Well here it is. Got bored while out
Frankly, you'd think this subject would have been exhausted by now. After all, how much "borderless education" can you absorb about such common and oft-explained engine functions as getting rid of combustion by-products? Well, this story offers you a challenge. Our plan is to integrate various header functions, dispel a few myths about how headers work, and simplify matching parts to engine size and rpm.
Basics
Many stock exhaust systems are not capable of transferring sufficient exhaust gas at high engine speeds. Restrictions to this flow can include exhaust manifolds, catalytic converters, mufflers, and all connecting pipes routing combustion residue away from the engine. As power levels increase, proportionate amounts of exhaust can also increase, placing added demands on systems that may be flow deficient. Header manufacturers, among other objectives, attempt to build systems that fit (or should) and provide bigger pipes for high-rpm power gains. Knowing how and why a system needs to work helps in the selection process.
Combustion by-products won't burn a second time. Therefore, an exhaust system that cannot properly rid cylinders of exhaust gas can cause contamination of fresh air/fuel charges. Residual exhaust material occupies space in the cylinders that prevents maximum filling during inlet cycles. As a rule, this problem grows with rpm, potentially reducing the benefits that can be derived from other performance-enhancing parts.As you will see, exhaust-flow velocity is an important component in an efficient exhaust system. Simply stated, at low rpm, the flow rate tends to be slow.
As engine speed increases, so does flow rate. Then, as restrictions increase, velocity slows again, reducing power accordingly. Interestingly, camshaft design, compression ratio, ignition-spark timing, and piston displacement affect all this if an accompanying improvement in the exhaust system isn't included with such changes. In fact, these types of modifications can cause exhaust problems to occur sooner in the rpm range.On the other hand, exhaust systems can be too big for engine packages that don't produce sufficient exhaust-flow volume to necessitate size increase. So we're back to the flow-velocity issue. Sizing of system components, such as headers, can be keyed to engine speed and piston displacement. We'll show you how this is done later in the story.
Graph A illustrates how merely changing pipe diameter affects an engine's output. Note that the smallest diameter creates good midrange torque yet falls off at the top, while the larger primary header pipes add more high-rpm power at the expense of low-speed torque.
Primary pipe length can also skew an engine's power curve based on length changes. Primary-pipe diameter establishes the peak torque point, so changing the pipe length will rock the output curve by pivoting it around that peak torque point. Graph B shows how longer tubes tend to increase power below peak torque while hurting power above peak torque. Shorter tubes tend to affect the engine in exactly the opposite way, hurting midrange torque in favor of increasing top-end power.
What Primary Pipes Do
The main function of primary pipes is to set the initial rpm point (engine speed) at which a torque boost is created, as contributed by the headers. Keep in mind, exhaust and intake systems can be tuned to different engine speeds. By so doing, an overall torque curve can be broadened or narrowed by the separate dimensioning of intake and exhaust systems.
In the case of headers, primary-pipe diameter determines flow rate (velocity). At peak torque (peak volumetric efficiency), the mean flow velocity is 240-260 feet per second (fps), depending upon which mathematical basis is used to do the calculation. But for sizing or matching primary pipes to specific engine sizes and rpm, 240 fps is a good number.
Changing the length of primary pipes generally affects the amount of torque produced above and below peak-torque rpm. For example, all else being equal, shortening primary pipes transfers torque from below to above the peak, not significantly shifting the rpm point at which peak torque occurs. Increasing primary-pipe length produces the opposite effect of shortening the length.
What Do Collectors Do?
Essentially, collectors have an impact on torque below peak torque. While the gathering or merging of primary pipes does affect header tuning, it is the addition of collector volume (typically changes to pipe length once a diameter is chosen) that alters torque. Engines operated above peak torque, particularly in drag racing, do not derive any benefit from collectors. Those required to make power in a range that includes rpm below peak torque do benefit. And the further below peak torque they are required to run (from 2,500-7,500 rpm for example), the more improvement collectors provide.
