THE MISUNDERSTANDING ABOUT BACK PRESSURE

I have felt the need and responsibility to explaining how an exhaust system on a 4-stroke engine works and to also expel any theory or belief that back pressure is required for an engine to run properly. At the same time, I have 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. a) The Misunderstanding of the Term Back Pressure in 4 Stroke Engines.
2. b) Two Stroke Engines – The requirement of Back Pressure.
3. c) The Ideal Exhaust System.
4. d) Harley-Davidson Performance Guide.
5. e) Blow Performance Exhausts 3-Step Header Design

1. The Misunderstanding of the Term Back Pressure in 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. I believe people have back pressure confused with Delta Pressure.

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. The sum of all these forces is called backpressure, and it is a measure of how hard the engine must work. Any time the engine is working, it is always working against some level of backpressure. So, technically, backpressure 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 moving through the exhaust tract for optimum performance. Now, for all the engineers out there, you understand that as a volume of air travels, 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 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 therefor basically affect your powerband, being too restrictive (stock setup) and will just increase backpressure (resistance to air flow) and hurt performance.

To summarize: BACKPRESSURE IS BAD and DELTA PRESSURE IS GOOD.

I believe it was coined by someone who did not understand how exhaust systems work. Production street legal exhaust systems are corked up to reduce sound. In an open, tuned system sound waves and pressure waves travel back and forth in the system along with the gasses. The sound waves have a large influence on the movement of gasses 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 the compression stroke and the exhaust stroke are the same thing, i.e. the exhaust valve is open while you're trying to compress the next dose of air/fuel mix. Without some back-pressure from the exhaust (which comes from the exhaust manifold and plumbing being sufficiently restrictive), you wouldn't get much compression - the fuel/air mix would just be forced out the exhaust valve. With a 4-stroke engine, the valves are all shut during the compression stroke, so you don't need any external pressure to get good compression - the cylinder is effectively sealed.

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.

Here is a short easy break down of exhaust design.

High rev torque = short, wide exhaust (allows better flow at high RPM meaning that the waves can be close together and pull each other out without slowing down)

Low rev torque = Thinner, longer (less flow meaning that the waves are longer and with the exhaust being longer they have more time to pull each other along)

Blow Performance Exhaust are a combination of the two. 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 to try and get it so that the pulses from each cylinder slow behind each other, so they pull each other out. As behind each wave is a vacuum.

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.

The idea behind an exhaust header is to eliminate the manifold's back pressure. Instead of a common manifold that all the cylinders share, each cylinder gets its own exhaust pipe

4. Harley-Davidson Performance Guide

(4a)        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 "tune" the exhaust length to the required power band.

(4b)        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 a custom build pipe.

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

The RPM's 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 Header Design:

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 RPM's 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.

Exhaust manifolds are generally the first line of disappointment when it comes to exhaust routing. Because the cast construction has been designed for ease of production, they are generally heavy, and do not offer desirable mingling of the exhaust pulses. Though some manufacturers have improved on the unequal length manifold, they are often discarded in favour of aftermarket solutions.

The most ubiquitous of which is the “header” — the term headers really refers to the first tubular exhaust manifolds that allow exhaust evacuation from the engine. These tubes are known in the exhaust industry as primaries because they are generally followed by subsequent tubes of varying size.

 When you have an exhaust header that does not have a collector, that 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. It sounds counter intuitive, but at the tuned length that wave is going to be plenty strong but when you’re off the rpm we’ve attenuated that wave such that it doesn’t hurt performance.

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 and the intake begin 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 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. Scavenging effect is created by implementing an appropriately sized exhaust system and collector combination.

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 centre (BDC) — this generates a very mild forced induction effect, something I call “Reverse Turbo”.

I hope this reading clarifies some of the myths about Back Pressure needed for Harley Davidson Exhaust Systems!

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