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2005 Mustang GT American Racing Headers Install - Waiting To Exhale
Choosing The Right Exhaust Is Critical To Performance. We Find Just How Critical On This 800hp Three-Valve Mustang.
Performance Enthusiasts seem to have a one-track mind. No, not the kind that constantly steers us to pictures of Jessica Alba and Pamela Anderson, but the one that provides tunnel vision in our quest for maximum performance-and that's not a bad thing.
Take the fancy '05 Mustang GT, for example. To maximize power from the 4.6L Three-Valve motor, forced induction is a must. Ditto for forged internals, ported heads, and even wilder cam timing. Adding a blower is oh-so common, as boost rules in these applications. In this case, knowing that positive-displacement blowers are ultrasensitive to inlet restrictions, we even modify the air-intake system with a larger throttle body, mass air meter, and filter assembly. In short, we do everything we can to maximize airflow into and out of the blower and into our modified motor. It's this methodical approach that produces exceptional results.
The one downside to this type of inductive reasoning is that we've forgotten one important thing about the internal combustion engine-all the air that goes in must also find its way out.
In our bigger-is-better world, we often forget that the power output of an engine is not determined by airflow readings. We know this runs contrary to the concern for such things as cylinder-head and carburetor-flow figures, but there's much more to the equation than simple cubic feet per minute. In reality, the power output of any internal combustion engine is a function of the amount of air it can process. For our simplistic definition, the term process simply means airflow both drawn in through the induction system and exhaled out the exhaust.
In our quest for performance, we often forget the second part of this equation, to say nothing of how to achieve airflow into and then out of a motor. Unfortunately, the same misguided theories that direct us to top our street motors with 400-cfm heads, 1050 Dominator carbs, and 0.750-inch lift cams are carried over to the exhaust system. As with the intake side, bigger is not always better when it comes to the exhaust, the possible exception being the actual after-cat exhaust. As with everything thing in life, there's something called overkill.
Maybe a couple of examples will help illustrate the bigger-is-better mentality and why it has no place in the performance world. On the surface, it's easy to see why we want to maximize airflow-after all, more airflow equals more power, right? Well, yes and no, as the power output of the motor is determined by the amount of airflow any engine can process. By process, we mean ingest air and fuel, burn the mix efficiently, and then expel the burnt gasses with minimal emissions.
With that, more airflow from an individual component may or may not increase the amount of air actually processed by the motor. A perfect example of this would be the installation of a dual 75mm throttle body on an otherwise stock 4.6L Three-Valve motor. The larger throttle body will certainly outflow the stock throttle body, but the motor can't take advantage of the additional airflow since the stock throttle body didn't represent a restriction (at stock power levels). This situation changes when we add a supercharger, but only once we reach a given power output (which translates directly into an airflow measurement). In this example, if we see vacuum present behind the throttle body at wide-open throttle, a larger throttle body will likely improve the power output.
Simple airflow devices such as a throttle body or even an after-cat exhaust are fairly straightforward, but more complicated are components like camshafts, intake manifolds, and (the subject of this test) long-tube headers.
In truth, this test involved both headers and an after-cat exhaust, but the significant midrange power gains came from the scavenging effect of the long-tube headers. Unlike air filters or throttle bodies that allow enough air past or not, cams, intakes, and headers have a decided tuning effect on the power curve. Camshaft timing dictates at which engine speed the motor will be most efficient, with higher-duration cams dictating higher engine speeds. Intake manifolds work much like long-tube headers in that longer runner lengths (primary lengths on the header) are optimized for lower engine speeds, while shorter lengths promote power higher in the rev range.
The same can be said of runner diameter (or cross section), as larger runners (or primary diameter pipes for headers) will increase the optimum engine speed. That is to say, a 2-inch by 34-inch primary header pipe will be optimized at a higher engine speed than a pipe that measures 1.75 inches by 34 inches. This tuning effect has nothing to do with the actual flow rate, as the diameter and length determine the travel speed of the resonance waves. Primary pipe length and diameter are but two of the many variables that can affect the performance of a set of headers.
Before getting to the test, perhaps a brief explanation of how this resonance occurs will shed some light on how difficult it is to produce the proverbial "ideal" or "best" set of headers for any given combination.
More than simple exhaust flow, true headers promote power production through two effective means of scavenging. In simplified terms, scavenging occurs when both the intake and exhaust valves are open (a position referred to as camshaft overlap). The outgoing exhaust flow helps draw in intake mixture by creating a low-pressure zone in the combustion chamber. This scavenging effect helps introduce more intake air and fuel mix, which allows the motor to make more power.
