There is too much misinformation regarding exhaust
theory. What kind of misinformation? For starters, there are a lot of people in
the "Bigger is Better" camp. We're talking about exhaust pipe
diameters. Even the big magazine editors are boldly smattering statements like,
"For a turbo car, you can't get an exhaust pipe that's too big."
Also, terms like "back pressure" and the statement, "An engine
needs back pressure to run properly!" really can be misleading.
Let's start from the beginning. What is an
exhaust system? Silly question? Not hardly. Exhaust systems carry out several
functions. Among them are: (1) Getting hot, noxious exhaust gasses from your
engine to a place away from the engine compartment; (2) Significantly
attenuating noise output from the engine; and (3) In the case of modern cars,
reduce exhaust emissions.
In order to give you a really good idea of what
makes up an exhaust system, let's start with what exhaust gas travels through
to get out of your car, as well as some terms and definitions:
After your air/fuel mixture (or nitrous/fuel
mixture) burns, you will obviously have some leftovers consisting of a few
unburned hydrocarbons (fuel), carbon monoxide, carbon dioxide, nitrogen oxides,
sulfur dioxide, phosphorus, and the occasional molecule of a heavy metal, such
as lead or molybdenum. These are all in gaseous form, and will be under a lot of
pressure as the piston rushes them out of the cylinder and into the exhaust
manifold or header. They will also be hotter `n Hades. (After all, this was
the explosion of an air/fuel mixture, right?) An exhaust manifold is usually
made of cast iron, and its' primary purpose is to funnel several exhaust ports
into one, so you don't need four exhaust pipes sticking out the back of your
Exhaust manifolds are usually pretty restrictive
to the flow of exhaust gas, and thus waste a lot of power because your pistons
have to push on the exhaust gasses pretty hard to get them out. So why does
virtually every new automobile sold have exhaust manifolds? Because they are
cheap to produce, and easy to install. Real cheap. Real easy.
"Ok," you ask, "so now what?"
Ah, good thing you asked. The performance alternative to the exhaust manifold is
a header. What's the difference? Where a manifold usually has several holes
converging into a common chamber to route all your gasses, a header has
precisely formed tubes that curve gently to join your exhaust ports to your
exhaust pipe. How does this help? First of all, as with any fluid, exhaust
gasses must be treated gently for maximum horsepower production. You don't
want to just slam-bang exhaust gas from your engine into the exhaust system. No
way, Jo-se'! Just as the body of your `94 Eclipse is beautiful, swoopy, and
aerodynamic, so must be the inside of your exhaust system.
Secondly, a header can be "tuned" to
slightly alter your engines' characteristics.
We'll go in-depth into header tuning a little
Next, exhaust gasses exit from your manifold or
header, travel through a bit of pipe, then end up in the catalytic converter, or
"cat". The cat's main job is to help clean up some of the harmful
chemicals from your exhaust gas so they don't end up in your lungs. In most
cars, they also do a great job of quieting things down and giving any exhaust
system a deeper, mellow tone. You'll see a lot of Self-Proclaimed Master
Technicians (SPMT's) telling people that removing a cat will get you tons of
power. There's room for debate on this, but in our experience, removing a
catalytic converter from a new car won't gain you much in the horsepower
department. If you drive an OBD-II equipped car, you'll also get that damn
annoying CHECK ENGINE light burnin' up your dashboard. (And for all you racers
concerned with OBD-II's fabled "limp mode", you can put your fears
From the catalytic converter, the exhaust gasses
go through a bit more pipe and then into a muffler, or system consisting of
several mufflers and/or resonators.
Are you a muff?
Exhaust gases leave the engine under extremely
high pressure. If we allowed exhaust gasses escape to the atmosphere directly
from the exhaust port, you can well imagine how loud and cop-attracting the
noise would be. For the same reason gunshots are loud, engine exhaust is loud.
Sure, it might be cool to drive around on the street with that testosterone
producing, chest-thumping, 150 decibel roar coming from your car... for about
5.3 seconds. (Not 5.2 or 5.4 seconds... 5.3.) Even the gentleman's gentleman
has gotta use a muffler, or system of mufflers, on their exhaust.
Again, you may hear a few SPMT's tell you that
"Borla mufflers make horsepower!" Or "An engine needs some
backpressure to run properly!" Nonsense. A muffler can no more
"make" horsepower than Wile E. Coyote can catch roadrunners. Any
technician with any dyno experience will tell you that the best mufflers are no
mufflers at all!
Types of Muff
Mufflers can take care of the silencing chores by
three major methods:
Absorption, Restriction, and Reflection. Mufflers
can use one method, or all three, to attenuate sound that is not so pleasing to
the ears of the Highway Patrol.
The absorption method is probably the least
effective at quelling engine roar, but the benefit is that "absorbers"
are also best at letting exhaust gas through. Good examples of absorbers are the
mufflers found in GReddy BL-series exhausts, DynoMax UltraFlow, and the good
old-fashioned Cherry Bomb glasspack.
