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 SizeConsider 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 ObjectivesIf 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 151/48 inches, 131/44 inches, and 171/48 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.