Given a particular amount of exhaust pressure at an exhaust valve, a smaller diameter header tube provides higher flow velocity than a larger diameter tube. The paradox is that the laws of physics don’t allow a small diameter tube to transfer sufficient volume to make maximum potential power at the higher RPM range. If you install a larger diameter tube, you’ll have enough flow volume for maximum power at mid-to-high RPM, but the velocity of the flow will decrease and low-to-mid range throttle response and torque suffer.
You see the problem, and the solution requires a compromise.
Still not, pardon the pun, baffled? Then peep this, there’s something called “scavenging” you have to consider as well, and it is seriously complicated to explain.
There are two types of scavenging, inertial scavenging and wave scavenging, and they’re different phenomenon but they both impact exhaust system efficiency and each other. “Scavenging” is gas extraction. The two scavenging effects are,not surprisingly, directly influenced by the diameter, length, shape and the thermal properties of the tube material used to make the system. When an exhaust valve opens, two things happen; an energy wave, or pulse, is created from the rapidly expanding combustion gases. When it does,that wave enters the header or “manifold” tube and starts traveling outward at something like 1,300 to 1,700 feet per second. That speed is of course dependent on engine design. The wave is pure energy and akin to a shock wave from an explosion because, well, it is a shockwave from an explosion.
At the same moment the energy wave is created, the spent combustion gases enter the header tube and travel outward at between 150 to 300 feet per second. Since maximum power is usually created at gas velocities between 240 and 300 feet per second, that’s good.
Here’s the rub, the “energy wave” is moving about five times faster than the exhaust gases, and that means it’ll get where it’s headed faster than the gases. When the outbound energy hits a low pressure area like a larger collector pipe, muffler or the atmosphere, a reversion wave, essentially a reversed or mirrored wave, is reflected back toward the exhaust valve at nearly the same velocity.
Here again, it gets complicated.
That “reversion wave” moves back toward the exhaust valve where it’s going to collide with the exiting gases. It’s where they they pass through each other that some energy loss and turbulence occurs. Where the reversion wave arrives back at the exhaust valve depends on whether the exhaust valve is open or closed. It’s a critical moment in the exhaust cycle as the reversion wave can be good or bad for the exhaust flow. It all comes down to the arrival time at the exhaust valve. If the exhaust valve is closed when the reversion wave hits, the wave is reflected toward the exhaust outlet and eventually dissipates its energy in the back and forth motion. If the exhaust valve is open when the wave hits, its effect upon exhaust gas flow depends on which part of the wave is hitting the open exhaust valve.
Waves are made up of alternating and opposing pressures. During one part of the wave cycle, gas molecules are compressed. During the other part of the wave, gas molecules are “rarefied.” Waves contain a compression area of higher pressure and a “rarefaction” area of lower pressure. An exhaust tube of the proper length for a specific RPM places the wave’s point of rarefaction at the exhaust valve at the proper time for the lower pressure to help fill the combustion chamber with fresh incoming gases and to extract spent gases from the chamber via a vacuum effect.
In a nutshell, it’s this collision of forces that make up “wave scavenging” or “wave tuning.”
The beneficial part of a rapidly traveling reversion wave is only present at an exhaust port during some portions of the power band since it’s relative arrival time changes with RPM. It’s what makes it difficult to tune an exhaust system to take advantage of reversion waves. There are various anti-reversion schemes designed into some header systems and exhaust ports, and these anti-reversion devices are made to weaken and disrupt any detrimental parts of the reversion waves such as when the wave’s higher-pressure node fights scavenging and intake draw-through. These sorts of mechanical schemes are done with merge collectors, cones or rings built into the primary tube entrance and sometimes with exhaust port ledges.
Reversion waves have no mass, but exhaust gases do have mass, and exhaust gases with mass in motion also have inertia, or “momentum,” as they travel out of the system. When the gases move outward as a gas column through the header tube, a decreasing pressure area gets created in the pipe behind them. This lower pressure area is essentiall a partial vacuum and you can visualize the lower pressure area by imagining that it’s pulling residual exhaust gases from the combustion chamber and exhaust port. Set up optimally, that pressure can help pull fresh air and fuel into the combustion chamber.
That, my friends, is inertial scavenging, and it’s got a big effect on engine power at low-to-mid range RPMs.
If properly timed with RPM and firing order, the low pressure which is built up from gas inertia can spill-over into other primary tubes, via the collectors, and aid the scavenging of other cylinders.
Is that complicated enough for you? Well, if you’d like more elements to consider, the behavior of exhaust gases is also affected by wave harmonics, wave amplification and wave cancellation effects.
The interaction of all the variables is so obtuse, so complex, that it’s difficult to wrap your head around without a slide rule and a theoretician standing by, and what it means is that there’s no absolute formulas which will produce a perfect exhaust design.
Factory systems designed on supercomputers still need to be tested on a dynamometer, and get track tested as well, to arrive at the proper exhaust tube adjustments for a particular results.
So the next time you decide to fabricate some pipes for your build, it might serve you to follow the example of a factory system if you want maximum performance.