Absorption mufflers are also the simplest. All of
the above named mufflers utilize a simple construction consisting of a
perforated tube that goes through a can filled with a packing material, such as
fiberglass or steel wool. This is similar to simply punching holes in your
exhaust pipe, then wrapping it up with insulation. Neat, huh?
Another trick absorption mufflers use to kill off
noise is, well, tricky. For example, the Hooker Aero Chamber muffler is a
straight-through design, with a catch. Instead of a simple, perforated tube,
there is a chamber inside the muffler that is much larger than the rest of the
exhaust pipe. This design abates sound more efficiently than your standard
straight-through because when the exhaust gasses enter this large chamber they
slow down dramatically. This gives them more time to dwell in the sound
insulation, and thus absorb more noise. The large chamber gently tapers back
into the smaller size of your exhaust pipe, and the exhaust gasses are sent on
their merry way to the tailpipe.
Doesn't that word just make your skin crawl? It's
right up there in the same league with words like "maim".
Obviously, a restrictive muffler doesn't require
much engineering expertise, and is almost always the least expensive to
manufacture. Thus, we find restrictive mufflers on almost all OEM exhaust
systems. We won't waste much time on the restrictive muffler except to say
that if you got `em, you might not want to flaunt `em.
Probably the most sophisticated type of muffler is
the reflector. They often utilize absorption principles in conjunction with
reflection to make the ultimate high-performance silencer. Remember any of your
junior high school math? Specifically, that like numbers cancel each other when
on a criss-cross? That's the same principal used by the reflective muffler.
Sound is a wave. And when two like waves collide, they will "cancel"
each other and leave nothing to call a corpse but a spot of low-grade heat.
There are numerous engineering tricks used in the
reflective muffler. Hedman Hedders makes a muffler that looks a lot like a
glasspack. In fact, it is a glasspack with a catch. The outer casing is sized
just-so, so that high-pitched engine sound (what we deem "noise") is
reflected back into the core of the muffler... where those sound waves meet
their maker as they slam right into a torrent of more sound waves of like
wavelength coming straight from the engine. And, this muffler is packed with a
lot of fiberglass to help absorb any straggling noise that might be lagging
The Exhaust Pulse
To gain a more complete understanding of how
mufflers and headers do their job, we must be familiar with the dynamics of the
exhaust pulse itself. Exhaust gas does not come out of the engine in one
continuous stream. Since exhaust valves open and close, exhaust gas will flow,
then stop, and then flow again as the exhaust valve opens. The more cylinders
you have, the closer together these pulses run.
Keep in mind that for a "pulse" to move,
the leading edge must be of a higher pressure than the surrounding atmosphere.
The "body" of a pulse is very close to ambient pressure, and the tail
end of the pulse is lower than ambient. It is so low, in fact, that it is almost
a complete vacuum! The pressure differential is what keeps a pulse moving. A
good Mr. Wizard experiment to illustrate this is a coffee can with the metal
ends cut out and replaced with the plastic lids. Cut a hole in one of the lids,
point it toward a lit candle and thump on the other plastic lid. What happens?
The candle flame jumps, then blows out! The "jump" is caused by the
high-pressure bow of the pulse we just created, and the candle goes out because
the trailing portion of the pulse doesn't have enough oxygen-containing air to
support combustion. Neat, huh?
Ok, now that we know that exhaust gas is actually
a series of pulses, we can use this knowledge to propagate the forward-motion to
the tailpipe. How? Ah, more of the engineering tricks we are so fond of come in
to play here.
Just as Paula Abdul will tell you that opposites
attract, the low pressure tail end of an exhaust pulse will most definitely
attract the high-pressure bow of the following pulse, effectively
"sucking" it along. This is what's so cool about a header. The
runners on a header are specifically tuned to allow our exhaust pulses to
"line up" and "suck" each other along! Whoa, bet you didn't
know that! This brings up a few more issues, since engines rev at various
speeds, the exhaust pulses don't always exactly line up. Thus, the reason for
the Try-Y header, a 4-into-1 header, etc. Most Honda headers are tuned to make
the most horsepower in high RPM ranges; usually 4,500 to 6,500 RPM. A good
4-into-1 header, such as the ones sold by Gude, are optimal for that high
winding horsepower you've always dreamed of. What are exhaust manifolds and
stock exhaust systems good for? Besides a really cheap boat anchor? If you think
about it, you'll realize that since stock exhausts are so good at restricting
that they'll actually ram the exhaust pulses together and actually make pretty
darn good low-end torque! Something to keep in mind, though, is that even though
an OEM exhaust may make gobs of low-end torque, they are not the most efficient
setup overall, since your engine has to work so hard to expel those exhaust
gasses. Also, a header does a pretty good job of additionally
"sucking" more exhaust from your combustion chamber, so on the next
intake stroke there's lots more fresh air to burn. Think of it this way: At
8,000 RPM, your engine is making 280 pulses per second. There's a lot more to
be gained by minimizing pumping losses as this busy time than optimizing torque
production during the slow season.
