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Submitted by: Larry Fasse
To many people involved in motorsports, particularly racers like
myself, headers and exhaust systems are like "black-art"; of course to
me, getting the VCR clock on the right date is like "black-art" so I
am not sure I am the best example. I do believe there is a lot of
misunderstanding about what it takes to get the best performance from
an exhaust system; back pressure, tuned exhaust, merged collector are
all terms you have heard about but what do they mean?
Loren
Barnes is the President and technical "guru" at S&S Headers in
Glendale, Arizona. Loren has years of experience as a "racer" and as
a manufacturer of performance exhaust systems. He has also
manufactured some very successful race cars. S&S manufacture headers
and complete exhaust systems for a variety of street and competition
applications. They are one of the few who manufacture off the shelf
performance headers for welter weight race vehicles such as Dwarf
Cars, Mini-Sprints etc. I have had several conversations with Loren
and recently he sent me the following article, which I found to be
very helpful in beginning to understand some of the basics. I felt it
would be something that many people may like to read as they struggle
with how to solve their exhaust system issues. While the article is
somewhat general it gives a good overview of why there are different
requirements for different types of engines.
Following is Loren's article.
HEADER
BASICS
Written
by: Loren Barnes - President S&S Headers
You
have probably heard words like; back pressure, scavenging, tuned
length, merged collector, rotational firing order, compatible
combination and many others that meant something, but how they relate
to a header may be a little vague. This article should give you a
basic understanding of how it plays a part in the header's performance
gains.
The
first misconception that needs to be cleared up is: that a header
relieves back pressure, but that a certain amount of back pressure is
needed for optimum performance.
Just
the opposite is true. A good header not only relieves the back
pressure, but goes one step further and creates a vacuum in the
system. When the next cylinder's exhaust valve opens, the vacuum in
the system pulls the exhaust out of the cylinder. This is what the
term "Scavenging" means.
The
first consideration is the proper tube diameter. Many people think
"Bigger is Better", but this is not the case. The smallest
diameter that will flow enough air to handle the engines c.c. at your
desired Red Line R.P.M. should be used. This small diameter will
generate the velocity(air speed) needed to "Scavenge" at low R.P.M.s.
If too small a diameter is used the engine will pull hard at low
R.P.M.s but at some point in the higher R.P.M.s the tube will not be
able to flow as much air as the engine is pumping out, and the engine
will "sign off" early, not reaching its potential peak R.P.M. This
situation would require going one size larger in the tube diameter.
The
second consideration is the proper tube length. The length directly
controls the power band in the R.P.M. range. Longer tube lengths pull
the torque down to a lower R.P.M. range. Shorter tubes move the power
band up into a higher R.P.M. range. Engines that Red Line at 10,000
R.P.M. would need short tube lengths about 26' long. Engines that are
torquers and Red Line at 5,500 R.P.M.s would need a tube length of
36'. This is what is meant by the term "Tuned Length". The tube
length is tuned to make the engine operate at a desired R.P.M. range.
The
third consideration is the collector outlet diameter and the extension
length. This is where major differences occur between four cylinder
engines and the V8 engines. The optimum situation is the four
cylinder engine because of it's firing cycle. Every 180 degree of
the crankshaft rotation there is one exhaust pulse entering the
collector. This is ideal timing because, as one pulse exits the
collector, the next exhaust valve is opening and the vacuum created in
the system pulls the exhaust from the cylinder. In this ideal 180
degree cycling the collector outlet diameter only needs to be 20%
larger than the primary tube diameter. (Example: 1 ¾ primary tubes
need a 2" collector outlet diameter.) The rule of thumb here is two
tube sizes. This keeps the velocity fast to increase scavenging,
especially at lower R.P.M.s. Going to a larger outlet diameter will
hurt the midrange and low R.P.M. torque
The
amount of straight in the collector extension can move the engine
torque up or down in the R.P.M. range. Longer extension length will
pull the torque down into the midrange.
Engines that "Red Line" at 10,000 R.P.M. would only need 2" of
straight between the collector and the megaphone. This is just enough
length to straighten out the air flow before it enters the megaphone.
This creates an orifice action that enhances exhaust velocity.
