Exhaust Scavenging 101
 

I just had a little bit of a debate with my buddy here about what "scavenging" was. After the fact I hit the web to make sure that I was in fact correct, turns out I was..HAHA. Anyway the following is an excerpt that I found to be a great read. Just thought I would share it with everyone.

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It ain't just pipes, it's science! Part 1

I’d like to try to explain some basic exhaust theory and clear up some
issues that may not be completely clear.

Everyone knows the purpose of an exhaust system is to provide a means for
the exhaust gases to be removed from the cylinder. You might wonder why
have an exhaust system at all. Other than the frequent need to muffle the
noise of rapid combustion, why not simply open the exhaust port to the
atmosphere, thereby saving both weight and expense?

Some time back in early internal combustion engine history, it was
discovered that attaching a length of pipe to the exhaust port (probably
to direct the noxious exhaust fumes away from a passenger compartment or
out of a room where a stationary engine was housed) often had an effect on
the performance of that engine. Depending on parameters such as pipe
diameter and length, the performance could be adversely or positively
impacted.

I expect it was clear from the very beginning that exhaust gases have
momentum. What may not have been known at the outset is that they also
exhibit wave properties, specifically those of sound. Both those
properties can be utilized to evacuate the exhaust gases more quickly and
completely. The usual term for this removal process is “scavenging.”

There are two types of scavenging: inertial and wave. Inertial
scavenging works like an aspirator whereby some of the kinetic energy of a
moving fluid stream (air, water, etc, generally in a pipe) is transferred
to the fluid in an adjacent pipe. You may remember from high school
chemistry lab class where you used water traveling through the top of a
“T” fixture to draw a quite powerful vacuum in an attached vessel.

The “T” can be likened to a merge collector as used in virtually all
successful racing cars (although often not in dragsters). The most
effective merge collectors minimize the volume increase at the juncture of
the pipes. If this volume is too large, gas speed is diminished and less
kinetic energy is transferred to the gases in an adjacent pipe. Thus, the
scavenging is less complete. Well, so what if there is a little gas left
in the pipes? Consider the engine cylinder as an extension of the exhaust
pipe. A cylinder with residual exhaust gases has less room available to
accommodate the incoming charge of gas and oxygen. Obviously, the more
gas and air you can get into a cylinder, the more power is developed; that
is why superchargers are so effective.

Not only can scavenging be utilized to empty the cylinders, it also can
help to draw in the new charge, by producing a negative pressure in the
cylinder. This gets tricky because there has to be adequate time in which
both the intake and exhaust valves are open, and there is the potential
problem of the new charge passing right through the cylinder into the
exhaust pipe! Gas is wasted and power is lost. Maybe you can design your
cam such that it closes at just the right time to prevent this from
occurring. Or maybe you can make the exhaust pipes just the right length
so that the reflected sound waves (at a particular engine speed) prevent
the incoming fuel and air from spilling out of the cylinder. More on this
later.

A stock S4 engine has very little valve overlap (some at small valve
openings) and therefore there is only a short time during which scavenging
of the cylinder can be accomplished. Even still, there is opportunity for
significant performance gains with effective scavenging of the primary
exhaust pipes (the first pipes that emanate from the ports) where it’s
possible to produce a negative pressure so that when the exhaust valve
opens, exit speed is increased. The result is increased momentum and
possibly improved cylinder evacuation.

On to wave scavenging. An analogy would be tuned organ pipes in which
their length is adjusted such that a standing wave of a particular desired
length (and frequency) is established. This means that some whole number
of waves will fit exactly within the length of the particular pipe. When
the point of maximum amplitude of a wave comes to the end of the pipe or a
change in diameter, the wave is reflected back up the pipe, but as its
mirror image. Thus a positive pressure wave is reflected as a negative
pressure, or rarefaction, wave which, in turn, helps to draw spent gases
from the pipe/cylinder. Wave scavenging is most effective over a narrow
speed range that can be adjusted by changing the primary pipe length.
Thus a torque or power peak can be designed to occur at a particular
engine speed to suit the application whether it is racing or everyday
driving.

What are crossover headers? There are numerous types of headers, tri-Y,
equal length, stepped, unequal length, crossover, etc. Unequal length
headers are by definition not tuned at a specific rpm; rather each pipe is
tuned for a different speed. They tend to perform better than the stock
manifold and may increase performance over a broad speed range. Because
of their unequal length, each pipe will utilize wave scavenging at a
different speed, thus reducing the effect at any single or narrow band of
speeds. They often have sub-optimal merge collectors and so, do not make
the best use of inertial scavenging. Equal length headers can be
excellent wave scavengers, but often have inferior collectors, so inertial
scavenging is not optimized. The tri-Y design is especially good on
4-cylinder engines and is now being used almost exclusively on NASCAR
engines with 8 cylinders. Stepped headers gradually increase the pipe
diameter going away from the port. I believe at least one of the purposes
is to inexpensively approximate a megaphone which is the most efficient
device for returning the pressurized gases back to the surrounding
atmosphere.

Re: Exhaust Scavenging 101
 

Thanks, it's difficult to get theory unless one is an automotive engineer. My concern was about the effect on scavenging of too large an exhaust pipe in a cat back system (say, a 75 mm pipe on an S2000 instead of the stock 60 mm pipe). I suspect from what you have posted that the effect may be minor because the main determinant on scavenging is the size of the primary pipes in the exhaust header, followed probably bny the secondary pipes.