A must read if you are thinking on custom exhaust!!!

[Diokaef]

I think you guys may like to know about this a bit better.

http://www.x-pipe.com/Scavenge.html

4into1 Urban Legends

Urban Legend #1:
Headers produce scavenge, thereby increasing exhaust airflow, allowing the engine to burn more air/fuel, and make more power.
The Truth of the matter:
Most headers designed to create scavenge actually slow down exhaust airflow, creating back pressure, which is why longer duration exhaust cams can improve the performance of a header equipped engine. There is only one way to make more horsepower; increase dynamic cylinder pressure.
FACT:
Only dynamic cylinder pressure can create horsepower. Scavenge is defined as a suction or a less than atmospheric pressure.
REASON:
There is no way that a suction can create an increase in dynamic cylinder pressure. Maybe headers do work, in some applications, but scavenge alone can not be credited with creating performance improvements.
Urban Legend #2:
Scavenge removes burned exhaust gases from combustion chambers, to allow more fresh air fuel to be burned, making more horse power. More scavenge is better. In fact, mankind has gone to the moon, but mankind has never, ever, created too much scavenge.
The Truth of the matter:
Everything has diminishing returns. More scavenge may not improve power. It seems illogical to state that more vacuum creates more dynamic cylinder pressure.
FACTS:
Scavenge, created and maintained by a resonant 4into1 header, does not remove burned gases from the combustion chamber of an internal combustion engine. As the combustion mixture is burned during the power stroke, the mixture begins its burn from the sparkplug and continues burning toward the descending piston. When the exhaust valve opens in the latter portions of the piston's descent, the gases nearest the valve, and sparkplug, are the first to leave the chamber, under hundreds or even thousands of pounds per square inch pressure. As the light, hot, energetic combustion product gases pass out of the chamber through the exhaust port, chamber pressure drops as potential energy (cylinder pressure) is converted into kinetic energy (exhaust gas velocity). This high velocity exhaust gas column then tends to stay in motion, even after all of the hot, burned, exhaust gases have left the chamber and the chamber pressure drops to zero. Scavenge is then created as the still rapidly exiting exhaust gas column evacuates the chamber by sucking any unburned air/fuel, remaining at the bottom of the chamber, out the exhaust port. No dynamic cylinder pressure increase here.
About 60 degrees before the end of the exhaust stroke, the intake valve opens, exposing the intake tract to the scavenge created vacuum now in the combustion chamber. If the scavenge vacuum is greater than manifold vacuum at that instant (say the throttle is wide open), air/fuel mixture will be sucked out of the intake manifold, through the combustion chamber, past the still open exhaust valve, and out the tail pipe. No dynamic cylinder pressure increase here. In fact, cylinder pressure will be reduced in the very next compression cycle because a portion of the available air/fuel has been lost, never to be burned. Opps, you have just lost dynamic cylinder pressure, the one thing needed to make power.
Urban Legend #3:
Fuel mixture must be made richer in fuel when a scavenge header is installed. The reason is that the header's powerful scavenge allows the engine to breath better, burning more air/fuel, making more horsepower.
The Truth of the matter:
Fuel must be added because the scavenge header is sucking the same amount of fuel, the amount that must now be added, out the tailpipe; the phenomenon is known as over-scavenge.
FACTS:
All intake systems compensate for airflow changes by proportionately adding fuel to air passing through the intake. Carburetors use a simple Ventura that generates a stronger vacuum signal with greater airflow, pulling more fuel through jet circuits, spraying it into the intake air stream. Fuel injection systems use various airflow sensors and air/fuel maps to precisely meter fuel to match air flow. So, even if intake air flow were to increase because of the installation of a scavenge header, the correct amount of fuel would always be added; there would be no need to rejet or remap air/fuel mixtures.
Typically, the more highly tuned the header, and the less the back pressure after the header (i.e. mufflers or catalytic converter restriction), the greater the over scavenge generated and the greater the amount of fuel that must be added to the fuel delivery curve. The additional fuel has to be added because an equal amount of the originally jetted or mapped fuel is lost. How? By over scavenge of the initial air/fuel charge, the richest portion of all air/fuel delivered to the combustion chamber. How do we know this? Because 1) fuel economy (BHPHR/#fuel) does not improve, 2) the mixture leans out only at the scavenge power peak (resonance) rpm, it becomes richer (needs less supplemental fuel) below and above that rpm, & 3) HC (unburned hydrocarbon) emissions go up within such tuned rpm range.