In
the case of the V-8 firing order, the five pulses fire alternately
back and forth from left to right collector, giving the ideal 180
degree firing cycle. Then it fires two in succession into the left
collector, then two in succession into the right collector. If the
proper collector diameter is being used(two sizes larger than
primaries) the two pulses in succession load up the collector with
more air than it can flow. This results in very strong midrange
torque, but causes the engine to "sign off" early, not reaching its
potential peak R.P.M. The improper firing order on a V-8 engine
results in the need to use large diameter collectors so the engine
will perform well at high R.P.M.s Unfortunately the large diameter
collectors cause a tremendous drop in air velocity, resulting in less
scavenging through the entire R.P.M. range.
Often
times the cams are used with extended vavle timing to help the exhaust
cycling. This results in the valve timing overlap (Intake and Exhaust
valves both open at T.D.C.) which causes a "Revesion" cycle in the
exhaust. When this happens, exhaust actually backs up into the
cylinder causing intake air to be pushed back out the intake. This
reversion causes "Standoff" (fuel blowing out of the Intake) at low
R.P.M.s. This whole improper cycling has resulted in a number of
"Cure Alls" to help stop this reversion and standoff.
The
plentum intake was created to stop the fuel "Standoff". Then came
"Anti Reversionary" Cones in the exhaust tubes, and stepped tube
diameter in the header, extended collector lengths and even plentums
in the exhaust tubes.
In
this chain of event of events beginning with the improper firing
order, a series of cures has developed, each one causing a new
problem.
The
optimum cure to this whole problem is to correct the exhaust firing
cycle. The two cylinders that fire in succession into each collector
have to be separated. This can be done partially by a "Tri-Y" header,
where the four primary tubes from each bank merge into two
secondary tubes (separating the two pulses firing in succession) and
finally collect into a single collector. This type of header helps,
but the two pulses are still coming back together at the collector.
The
second optimum cure is to cross the two center tubes from each bank,
across the engine running them into the collector on the opposite
side. This makes the firing cycle in each collector 180 degrees
apart, the same as a four cylinder engine. Once this firing order is
achieved, the small collector outlet diameter can be used and the
"High Velocity Scavenging" at low R.P.M.s cures the reversion problems
and eliminates the need for extreme cam duration.
This
sounds so easy, you are probably asking yourself why wasn't this done
from the start? If you have ever seen a set of 180 degree headers
you would understand.
On
today's cars, with space virtually nonexistent, crossing four tubes
either under the oil pan or around the front or rear of the engine
presents major problems. On racing applications where it is possible,
there is still the problem of keeping the tube length down to a
reasonable 32' long. If that's not enough challenge, then try to
arrange the tubes into each collector so they fire in a "Rotational
Firing" pattern. Then you have what is called "A Bundle of Snakes".
Arranging the tubes to fire rotationally adds to the scavenging
capabilities. The exhaust gas exiting one tube, passing across the
opening of the tube directly beside it, creates more suction on that
tube than it would on a tube on the opposite side of the collector.
The
next problem is "Turbulence" in this collector. When four round tubes
are grouped together in a square pattern, so a collector can be
attached, you notice a gapping hole in the center of the four tubes.
The standard method in manufacturing headers is to cap this hole off
with a square plate. This plate in the center of the four tubes
creates dead air space, or turbulence, disrupting the high velocity in
the collector. This problem is solved by using a "Merge Collector".
This collector is formed from four tubes, cut at approximately an 8
degree angle on two sides. When the tubes are all fit together they
form a collector with a :Pyramid" in the center. This has eliminated
the need for the square plate and has taken up some of the volume
inside the collector, speeding the air velocity.
Other
methods of curing this problem are: fabricating a pyramid out of sheet
metal and welding it over the hole between the tubes, or squaring the
tubes on two sides so they fit together forming a "+" weld in the
center eliminating the hole all together.
You
can see there are a great many factors that go into making a good
header. When the header, intake system, and cam timing are all
designed to operate to their maximum in the same R.P.M. range, then
you have a "Compatible Combination". This combination can be tuned to
deliver maximum power at any desired R.P.M. range.
These are some of the "Basics" you need to know about building a good
high performance header. There are many other adjustments that can be
made to fine tune a header, but this should give you a basic
understanding of how all of the components work together.
Information is available about the S&S Header product line by calling
Speed Partz at 513-755-7205. You may also request information about
your application by sending a FAX to 513-777-6804
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