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.
BasicsMany 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.
What Primary Pipes DoThe 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.