Joining collectors, cross-pipe science notwithstanding, tends to further boost low-rpm torque by the increase in total collector volume. Generally, crossover pipes become less effective at higher rpm, as you might expect, although some manufacturers of the more scientific cross-pipes claim power gains as engine speed increases. The mere joining of collectors in a dual-collector system does not appear to produce this improvement.
Header Size
Consider this: It is the downward motion of a piston that creates cylinder pressure less than atmospheric. Intake flow velocity then becomes a function of piston displacement, engine speed, and the cross-section area of the inlet path. On the exhaust side, a similar set of conditions exists. In this case, exhaust-flow velocity depends on piston displacement, engine speed, the cross-sectional area of the exhaust path, and cylinder pressure during the exhaust cycle.
Of the similarities between the intake and exhaust process, piston displacement, engine speed, and flow-path cross section are common. Therefore, there must be a functional relationship among rpm, piston displacement, and flow-path section area, and there is (see the section on calculating pipe sizes).
This suggests the possibility of sizing primary-pipe diameter to produce torque boosts (as contributed by the exhaust system) to an engine's net torque curve. The previously mentioned mean flow velocity (240-260 fps) found in primary pipes around peak torque rpm is a function of pipe diameter. So, selecting diameters that correspond with the rpm at which torque boosts are desired is one method of header selection or sizing.
Matching Headers to Objectives
If we know any two of the three previously mentioned variables (piston displacement, rpm, or primary-pipe diameter), we can apply some simple math to solve for the other. Here's how that works.
1. Peak torque rpm = Primary pipe area x 88,200 /displacement of one cylinder. Given this relationship, we can perform some transposition to solve for the primary-pipe cross-section area.
2. Primary pipe area = peak-torque rpm / 88,200 x displacement of one cylinder. We can also determine the required displacement of one cylinder (multiplied by the number of cylinders for total engine size) by:
3. Displacement of one cylinder = Primary pipe area x 88,200 / peak-torque rpm.
Equations 1 and 2 provide a method for determining peak-torque rpm (as contributed by the primary pipes) if you have already selected a set of headers and know the engine size. In equation 3, primary-pipe area can be determined if the desired peak-torque rpm and engine size are already known. It will also calculate engine size based on a known set of headers and rpm at which peak torque is desired.
Here's an example of how this approach can work. Suppose you have a 350ci small-block (43.75 cubic inches per cylinder). A primary-pipe torque boost around 4,000 rpm is your target engine speed. The choices for pipe size are 15⁄8 inches, 13⁄4 inches, and 17⁄8 inches. If we assume a tubing wall thickness of 0.040 inch, each of these od dimensions requires subtracting 0.080 inch when computing cross-section areas.
Using the formula, Area = (3.1416) x (id radius) x (id radius), we obtain the following cross sections: 15⁄8 inches = 2.07 square inches; 13⁄4 inches = 2.19 square inches; 17⁄8 inches = 2.53 square inches.
Plugging each of these values into equation 1, we find the selection of peak torque becomes (in the same order of pipe sizes), 4,173, 4,415 and 5,100 rpm. Based on an intention to provide a torque boost around 4,000 rpm, 15⁄8-inch-diameter primaries appears to work. In accord with our previous comments about primary-pipe length, extending these primaries will increase torque below 4,000 rpm at the expense of torque above this point, which is an additional tool to manipulate a torque curve about its peak (see "Torque Peaks").
While this method will not predict header-pipe area as precisely as some contemporary computer-modeling programs, it can be a valuable quick-and-dirty tool when making decisions about header choice or application of sets already on hand.
Conclusion
There is much more to the science of exhaust-system tuning and headers that space does not allow us to include. It's worth noting once again that the final combination of parts must take into account all the components as a system, rather than looking at the headers as a separate entity. Any engine will make its best overall power when treated as a complete system.
I had to.