Hot Rods 101
|Start with a late model Camaro powered by GM's marvelous LS1 aluminum engine.|
|Remove the catalytic converters and exhaust system. Don't tell EPA.|
|Install custom exhaust headers and chassis "stiffeners".|
|Fabricate a free-flowing exhaust with straight-through "mufflers".|
|Install the exhaust system and buy good ear plugs.|
|Stiffen the chassis some more. You want it to launch hard, don't you?|
|Install an effective traction device to further aid the launch.|
|Install desired gear ratio.|
|Relocate the battery to the trunk for even MORE traction.|
|Install lightweight, billet shocks for better weight transfer during launches.|
|Hide a Nitrous Oxide bottle on top of the battery. You DO want big power, right?|
|Carefully plumb and "jet" your nitrous system for... 150 extra HP.|
|Add nitrous arming switches to the dash.|
|Install high strength axles to handle the big power and traction.|
|Put a shift light where you can't miss it.|
|Mount wide, sticky, "cheater" slicks for wheelies.|
|Add light, custom rims for dazzle.|
|Give it a little "character".|
|Polish it to perfection.|
|Polish it some more.|
|Take a picture. It might never look this good again.|
Hang with friends and enjoy casual "kills" of $150K Turbo Porsches.
Launch with 18" wheelies and run 11-second/120+ 1/4-miles.
Be humble. There's always someone quicker, especially sport bikers!
|Show them how "little" power it made from the factory.|
|They won't believe you so show them how much power your custom exhaust gained.|
|For your finale, rattle them with your new nitrous power while retaining 20+ MPG.|
The following information is the result of several decades of research and experimentation. It was compiled from various sources including the author’s personal experience and theories as a pipe fabricator and drag racer. Considerable effort has been invested to assure it's technical accuracy. It is a work-in-progress that is updated when relevant data presents itself. It was written non-mathematically to facilitate an overall understanding of exhaust physics in the quest for higher performance.
"Exhaust Theory" © Tom Bumpous 1999
No header/exhaust system is ideal for all applications. Depending on their design and purpose, all headers compromise something to achieve something else. Before performing header or other exhaust modifications to increase performance, it is critical to determine what kind of performance you want.
Without careful thought about these variables, a header/exhaust system can yield disappointing results. Conversely, a properly designed system that is well-matched to the engine can provide measurable power gains.
The distinction between "maximum power" and "maximum performance" is significant beyond semantics. Realistically, one header may not produce both maximum horsepower and maximum performance. For a vehicle to cover "X" distance as quickly as possible, it is not the highest peak power generated by the engine that is most critical. It is the highest average power generated across the distance that typically produces the quickest time. When comparing two power curves on a dynamometer chart (assuming other factors remain constant), the curve containing the greatest average power is the one that will typically cover the distance in the least time and that curve may, or may not, contain the highest possible peak power.
In the strictest technical sense, an exhaust system cannot produce more power on its own. The potential power of an engine is determined by the amount of fuel available for combustion. More fuel must be introduced to increase potential power. However, the efficiency of combustion and engine pumping processes is profoundly influenced by the exhaust system. A properly designed exhaust system can reduce engine pumping losses. Therefore, the design objective for a high performance exhaust is (or should be) to reduce engine-pumping losses, and by so doing, increase volumetric efficiency. The net result of reduced pumping losses is more power available to move the vehicle. As volumetric efficiency increases, potential fuel mileage also increases because less throttle opening is required to move the vehicle at the same velocity.
Much controversy (and apparent confusion) surround the issue of exhaust "back-pressure". Many performance-minded people who are otherwise well-enlightened still cling tenaciously to the old cliché.... "You need some back-pressure for best performance."
For virtually all high performance purposes...
Backpressure in an exhaust system increases engine-pumping losses and thereby decreases maximum engine power.
It is true that some engines are mechanically tuned to "X" amount of backpressure and can show a loss of low-end torque when that backpressure is reduced. It is also true that the same engine that lost low-end torque with reduced back-pressure can be mechanically re-tuned to show an increase of low-end torque with the same reduction of back-pressure. More importantly, maximum mid-to-high RPM power will be achieved with the lowest possible backpressure. Period!
