The debate about whether or not exhaust backpressure is required for an engine to develop peak power is, and has been probably one of the most hotly contested controversies among car enthusiasts the world over. However, the image above represents a definitive and unambiguous answer; excessive backpressure is bad, and it will choke off the exhaust flow in any exhaust system just as effectively as the knot in this exhaust tube will. In this article, we will take closer look at the phenomenon of exhaust backpressure in terms of what it is, what it does, why it is bad for positive exhaust flow, and how it is often confused with exhaust scavenging, starting with this question-
In simple terms, it is not possible to eliminate exhaust pressure completely, since in any fully functional exhaust system, a certain amount of friction exists between the flowing exhaust gas and the inner walls of the exhaust tube, as well as between the flowing exhaust gas and the inner structures of catalytic converters and mufflers.
Many car enthusiasts interpret exhaust pressure as exhaust backpressure, and according to many, this backpressure is essential for an engine to make peak power. However, exhaust pressure is normal and to be expected because it is a function of the relationships between the length and diameter of the total exhaust system, the velocity of the exhaust gas, and how efficiently (or otherwise) the exhaust manifolds scavenge exhaust gas from the cylinders.
Exhaust backpressure on the other hand, is a rather nebulous concept that this writer has yet to find a satisfactory explanation for from proponents of the notion that exhaust backpressure is essential for efficient engine operation- and especially among performance enthusiasts. However, to dispel the notion that exhaust backpressure is needed for efficient engine operation, we need to state-
This writer has replaced many so-called performance exhaust systems with standard systems at the request of disappointed car owners who have experienced serious power losses after the upgrade. Their argument usually follows this pattern-
“I thought that upgrading my system from 50mm pipes to 75mm pipes would extract the exhaust gasses more efficiently, which according to the people who sold me the upgrade, would increase engine power. However, instead of increasing engine power, I have lost engine power, which proves that backpressure is required to make peak power.”
Well, it is easy to see how an uninformed car owner can be bamboozled by an unscrupulous vendor out to make a quick buck, so let us state-
The exhaust systems on standard road-going vehicles are designed to extract exhaust gas from the cylinders as quickly and efficiently as possible. Provided there are no restrictions or leaks in the exhaust system, the extraction of exhaust gas produces distinct pulses in the exhaust flow; for instance, a 4-cylinder engine will deliver 4 high-pressure pulses per engine cycle (4 cylinders have fired in their correct order), while a 6-cylinder engine will deliver 6 high-pressure pulses per cycle, etc.
The more distinctly separate pulses an exhaust system delivers, the more positive the exhaust flow is, and therefore, the more efficiently exhaust gas is scavenged from the cylinders. In reality, exhaust backpressure is a resistance to the positive flow of the exhaust stream through the exhaust system that actively prevents effective exhaust gas scavenging. In terms of symptoms, poor exhaust gas scavenging can cause engine overheating, poor fuel economy, severe power losses, and in severe cases, it can prevent the engine from idling or even starting.
In practice, it is also easy to see how uninformed car owners can confuse excessive exhaust pressure with exhaust backpressure, but in order to correct this misconception in the minds of some car enthusiasts, we need to understand at least the basics of-
When a piston rises on the exhaust stroke, it pushes the gas in the cylinder out through the exhaust valve(s), but this is where things get complicated. In practice, the speed at which the slug of exhaust gas enters the exhaust system has less to do with the speed of the piston, than it has to with the relationship between the effective diameter of the exhaust port(s), the exhaust valve duration, the firing order, and the design of the exhaust manifold. In addition, the speed of the exiting exhaust gas also depends on the length and diameter of the total exhaust system, and if the application has a turbocharger, the position and condition of the wastegate.
Assuming that both the engine and the exhaust system are standard, fully functional and in good condition, the slug of exhaust gas from a cylinder enters the exhaust system in a slightly pressurised condition, and at a speed that is high enough to create a low-pressure area behind it as it races through the system. Although the tail end of the slug of exhaust gas does decay somewhat as it travels down the exhaust system, it generally does not decay enough to raise the pressure behind it to the point where the slug of exhaust gas that follows would slam into it.
