EXHAUST SYSTEM

You always want the lowest pressure possible in the relevant branch of the manifold as the exhaust valve opens as this will allow as much of the contents of the cylinder to be ejected as possible before the valve closes, thus allowing more oxygen to be drawn into the cylinder on the next intake stroke.

Well-tuned NA. EFI or SC engine exhausts use the kinetic energy of the exhaust gas from the previous cylinder to create a partial vacuum in the manifold at exactly the right moment (this is called exhaust gas scavenging).

We’ve seen too much misinformation regarding exhaust theory. “Back pressure” and the statement, “An engine needs back pressure to run properly!” is absolute nonsense. Any technician with any dyno experience will tell you that the best mufflers are no mufflers at all!

Proper exhaust manifold/header tuning creates a vacuum, which helps to draw exhaust out of the cylinders and improve volumetric efficiency, resulting in an increase in horsepower.

More back pressure has multiple adverse effects on engine performance. Most importantly, high back pressure leads to poor scavenging, causing a drop in the volumetric efficiency of the engine.

 

4. HARLEY-DAVIDSON PERFORMANCE GUIDE

(a)     Maximum Horsepower Output.

Tuning the exhaust system is an important component in achieving maximum power when optimizing the performance of an engine. While not recommended for street bikes, the use of drag or straight pipes can maximize the horsepower produced by any specific engine combination. The RPM range that the straight pipes produce their maximum power is very narrow. The best way to improve the performance of straight pipes is to include steps of increasing pipe diameter at a calculated length.

(b)      Determining Exhaust Pipe Length.

Any formula that calculates header pipe lengths must consider conditions such as exhaust temperature, gas speed, exhaust valve duration and the RPM the engine is running at. Each formula makes different assumptions about these items resulting in different results from the same basic input parameters. The formulas used here result in short and long pipe length being calculated.

For serious performance efforts, the pipe lengths are calculated for a 3-step pipe. This 3-step design has generally proven to give the highest horsepower results over any other design. 3-step pipes are generally custom build pipes.

 There are two pieces of information that must be supplied to determine the exhaust pipe lengths for an engine.

The RPMs for the middle of the desired power band is needed. For Harley-Davidson applications, the following RPM values would be typical: a street engine will be 4000-5000, a street/strip engine 4500- 5500, race engines 5000-6000 and dyno shootout engine 5500-6500.

 

5. BLOW PERFORMANCE EXHAUSTS 3-STEP DESIGN

There are a lot of things that can affect the exhaust performance such as header length before you merge the cylinders together. Merging at different points can affect how well the exhausts scavenge each other. The best thing is having each cylinder its own individual tuned pipe.

During the exhaust stroke, a good way for an engine to lose power is through back pressure. The exhaust valve opens at the beginning of the exhaust stroke, and then the piston pushes the exhaust gases out of the cylinder. If there is any amount of resistance that the piston must push against to force the exhaust gases out, power is wasted.

Pro Racing and Performance teams use a Stepped Designed Header -a thinner Header from the cylinder going into a bigger one then merging into an even bigger one.

Some pipes use multiple steps along the length of the header. Typically, there is a step where the exhaust port ends, and the exhaust pipe begins. This is left as a step, rather than making the pipe diameter match the port diameter to help prevent reversion, that is prevent exhaust gases from flowing backwards into the combustion chamber after the piston reaches TDC.

If the pipe is designed to be used at high RPM's, then the exhaust gases may not have enough velocity at low RPMs to prevent reversion, so this step helps prevent it, thus helping power at low RPM's. Steps placed further down the pipe create pressure drops in the flow of the gases and can help increase power at higher RPM's depending on where they are placed along the pipe.

The “header” — the term headers really refer to the first tubular exhaust manifolds that allow exhaust evacuation from the engine. These tubes are known in the exhaust industry as primary because they are generally followed by subsequent tubes of varying sizes.

 When you have an exhaust header that does not have a collector, the scavenging wave hits the end of the pipe and comes back. There’s an important ratio that comes into effect. The greater the area ratio, the stronger that vacuum wave is. When you have a single pipe the area ratio at the end of the pipe is infinitive because you’re opening it up to the atmosphere.

When you fire that same tube into a collector the area ratio becomes a finite number, and we reduce the strength of that wave.

When the exhaust valve opens, you have a pressure wave that begins to travel down the tube, when it reaches the end of the tube it reverses as a vacuum wave and comes back and hits the cylinder. You want to be able to time that wave to hit right around the closing of the exhaust port, what that helps us do is scavenge residuals out of the cylinder as the intake begins to fill.

It is a common practice to size the first length of primary to as close to the exhaust valve diameter as is reasonably available. This way there is no sudden drop in velocity due to a volume increase from head port to the exhaust tube. It is then common to start stepping up the diameter. The term back pressure is far and away the most misused phrase to illustrate the importance of scavenging. Scavenging is the effect generated by harnessing the inertial energy of a high velocity exhaust gas pulse.

