Muscle Mustangs & Fast FordsHow To Engine
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.