General Rules of Thumb with Headers
You will undoubtedly see a variety of headers at
your local speed shop. While you won't be able to determine the optimal power
range of the headers by eyeballing them, you'll find that in general, the best
high-revving horsepower can be had with headers utilizing larger diameter,
shorter primary tubes. Headers with smaller, longer primaries will get you
slightly better fuel economy and better street drivability. With four cylinder
engines, these are also usually of the Tri-Y design, such as the DC Sports and
Do Mufflers "Make" Horsepower?
The answer, simply, is no. The most efficient
mufflers can only employ the same scavenging effect as a header, to help
slightly overcome the loss of efficiency introduced into the system as back
pressure. "So," you ask, "what the is the best flowing muffler
I can buy?"
According to the flowbench, two of the best
flowing units you can buy are the Walker Dyno Max and the Cyclone Sonic. They
even slightly out flow the straight through designs from HKS and GReddy BL
series. Amongst the worst, are the Thrush Turbo and Flow Master mufflers.
On your typical cat-back exhaust system, you'll
see a couple of bulges in the piping that are apparently mini-mufflers out to
help the big muffler that hangs out back. These are called Helmholtz Resonators
and are very similar to glasspacks. The main difference is that firstly, there
is no sound-absorbing fiberglass or steel wool in a Resonator. And secondly,
their main method of silencing is the reflective principle, not absorption. An
easy way to tell the difference between a glasspack and a true Helmholtz
Resonator is to "ping" one with your finger. A glasspack will make a
dull thud, and a true Resonator will make a clear "ping!" sound.
Another object that might be sitting in your
exhaust flow is a turbine from a turbocharger. If that is the case, we envy you.
Not only that, but turbos introduce a bit of
backpressure to your exhaust system, thus making it a bit quieter. All of the
typical scavenging rules still apply, but with a twist. Mufflers work really
well now! Remember, one of the silencing methods is restriction, and a turbine
is just that, a restriction.
This is actually where the term "turbo
muffler" is coined. Since a turbine does a pretty good job of silencing,
OEM turbo mufflers can do a lot less restricting to quiet things down. Of
course, aftermarket manufacturers took advantage of this performance image and
branded a lot of their products with the "turbo" name in order to drum
up more business from the high performance crowd. We're sad to say that the
term "turbo" has been bastardized in this respect, and would like that
to serve as a warning. A "turbo" muffler is not necessarily a
We've seen quiet a few "experienced"
racers tell people that a bigger exhaust is a better exhaust. Hahaha... NOT.
As discussed earlier, exhaust gas is hot. And we'd
like to keep it hot throughout the exhaust system. Why? The answer is simple.
Cold air is dense air, and dense air is heavy air. We don't want our engine to
be pushing a heavy mass of exhaust gas out of the tailpipe. An extremely large
exhaust pipe will cause a slow exhaust flow, which will in turn give the gas
plenty of time to cool off en route. Overlarge piping will also allow our
exhaust pulses to achieve a higher level of entropy, which will take all of our
header tuning and throw it out the window, as pulses will not have the same
tendency to line up as they would in a smaller pipe. Coating the entire exhaust
system with an insulative material, such as header wrap or a ceramic thermal
barrier coating reduces this effect somewhat, but unless you have lots of cash
burning a hole in your pocket, is probably not worth the expense on a street
Unfortunately, we know of no accurate way to
calculate optimal exhaust pipe diameter. This is mainly due to the random nature
of an exhaust system--things like bends or kinks in the piping, temperature
fluctuations, differences in muffler design, and the lot, make selecting a pipe
diameter little more than a guessing game. For engines making 250 to 350
horsepower, the generally accepted pipe diameter is 3 to 3 ½ inches. Over that
amount, you'd be best off going to 4 inches. If you have an engine making over
400 to 500 horsepower, you'd better be happy capping off the fun with a 4 inch
exhaust. Ah, the drawbacks of horsepower. The best alternative here would
probably be to just run open exhaust!
A lot of the time, you'll hear someone talking
about how much hotter the exhaust system on a turbo car gets than a naturally
aspirated car. Well, if you are catching my drift so far, you'll know that
this is a bunch of BS. The temperature of exhaust gas is controlled by air/fuel
mixture, spark, and cam timing. Not the turbo hanging off the exhaust manifold.
When designing an exhaust system, turbocharged
engines follow the same rules as naturally aspirated engines. About the only
difference is that the turbo engine will require quite a bit less silencing.
Another thing to keep in mind is that, even though
it would be really super cool to get a 4 inch, mandrel bent exhaust system
installed under your car, keep in mind that all of that beautiful art work won't
do you a bit of good if the piping is so big that it gets punctured as you drag
it over a speed bump! A good example of this is the 3 inch, cat back system sold
by Thermal Research and Development for the Talon/Laser/Eclipse cars. The piping
is too big to follow the stock routing exactly, and instead of going up over the
rear suspension control arms, it hangs down below the mechanicals, right there
in reach of large rocks! So when designing your Ultimate Exhaust System, do be