REASON:
Scavenge quickly becomes over scavenge, sucking the richest portion of delivered air/fuel out of the intake manifold and through the combustion chamber during the 60 or so degrees of valve overlap, when both intake and exhaust valves are open at the same time. Ironically, 3, 4 and 5 valve engines are the most likely engines to suffer over scavenge due to the much greater valve curtain from which air/fuel is allowed to escape under minimal vacuum conditions. High flow or ported 2 valve heads suffer the same fate due to increased air flow propensity. So, the better the head, the more likely that scavenge will become over scavenge, and dynamic cylinder pressure will drop.
Urban Legend #4:
4into1 headers flow better than cast iron manifolds.
The Truth of the matter:
A smaller, well designed iron manifold can perform better than most headers.
FACTS:
1. Airflow velocity is generally limited to the speed of sound in that medium. The speed of sound at STP (standard temperature and pressure: sea level) is about 1100 ft/sec. The speed of sound increases to about 1400 ft/sec at about 1100 degrees Fahrenheit.
2. As air flows into a pipe with a different cross sectional area, airflow velocity changes according to that change in area. If the area increases, airflow velocity drops and pressure goes up. If the area decreases, airflow velocity increases and pressure goes down. This is known as Bernoulli's Law.
REASON:
Headers typically flow less than OEM manifolds, because they typically increase the cross sectional area more than manifolds. More area increase means that as the exhaust gases travel through the header, they must expand more, causing the exhaust gas velocity to drop, resulting in back pressure.
A 4into1 also creates additional back pressure at high rpms because the exhaust pulses are slowed down and can't get out of the header before more pulses stack up behind them in the exhaust system.
Say you have a 30mm (1.18" dia.) exhaust valve. Say that the area under that valve, the seat throat region is 80% of the area of the valve head; .7 square inch area. If that is the smallest cross sectional area in the exhaust flow path, it becomes the "choke point", meaning that once the air in that region reaches the speed of sound, it becomes the limit of air flow for the entire system: the "weak link" in the chain of flow elements. Each increase in area brings about a reduction in subsequent flow velocity.
Next, say that the head pipe is 1.5"o.d. & 1.4" i.d. When the air flow passes from .7 square inch area to 1.543 square inch area, the velocity drops from say 1400 ft/sec to 634.8 ft/sec. Likewise, when that air flow then passes into a 3" collector section (7.069 square inch area), airflow drops to 138.57 ft/sec. And finally, when the airflow continues on to a 4" megaphone (12.566 square inch area), airflow makes its final drop to 77.95 ft/sec.
A 4into1 on an engine running 9,000 rpm produces 75 exhaust pulses per second. So, if airflow velocity is reduced to 77.95 ft/sec., each exhaust pulse is compressed making it no longer than 1.04' long. Bernoulli's law predicts that as the air flow is slowed, its pressure increases. This pressure increase is high rpm back pressure!
A 4into1 slows exhaust gas flow and creates back pressure at high rpm, even though it produces overscavenge during the overlap period at the "tuned" rpm.
Urban Legend #5:
Step headers flow more because they increase scavenge.
The Truth of the matter:
Each time that the cross sectional area of the exhaust system increases, exhaust gases are forced to expand to fill that new, large area. When gases expand they cool down, contract and become denser. Expanding/cooling gases slow down as their energy is lost. Each transition in size creates sonic shock waves that travel throughout the system, absorbing even more kinetic energy, slowing the exhaust gas flow further, in jerks.
FACTS:
1. As airflow expands into a greater cross sectional area pipe, it tends to cool down.
2. Exhaust airflow is driven by combustion chamber pressure, released into the exhaust when the exhaust valve opens. The potential energy in the combustion chamber (high pressure) is converted into kinetic energy (velocity, up to the speed of sound), when the exhaust valve is opened.
3. When the cross sectional area is increased in the header, velocity drops, see above.
4. When the exiting, high velocity, gases pull a vacuum, energy is converted from kinetic energy (velocity) into potential energy (scavenge: negative pressure), thereby slowing the exiting exhaust gases by absorbing some of their energy (velocity is reduced).
5. When a gas stream expands into a larger cross section pipe, it cools, due to that expansion.
6. The speed of sound in the exhaust gases is reduced each time that those gases expand and cool. (The velocity of the exhaust gases is limited to the speed of sound. When gases cool, they can not exceed that cooler, slower, speed of sound. They are forced to slow down, preventing exhaust gases from exiting at a higher velocity, creating back pressure.)
REASON:
There is no free lunch, even in a step header. The valve seat I.D. limits the rate of exhaust gas escape, because gas flow rates can not exceed the speed of sound. As exhaust gases enter the head pipe, they are forced to expand, cool, and slow down. With each increase of inside diameter of the exhaust system, the process is repeated. Even though the pipe is larger, the gases can not flowing faster. In fact, the gases slow down with each step. A larger diameter pipe has a greater circumference, and therefore greater surface area to allow exhaust gas heat to escape. The cooler the gas becomes, the more energy it loses from its velocity (thermal energy is lost, allowing the exhaust gases to contract, becoming denser and heavier) and the slower its speed of sound (maximum velocity) in those gases.
Urban Legend #6:
Long duration exhaust cams allow headers to make the most horsepower because headers flow better than stock cast iron exhaust manifolds.
The Truth of the matter:
Long duration exhaust cams are necessary with scavenge headers because such headers convert exhaust gas velocity into scavenge, slowing the rate at which exhaust gases can escape from the engine. Longer duration exhaust cams give the exhaust stroke more time to blow down exhaust gases. Longer duration exhaust cams give that unmistakable big cam (stumbling) idle, because at idle the intake vacuum is the greatest & scavenge is the weakest, allowing exhaust gases to be sucked into the intake manifold during valve overlap, diluting intake manifold mixture, which in the subsequent intake stroke, fills the combustion chamber with inert exhaust gases that don't burn, causing a misfire (stumble).
FACTS:
1. Exhaust blow down rate is usually limited by exhaust valve seat inside area. Exhaust gases can not exceed their speed of sound: 1400 ft/sec @ 1100 degrees f.
2. Headers can work better with long duration exhaust cams. But, not because headers flow better. Rather, headers flow poorly and need more time to flow the same amount of exhaust.
3. Long duration exhaust cams open exhaust valves earlier, dumping combustion energy into the exhaust system. The extra combustion energy and the extra exhaust valve open time (duration) allows the header to expel more exhaust gases than it otherwise could with the shorter duration stock cam.
4. Opening the exhaust valve earlier shortens the power stroke, drastically reducing thermodynamic efficiency of the engine.
5. Opening the exhaust valve earlier dumps hotter, still burning air/fuel into the exhaust port and header, making them hotter. Hotter exhaust gases shorten the life of exhaust valves, cook headers, and add heat load to the cylinder head.
6. If headers really flowed so well, a shorter duration exhaust cam, one more like stock, would allow a longer, more efficient, power stroke, kind of like a stroker crank vs. a stock crank. The longer stroke usually makes more power.
7. Hp = Work/sec (1 HP = 33,000 ft lbs./minute = 550 ft lbs./ second)
8. Work = Force X Distance (Work = (area of bore X combustion chamber pressure) X (power stroke length))
9. If all else were equal, reducing the power stroke will reduce Work done by each cylinder and total engine HP.
10. In a four stroke engine, the crankshaft rotates 720 degrees in order to complete all four of its strokes (4 X 180 degrees).
11. If one degree of duration is added to the exhaust stroke, one degree must be subtracted from one of the other three strokes (intake, compression, or power).
REASON:
Since the exhaust stroke generally can not be expanded into the intake stroke, due to the increased probability of valve to valve and valve to piston intersection during the valve overlap period, degrees are usually subtracted from the power stroke. If a long duration exhaust cam is, say, 20 degrees longer than stock, usually it also makes the power stroke become 20 degrees shorter than stock.
For example, a Ford 302 has a 3" stroke. If the stock exhaust cam forces the valve open at 70 degrees BBDC (before bottom dead center), the power stroke is 2.013". If the long duration exhaust cam opens the valve only 20 degrees earlier (in order to add 20 degrees of exhaust duration) the power stroke is reduced to 1.5". The long duration exhaust cam, with just 20 degrees more duration, has reduced the power stroke by .513"! That is the opposite effect of a 1/2" stroker kit!
People spend thousands to put even a 1/4" stroker crank in their engine, to make more power. You've just done the opposite. You've just cut your stroke by 25%. All else being equal, you can expect to lose about 25% of your engine's thermal efficiency (its ability to convert heat (thermal energy) into horsepower (mechanical energy)).
If a header really did flow better, wouldn't a person be smarter to leave the power stroke duration stock?
The reality is, a header does not flow very well at all, because it expands, cools and therefore slows the exiting gases. The long duration exhaust cam is really just a marketing band aid for the very poor flowing 4into1.
Enough with the Urban Legends. You are set free. Now, you know he truth of the matter.

[Diokaef]

4into1 Urban Legends you can also read as 3into1.