Not only "smooth tube" resonance, but intake resonance also comes into play. When everyone gets a look at the 2003 intake, they'll rethink a few things.
Taking resonance out of the picture, the "effective" cross sectional flow area is what counts, a smooth tubes effective flow area is greater than the stock ribbed tube because the thickness of the turbulent boundry layer is smaller.
With all that said, remember, Oreca's Vipers breathed through restrictor plates, much smaller than the throttle bodies, and got PLENTY of air......(think venturi)
As long as the engine gets enough air for complete cumbustion..... life is good.
Kid97 posted that after several additional intake changes, he did not see any addtional Dyno HP. It appears he was getting enough air for complete combustion, and nothing downstream was inhibiting the exhaust.... hence upstream changes had a minimal effect. We spent a lot of time running engines on dynos back in my copllege days at the UW thermal labs, combustion is a pretty neat science.
Here is a pretty good article on exhaust theory, talks a lot about things that are applicable to intake theory as well:
Introduction
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.
Do you want the best possible low-end power, the best mid-range power
or maximum top-end power?
Do you plan to use nitrous oxide or forced induction (supercharger or
turbocharger)?
Are you going to increase displacement?
Will you be using an aftermarket cam with different lift, duration, timing
and overlap?
Do you understand the relationship between torque (force) and
horsepower (amount of work within time)?
Can you distinguish between cosmetic headers and performance
headers?
Have you considered vehicle weight, transmission (stall speed, if
applicable) and gear ratios?
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 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?
Longer header tubes tend to increase power below the engine’s torque
peak and shorter header tubes tend to increase power above the torque
peak.
Large diameter headers and collectors tend to limit low-range power and
increase high range power.
Small diameter headers and collectors tend to increase low-range power
and limit high-range power.
"Balance" or "equalizer" tubes between the collectors tend to flatten the
torque peak(s) and widen the powerband (increase average 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.
Exhaust Scavenging
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.
Mandrel Bends
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.
Welds
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
Stainless 304 to 321
Aluminized Steel
Stainless 308
Stainless 308
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.
©TomBumpous 1999