As a practical matter, the slugs of exhaust gas from each cylinder are all separated from each other by a low-pressure area between them, and each area of low pressure acts as a scavenging mechanism in the sense that each low-pressure area “pulls” the slug of exhaust gas that follows it through the exhaust system. However, how well (or otherwise) this works depends on-
This relationship is a critically important aspect of any exhaust system design, regardless of the application, since it must represent a perfect balance between the flow capacity of the system and the velocity of the exhaust gas, both of which must in turn, be balanced against the volume of exhaust gas the engine produces.
In the days before emissions regulations, none of these factors was particularly important; neither in isolation, nor in combination, but this is no longer the case. Let us look at the basic requirements a modern exhaust system must satisfy, starting with-
Flow capacity
Put simply, this refers to the exhaust system’s ability to allow the maximum volume of exhaust gas the engine can produce to flow through the system freely, without raising the exhaust pressure above the maximum allowable limit set by the manufacturer. Note that this aspect of an exhaust system is more closely related to the diameter of the exhaust tubing than it is to the length of the system, or the number or restrictions built into the exhaust system.
Flow velocity
In order for the whole exhaust system to function as an effective scavenging mechanism, the exhaust gas must travel through the system at the highest possible velocity. For instance, if you have two identical engines running at the same speed, and both exhaust systems are also identical save for their diameters, say 50mm vs. 75mm, the exhaust flow through the smaller diameter system will be considerably faster than it would be though the larger system, but herein lays a potential problem.
Even though the flow through the smaller-diameter system may be faster, that system’s flow capacity may be inadequate for the engine it is fitted to, with the result that the system’s scavenging ability will almost certainly be destroyed by successive slugs of exhaust colliding and piling up in the system, thereby raising the overall pressure in the exhaust system to unacceptable levels.
Put simply, this scenario represents a classic case of excessive exhaust backpressure, but instead of improving engine performance, engine performance suffers severely because the exhaust system now chokes the engine to death because it cannot evacuate the exhaust stream fast enough, which begs this question-
Well, yes, and no. It all depends on the application and its intended use. In the case of standard, street legal vehicles that will never see any modifications of any kind, there is no doubt that a standard exhaust system is the best choice, although many purveyors of so-called “performance exhaust systems” would claim otherwise.
The point is that since standard engines produce both their peak power and most optimal fuel efficiency between clearly defined points in their operating ranges, their exhaust systems are designed to operate at maximum efficiency precisely at these points in the power band. Of course, this is not to say that a standard exhaust system does not perform well at points below, between, and above these points in the power band.
However, it must be understood that at points below, between, and above maximum power and optimal fuel efficiency, the operation of the exhaust system represents several compromises between effective exhaust gas extraction, fuel efficiency, power delivery, effective exhaust gas pressure, and noise suppression. In practical terms, there is simply no way a “performance exhaust” can improve meaningfully upon any of the above characteristics of a standard exhaust system without creating a penalty in another characteristic, because changing any aspect of an exhaust system’s design necessarily affects one or more other aspects.
Any modification of, or alteration to an engine that effects either the volume or velocity of exhaust gas that engine produces has the potential to create excessive exhaust gas pressures that in turn, have the potential to cause severe power losses and other symptoms unless the exhaust system is tuned and matched to the engine.
Similarly, on standard exhaust systems, even relatively slight modifications such as extending the length of the system by fitting an exhaust diffuser to the end of the system, or fitting a non-standard resonator or muffler can produce measurably negative effects- simply because the relationship between the length and diameter of the system had been disturbed.
In the final analysis, the efficiency of any exhaust system depends on the overall pressure in the system being as low as possible, regardless of whether the exhaust system is fitted to a suburban runabout, or to a highly modified track racer. Anything that raises the overall pressure in the exhaust system is therefore bad, and must be avoided as far as possible.