The objective of most engine modifications is to maximize air and fuel flow into, and exhaust flow out of the engine. The inflow of an air/fuel mixture is a separate issue, but it is directly influenced by exhaust flow, particularly during valve overlap (when both valves are open for "X" degrees of crankshaft rotation). Gasoline requires oxygen to burn. By volume, dry, ambient air at sea level contains about 21% oxygen, 78% Nitrogen and trace amounts of Argon, CO2 and other gases. Since oxygen is only about 1/5 of air’s volume, an engine must intake 5 times more air than oxygen to get the oxygen it needs to support the combustion of fuel. If we introduce an oxygen-bearing additive such as nitrous oxide, or use an oxygen-bearing fuel such as nitromethane, we can make much more power from the same displacement because both additives bring more oxygen to the combustion chamber to support the combustion of more fuel. If we add a supercharger or turbocharger, we get more power for the same reason…. more oxygen is forced into the combustion chamber.
Theoretically, in a normally aspirated state of tune without fuel or oxygen-rich additives, an engine’s maximum power potential is directly proportional with the volume of air it flows. This means that an engine of 350 cubic inches has the same maximum power potential as an engine of 454 cubic inches... if they both flow the same volume of air. In this example, the powerband characteristics of the two engines will be quite different but the peak attainable power is essentially the same. In view of this, the author has amended the old hot rod proverb "There's no substitute for cubic inches." to include..... "except more efficiency!"
Flow Volume & Flow Velocity
One of the biggest issues with exhaust systems, especially headers, is the relationship between gas flow volume and gas flow velocity (which also applies to the intake track). An engine needs the highest flow velocity possible for quick throttle response and torque throughout the low-to-mid range portion of the power band. The same engine also needs the highest flow volume possible throughout the mid-to-high range portion of the powerband for maximum performance. This is where a fundamental conflict arises. For "X" amount of exhaust pressure at an exhaust valve, a smaller diameter header tube will provide higher flow velocity than a larger diameter tube. Unfortunately, the laws of physics will not allow that same small diameter tube to flow sufficient volume to realize maximum potential power at higher RPM. If we install a larger diameter tube, we will have enough flow volume for maximum power at mid-to-high RPM, but the flow velocity will decrease and low-to-mid range throttle response and torque will suffer. This is the primary paradox of exhaust flow dynamics and the solution is usually a design compromise that produces an acceptable amount of throttle response, torque and horsepower across the entire powerband.
A very common mistake made by some performance people is the selection of exhaust headers with primary tubes that are too large in diameter for their engine's state of tune. Bigger is not necessarily better and is often worse simply because of the loss of gas velocity.
Equal Length Primary Tubes
The effectiveness of equal length header tubes is widely debated.
Assuming that a header is otherwise properly designed (and many headers are not), equal length primary tubes offer some benefits that are not present with unequal length tubes. Those benefits are smoother engine operation, tuning simplicity and increased low-to-mid range torque.
If the header tubes are not equal length (many, if not most, commercial headers are not equal length), both inertial scavenging and wave scavenging will vary among engine cylinders, often dramatically. This, in turn, causes different tuning requirements for different cylinders. These variations affect air/fuel mixtures and timing requirements, and can make it very difficult to achieve optimal tuning. Equal length header tubes eliminate these exhaust-induced difficulties. "Tuning", in the context used here, does not mean installing new sparkplugs and an air filter. It means configuring a combination of components to maximum efficiency for a specific purpose and it can not be overemphasized that such tuning is the path to superior performance with a complex system of parts that must work together in a complimentary manner.
If a header is otherwise properly designed for it’s application, equal length header tubes are usually longer than unequal length tubes. The lengths of both primary and collector tubes strongly influence the location of the torque peak(s) within the powerband. In street and track performance engines, longer header tubes typically produce more low-to-mid range torque than shorter tubes. This begs the question... Where in the powerband do you want to maximize torque?
There is limited space in most engine compartments for header tubes and equal length tubes complicate the design process and are more costly to build than "convenient" length or cosmetic headers. Exhaust header designers are severely compromised by these limitations. Among the more astute (and responsible) professional header builders, it is more-or-less understood that header tube length variations should not exceed 1" to be considered equal. Even this standard can result in a 2" difference if one tube is an inch short and another tube is an inch long. By this definition, equal length headers are quite rare. By absolute measurement, it may be impossible to find equal length headers from a commercial manufacturer. Because of this, it is no surprise that many people have little knowledge of the benefits of equal length headers since the average user is unlikely to have experience with them. If you have headers that are supposedly equal length, carefully measure each tube and you will know the truth. Tube measurements should always be calculated from the tube centerline.