And this relatively simple scavenging effect is accomplished through two somewhat sophisticated mechanisms-the first being the kinetic energy of the outgoing gases. Since we lack the space for a detailed description of exhaust theory, we will have to revert to the Reader's Digest version. The opening of the exhaust valve produces a compression pulse. The release of this compression pulse creates high pressure in front of the wave but a depression on the backside of the wave. Since the speed of this wave exceeds that of the exhaust gas flow through the pipe, the depression or low-pressure zone produces a scavenging effect. This helps rid the combustion chamber of residual exhaust gases and, in turn, helps pull a fresh air/fuel charge in from the induction system.
The second method of scavenging produced by the long-tube header is called reflected wave scavenging. Once the pressure pulse has been released by the opening of the exhaust valve, the wave travels the length of the exhaust pipe. Upon reaching the end of the pipe (typically the collector), something magical happens. The positive pressure wave is allowed to expand into the relatively larger collector. This expansion causes a momentary drop in density of the air surrounding the end of the primary pipe. The elasticity of the air causes it to rebound toward the pipe exit. This creates a new negative pressure wave that then travels back up the primary pipe to the awaiting exhaust port. This reflection of the positive and negative pressure waves continues indefinitely, though the waves decrease in amplitude (or effective strength). For optimum performance, the exhaust-pipe length should be selected to produce the primary (first order) reflected (negative) pressure wave (at its lowest pressure) when the piston just passes TDC at the end of the exhaust stroke.
Since these waves travel at the speed of sound (which is pressure and temperature dependent), tuning this event for a specific engine speed requires changing the length of the primary pipes. Short primary runners employed on stock exhaust manifolds don't allow sufficient time for the compression wave to leave behind a depression capable of improving scavenging. The short primary lengths also promote early arrival of the reflected wave, which minimizes effective intake and exhaust scavenging.
It's important to note that no header yet produced is optimum for all combinations. The laws of physics dictate otherwise, as the scavenging effect of the headers is initiated by the opening of the exhaust valve, which is also dependent upon the overlap-which is also dependent on the reflected waves produced by the intake design. You can see that this is one fairly complex dynamic system, and that header choice comes down to not only your particular combination, but the point in the rpm curve that you'd like to optimize power production. On any given combination, it's possible to design the headers to improve power down low, in the midrange, and even at high rpm.
The possible exception is the installation of headers in place of stock exhaust manifolds, as the stock manifolds provide almost no scavenging effect due to their excessively short primary length. Even by replacing the stock manifolds with a quality set of long-tube headers, the gains offered have as much to do with the exact combination tested as the design of the headers themselves.
That said, we can now better understand the sizable power gains offered by not only a quality set of headers, such as the ones chosen for this article from American Racing, but also a 3-inch after-cat exhaust system from the exhaust experts at MagnaFlow. While such an exhaust upgrade would be a welcome addition to any modern Mustang, we applied these to what can best be described as a wildly powerful, supercharged '05 GT.
The GT in question was equipped with a 4.6L Three-Valve motor that had been upgraded with a forged reciprocating assembly from Sean Hyland Motorsports, a set of Stage 3 ported heads from Livernois, and matching Stage 3 cams from the Comp Cams catalog. The highlight of the mod motor was the Kenne Bell 2.8L H-series blower. Feeding the blower was a custom 4 1/2-inch MAF and air inlet, a dual 75mm throttle body, and a prototype of the new Mammoth intake manifold from Kenne Bell.
By minimizing inlet restrictions, we were able to coax just over 25 psi from the twin-screw blower. That the '05 GT ripped off plenty of nine-second quarter-mile times after the exhaust upgrade is a testament to the work put into the impressive Three-Valve motor.
With the induction system well sorted, we decided it was time the supercharged beast was allowed to exhale. This provided an excellent opportunity to test the effectiveness of exhaust upgrades on such a high-horsepower Three-Valve combination.
For our test, the supercharged mod motor was equipped with the stock exhaust manifolds feeding the stock cat pipes, after-cat exhaust, and aftermarket mufflers. The cats had long been removed from the cat pipe, while the stock mufflers had been replaced by a set from Jardine. In this configuration, the supercharged mill produced 779 hp and 691 lb-ft of torque.
The stock components were then replaced by the 1 3/4-inch stainless steel headers (with 3-inch merge collectors) from American Racing Headers, along with the 3-inch after-cat system from MagnaFlow.
MagnaFlow also supplied a universal 3-inch x pipe system that allowed us to connect the American Racing headers to the MagnaFlow after-cat. After the installation of the exhaust components, the peak power numbers jumped to 802 hp and 714 lb-ft of torque. This test is a perfect example of why we include graphs (not just peak numbers), as the exhaust upgrades improved power production throughout the rev range with a maximum gain of 62 hp and 60 lb-ft of torque at 5,400 rpm. While we changed two components at once for this test, the huge gains in the midrange certainly indicate the scavenging effect produced by the long-tube headers was present and accounted for.