A high velocity pulse of exhaust gas carries with it energy, as the pulse moves through space it displaces the following volume behind it. This generates a low-pressure zone like a weak vacuum. The scavenging effect is created by implementing an appropriately sized exhaust system.

When executed correctly, a low-pressure area is left in the vacated cylinder, ready for the incoming intake charge. When the intake valve opens the air/fuel mixture can cram in, even before the piston begins to travel toward bottom dead center (BDC) this generates a very mild forced induction effect.

Back pressure fights against performance. Back pressure holds back power. 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, it’s not running stronger 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.

 

6. REVERSION

Buying an exhaust system can be a daunting experience. For example, there is an overwhelming selection of exhaust system designs, from staggered 2-into-2s, 2-into-1s, and true duals for baggers to straight pipes, big and small diameter headers, short and long header lengths, and stepped headers, just to mention a few. Not surprisingly, it is hard to determine what pipe works best, let alone which one looks best. Then there are a multitude of engine displacements from 80ci to 135ci and even larger that have varying performance requirements.

A slow-flowing port typically allows excessive amounts of exhaust gases to back up in the port and re-enter the combustion chamber (called reversion). The exhaust gases dilute the intake charge and ruin carburetion and throttle response. An anti-reversionary (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 ports are designed with a miss-match at the bottom of the exhaust header pipe. The miss-match 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 diameter (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 favors top-end horsepower while a longer pipe caters toward low-end torque. 3-step headers of specific tailored design will overcome many of the issues.

 

 7. EXHAUST SYSTEM ELEMENTS

The exhaust system is an integral part of the components that regulate airflow through the engine. Other key components include the induction system, cylinder heads, and camshaft. To achieve maximum performance, these components must be tuned together as a system for maximum performance within a given rpm range. If one part is changed or modified, the entire group of components must be returned for maximum performance.

a)      HEADER PIPE DIAMETER

Header pipe diameter has a major effect on exhaust gas velocity. Pipe diameter is determined by engine displacement (bore and stroke), compression ratio, valve diameter, camshaft specifications (lift, duration and timing), and the cuticle rpm band. 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 loses will be, since the piston must physically force the residual gases out of the cylinder during the exhaust cycle.

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 the blowdown. By using blowdown to remove exhaust gases, pumping losses are reduced because fewer gases remain for the piston to physically dispel from the cylinder.

If the header diameter is too large, exhaust gas velocity will be low, thereby weakening the scavenging wave and reducing its effect during valve overlap. As such, it is important to note that as blowdown pressure declines, there is an increase dependency on the exhaust system to scavenge cylinders of spent exhaust gases. Ideally, you want a balance between back pressure and velocity.

b)     HEADER PIPE LENGTH

Pipe length is determined by the engine's application and the most important rpm range. Pipe length is important for perfecting inertia and wave tuning, which determines the effect scavenging has on power production. Scavenging refers to the process of where a column of fast-moving exhaust gases (inertia scavenging) supersonic energy pulse (wave scavenging) aids the removal of combustion residue from the cylinder while assisting the intake charge into the cylinder. Because pressure waves can only be timed to help exhaust scavenging over a narrow rpm band, the engine's most critical rpm range must be first determined so pipe length can be matched to the rpm band. A longer pipe length optimizes power at low rpm. Conversely, a shorter pipe length improves upper-end rpm performance.

c)      STEPPED PIPES

Various exhaust systems are designed with a stepped-header pipe. A stepped header includes the placement of pipe diameter differentials in the pipe. The differentials are referred to as steps. Stepped headers are divided into two or more pipe sections.

Stepped headers are most beneficial when used on large-displacement and/or high-rpm engines to achieve maximum performance and cover a wider power band.

A camshaft with a long overlap can benefit from a pipe design that produces a wide exhaust-scavenging wave because a greater portion of the overlap period is effectively covered. A stepped header is an excellent design for this application.

 

Disclaimer

Should you use Blow Performance Exhaust Systems for street use we make no claim that our Catalytic Converter Kits or Baffle Kits meet your local EPA Emissions regulatory Levels. We advise you to conduct your own individual Emission Testing and compare the results to your local regulatory standards.

We are committed to our customers’ satisfaction and stand proud behind our product and service. We commenced in 2012 with design and testing, we are proud of what we have created for our biker communities worldwide.

Blow Performance Exhausts is an Australian Owned & Operated Company with design and manufacturing affiliates in the United States & Japan.

We currently have distribution warehouses in the USA & Australia. We ship to most countries.

Blow Performance Exhaust ships product worldwide, our products are designed and used for closed course competition, for street use, please check with your governmental regulatory authority that monitor and enforce EPA Emission Regulations in your area.

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