| Magnaflow's in-house flow bench putting a stainless muffler through its paces. Measures like this ensure a quality power-adding exhaust experience.
We're sure that most of you have thought about the reasoning behind having an aftermarket intake and exhaust system of some sorts. For most, adding these parts is just a standard affair with little to no reasoning behind it. But for those of you who question why the manufacturer built the airbox with all those twists and baffles or why an aftermarket exhaust system does not give you the horsepower advantage it once did, then this is the article for you.
Regarding the factory intake and exhausts, questions always seem to arise as to why this or why that. To best be able to sum up the whys, you first have to understand that the factory engineers are tasked with finding a happy medium when building parts. Factors such as cost, weight, noise, and performance are crunched through some poor engineers' brains, and the spew that comes forth is generally some major boring stuff. But, in the interim, a part is created that meets the EPA's stringent standards and conforms to the said manufacturer's list of requirements for that item. What does it all mean? It means your intake is quiet and makes enough room for air to feed the stock motor and also your exhaust is quiet and doesn't choke out the performance of the stock motor much. And then we have the aftermarket pieces. Let the performance begin.
Now for the boring stuff. We already know that the factory airbox is set and tuned for the factory engine's needs, but we're enthusiasts, so we want to know how the engineers equate an engine's needs. Since an engine is little more than an elaborate air pump, its ability to perform work is a product of its capacity to inhale and exhale air. Horsepower and torque are how we measure the work performed. An engine's air capacity is a product of its rpm and displacement divided by two (since only half of the engine's cubic capacity is being displaced during each stroke). For purposes of rating airflow, this formula is converted to a quotient reflecting cubic feet per minute (cfm). The reduced formula for cfm is: rpm x displacement/3,456cfm.
With the known cfm needs of your motor, we can then turn around and equate how large the filtering area needs to be to adequately allow enough air to enter the motor at any given rpm level. Let's do a motor equation. We'll keep it simple with a 350ci engine. Redline is 6,000 rpm: 350 x 6,000/3,456 = 607.64. Our 350 engine at 6,000 rpm requires 607.64 cfm to run correctly. On average, a good running engine will run at about 85 percent efficiency. 607.64 x 85 percent = 516.49 cfm. Our air filter needs to pass that much air or more to sufficiently feed our mill, even while dirty. With the known flow rates for air filter materials, it becomes a simple matter of plugging in the numbers to fully understand why aftermarket filters are so large. Knowing the cfm requirements of your motor, you just need to divide the material flow rates into that number to come up with the absolute minimum size filter necessary to do the job. We'll provide them for you just to make it easy. These are the flow rates for each of the three air filter materials used:
Foam - 117.9207889396246 cfm per sq. in.Paper - 104.3420314253648 cfm per sq. in.Gauze - 85.65390639395615 cfm per sq. in.
More flow equals more go, and more air in means more air out. That's where your headers and after-cat exhaust come into play. The factory engineers have come a long way in the performance versus noise engineering in OE exhaust systems. We have proven that again and again with dyno tests of everyone's exhaust systems. What little aftermarket exhaust does to contribute to your vehicle's horsepower profile is just a starting point. Additional breathing room created a bit of power, but you have the starting point for additional add ons such as a cam, headwork, supercharger, or turbo. With any of those parts in place, an aftermarket cold-air intake and exhaust become mandatory. For those of you who have put in the time to check out your stock exhaust pieces, this will be no surprise, but for you others, this will be eye opening. Those bulky anchors hanging off the exhaust side of your motor are exhaust manifolds. Generally cast iron, they are pretty much redheaded stepchildren in the performance department. A side-by-side comparison of a manifold next to a replacement header is like looking at a lump of coal next to a diamond. Whether shorty-style or full length, headers help the air-in/air-out needs of your hot rodded truck.
To reduce additional backpressure, the OEM exhaust tubing can be replaced with mandrel-bent tubing of the same diameter or one size up. Non-mandrel-bent tubing creates substantial restriction and adds turbulence to the flow stream. A fine line exists between reducing backpressure and maintaining good exhaust velocity. It's not about the biggest pipe; it is about the most efficient pipe. Generally, the factory is going to use the bare minimum tubing size to create an exhaust, but most have gotten better about restrictions at bends. Many of you have heard the reference to engine breathing using a straw. Imagine blowing air through a straw (a smaller diameter pipe). This would take time to release all the air from your mouth, and you would feel pressure in your mouth while doing so. Now imagine blowing air through a paper towel roll (large diameter pipe). You will relieve all your air much faster and feel little or no air pressure in your mouth because of the larger capacity of the tube. This is why it is important to get the correct size piping in order to relieve backpressure while maintaining efficiency. The same theory applies to the air coming in your motor. Instead of blowing, try sucking air through that same straw. Good luck with that.
In your highly enlightened state, you are now prepared to do battle with the next guy over the hows and whys of aftermarket intake and exhaust systems. Although not the super horsepower adders they once were thanks to fresh OEM engineering, every engine can benefit from a little breathing upgrade. And like we've said before, there is no excuse for leaving your truck stock. Stay tuned for a future issue in which we delve into the deep, dark underbelly of air filter construction and shed some light on what actually goes into that little piece of insurance standing between your truck's motor and the dirt that wants to get in and do it harm.