Inertial scavenging and wave scavenging are different phenomenon but both impact exhaust system efficiency and affect one another. Scavenging is simply gas extraction. These two scavenging effects are directly influenced by tube diameter, length, shape and the thermal properties of the tube material (stainless, mild steel, cast iron, etc.). When the exhaust valve opens, two things immediately happen. An energy wave, or pulse, is created from the rapidly expanding combustion gases. The wave enters the header tube (or manifold) traveling outward at a nominal speed of 1,300 - 1,700 feet per second (this speed varies depending on engine design, modifications, etc., and is therefore stated as a "nominal" velocity). This wave is pure energy, similar to a shock wave from an explosion. Simultaneous with the energy wave, the spent combustion gases also enter the header tube and travel outward more slowly at 150 - 300 feet per second nominal. Maximum power is usually made with gas velocities between 240 and 300 feet per second. Since the energy wave is moving about 5 times faster than the exhaust gases, it will get where it is going faster than the gases. When the outbound energy wave encounters a lower pressure area such as a larger collector pipe, muffler or the ambient atmosphere, a reversion wave (a reversed or mirrored wave) is reflected back toward the exhaust valve with little loss of velocity.
The reversion wave moves back toward the exhaust valve on a collision course with the exiting gases whereupon they pass through one another, with some energy loss and turbulence, and continue in their respective directions. What happens when that reversion wave arrives at the exhaust valve depends on whether the exhaust valve is still open or closed. This is a critical moment in the exhaust cycle because the reversion wave can be beneficial or detrimental to exhaust flow, depending upon its arrival time at the exhaust valve. If the exhaust valve is closed when the reversion wave arrives, the wave is again reflected toward the exhaust outlet and eventually dissipates its energy in this back and forth motion. If the exhaust valve is open when the wave arrives, its effect upon exhaust gas flow depends on which part of the wave is hitting the open exhaust valve.
A wave is comprised of two alternating and opposing pressures. In one part of the wave cycle, the gas molecules are compressed. In the other part of the wave, the gas molecules are rarefied. Therefore, each wave contains a compression area (node) of higher pressure and a rarefaction area (anti-node) of lower pressure. An exhaust tube of the proper length (for a specific RPM) will place the wave’s anti-node at the exhaust valve at the proper time for it’s lower pressure to help fill the combustion chamber with fresh incoming charge and to further extract spent gases from the chamber via vacuum effect. This is wave scavenging or "wave tuning".
From these cyclical engine events, one can deduce that the beneficial part of a rapidly traveling reversion wave can only be present at an exhaust port during portions of the powerband since it's relative arrival time changes with RPM. This makes it difficult to tune an exhaust system to take advantage of reversion waves which is one reason why there are various anti-reversion schemes designed into some header systems and exhaust ports. These anti-reversion devices are designed to weaken and disrupt any detrimental reversion waves (when the wave's higher-pressure node impedes scavenging and intake draw-through). Such anti-reversion schemes include merge collectors, truncated cones/rings built into the primary tube entrance and exhaust port ledges.
Unlike reversion waves that have no mass, exhaust gases do have mass. And since they are in motion, they also have inertia (or "momentum") as they travel outward at their comparatively slow velocity of 150 - 300 fps. When the gases move outward as a gas column through the header tube, a decreasing pressure area is created in the pipe behind them. It may help to think of this lower pressure area as a partial vacuum and one can visualize the vacuous lower pressure "pulling" residual exhaust gases from the combustion chamber and exhaust port. It can also help pull fresh air/fuel charge into the combustion chamber. This is inertial scavenging and it has a major effect upon engine power at low-to-mid range RPM.
If properly timed with RPM and firing order, the low pressure that results from gas inertia can spill-over into other primary tubes, via the collector, and aid the scavenging of other cylinders in that bank.
There are other factors that further complicate the behavior of exhaust gases. Wave harmonics, wave amplification and wave cancellation effects also play into the scheme of exhaust events. The interaction of all these variables is so abstractly complex that it is difficult to fully grasp. The author is not aware of any absolute formulas/algorithms that will produce a perfect exhaust design. Even factory super-computer exhaust designs must undergo dynamometer and track testing to determine the necessary adjustments for the desired results. Although there are some exhaust design software packages available, the author has found none that embrace all aspects of exhaust physics.
The Effects of Tube Size
Statistical Approximations for a Typical 350" GM V-8 Engine
|Horsepower >||300 - 375||375 - 475||475 - 580|
|Header Tube Diameter >||1.625"||1.75"||1.875"|
|Collector Tube Diameter for Max Low-RPM Power >||2.5"||2.75"||3"|
|Collector Tube Diameter for Max Mid-RPM Power >||2.75"||3"||3.25"|
|Collector Tube Diameter for Max High-RPM Power >||3"||3.25"||3.5"|
Tube dimensions (as opposed to "pipe" dimensions) are measured by Outside Diameter (OD).
Tube Inside Diameter (ID) is determined by subtracting the tube wall thickness (x 2) from the OD.
A mandrel tube bend maintains the same diameter as straight sections of the tube (within a few thousandths of an inch). A "mandrel" is just a rounded die that is inserted into the tube before it is bent, around which, the bend is made. The mandrel supports the inside tube wall during bending and prevents it from collapsing or kinking into a smaller diameter of less cross sectional area that could impede gas flow. A typical muffler shop bend is a press (or crush) bend, not a mandrel bend. Because of that (and depending upon wall thickness), the tube often crushes at the radius and the crushed area decreases the tube diameter. The amount of crushing that results from a press bend is proportional with the bend radius. Crushed bends reduce exhaust flow capacity and typically increase engine pumping losses. Mandrel bends are smooth in appearance and do not introduce unnecessary flow bottlenecks into an exhaust system. Virtually all headers are mandrel-bent but some intermediate tubes and tail pipes are not. The radius of a tube bend is measured from the center of the tube.
Quality exhaust welds are important for leak-free joints, structural integrity, longevity and, for some people, appearance. Tubes can be welded by several common welding processes including Oxy-Acetylene, Tungsten Inert Gas (TIG/Heli-arc), Metal Inert Gas (MIG), Gas Metal Arc Welding (GMAW/wire feed/flux-core) and Shielded Metal Arc Welding (SMAW/electrode/stick). Each type of welding has it's own characteristics. Inert gases and fluxes are used in welding to shield the molten weld puddle from atmospheric oxygen that would otherwise oxidize and weaken the weld. MIG welding is commonly used for production purposes and is usually adequate for exhaust components when skillfully done (as with all welding processes). Although expensive and talent-intensive, TIG welding offers the potential of very precise weld control.
For steel (ferrous) tube welds, the areas immediately adjacent to a weld are prone to heat-zone failure. This is because heat from the molten weld puddle de-tempers the base metal and reduces it's malleability. A condition called "hydrogen embrittlement" can also occur adjacent to welds that has a crystallizing effect which is prone to fractures. Proper welding techniques and post-weld heat treatments can reduce premature metal failure. These treatments are variously referred to as normalizing, stress relieving and tempering. Unfortunately, the author is not aware of any header manufacturers who employ these preventive treatments after fabrication of their exhaust products. It is not uncommon for exhaust header welds to fail at the point where the primary tubes enter the collector tube because this is the hottest area of a "collected" system.
The installation of headers, mufflers, cats and exhaust tubes is usually done by a local shop or the vehicle owner. When a weld must be made, the weld "filler" metal should match the molecular characteristics of the base metal(s) for proper fusion, strength and corrosion resistance.
Exhaust Tube Weld Reference
|Metals To Be Welded||Recommended Filler Metal|
|Mild Steel to Mild Steel||60xx or 70xx|
|Stainless 304 to 304||Stainless 308|
|Stainless 321 to 321||Stainless 347|
|Mild Steel to Stainless 304||Stainless 309|
|Mild Steel to Stainless 321||Stainless 309|
304 to 321
It should be noted that an over-penetrated weld that slightly sags into an exhaust tube is not cause for great alarm. Gas flow velocities within a tube are highest at the center of the pipe and decrease greatly near the wall of the tube where a minor weld protrusion would have negligible effect upon gas flow.