Have you ever wondered why some hot rods are loud? You may not even know what a hot rod is but an example of a popular hot rod today is the older 302 Cubic Inch 5.0 liter mustang like I have. They are cheap to build and have readily available parts. Typically motors in hot rods are modified to attain increased horsepower thus increasing acceleration capabilities and increase efficiency. One problem with do it yourself modifications is the fact that high horsepower engines are generally difficult to muffle. Have you ever noticed just how loud a semi truck is? They are terribly loud! Why? Consider the sheer size of the cylinders in their enormous engine blocks with an extra huge super charger or turbo attached to it. Yet somehow their true sound is muffled through the science of mufflers. The same is true for a hot rod. It is true that mufflers do cut down on horsepower but it has become a science in recent years to have a highly modified engine and yet have a tolerable db level in both the interior and exterior. For some hot rodders their budget is slim and therefore they have to do the best they can with what they have. Below is a basic explanation to the science behind the exhaust system. You can read more at Mikekemper.com
“First, we’ll explain how the different parts of an exhaust work so you can
choose the best pieces, then show you how you can build a high-performance
system with perhaps some help from your local muffler shop or a friendly welder.
No matter how small a town you live in, you should still be able to get this
stuff done.
THE MUFFLER
The key part of your exhaust
system is the muffler. The muffler is the can at the end of your exhaust whose
main purpose in life is to make the exhaust noise quiet. To be the whisper-quiet
device most car owners demand, a typical stock muffler must have an intricate,
labyrinthine flow path to help slow and cool the hot, vibrating exhaust gas. It
contains baffles that cause the exhaust flow to reverse direction and intermix.
These are great for reducing noise but are not so great for flow. The twists and
turns the exhaust must endure in a stock muffler are restrictions that cause
excess backpressure. You can run in a straight line faster than you can run in a
tight, fun-house maze, right? The same goes for your exhaust gas.
To produce the most power, an exhaust should have minimal restriction on the
exhaust flow. Restriction hampers the burned exhaust gases from exiting your
engine, causing some charge dilution with the incoming fresh fuel air mixture.
If all the exhaust gas cannot escape from your cylinders, it dilutes the
flammable power-producing intake mixture that is trying to come in. The diluted
mixture does not burn as well as a pure mixture. This causes a loss of power.
You don’t feel so energetic at a packed club with lots of cigarette smoke,
sweaty bodies and hot stuffy air right? Neither does your engine.
With greater restriction, backpressure is generated, making the engine work
harder to pump the exhaust out of the cylinders. That work could be used to turn
the wheels instead.
BACKPRESSURE = TORQUE?
An old
hot-rodder’s tall tale: Engines need some backpressure to work properly and make
torque. That is not true. What engines need is low backpressure, but high
exhaust stream velocity. A fast-moving but free-flowing gas column in the
exhaust helps create a rarefaction or a negative pressure wave behind the
exhaust valve as it opens. This vacuum helps scavenge the cylinder of exhaust
gas faster and more thoroughly with less pumping losses. An exhaust pipe that is
too big in diameter has low backpressure but lower velocity. The low velocity
reduces the effectiveness of this scavenging effect, which has the greatest
impact on low-end torque.
Low backpressure and high exhaust stream velocity can be achieved by running
straight-through free-flowing mufflers and small pipe diameters. The only two
exceptions to this are turbocharged engines and engines optimized for large
amounts of nitrous oxide. Both of these devices vastly increase the exhaust gas
volume and simply need larger pipes to get rid of it all.
Some stock mufflers and exhaust systems have up to 18psi of choking,
power-robbing backpressure. In direct contrast, a well-designed,
high-performance street exhaust system typically has about 2 to 6 psi of
backpressure. For an interesting comparison, an un-muffled straight pipe on a
real racecar usually has 1 to 3 psi of backpressure.
To get the least amount of backpressure, most of the good, high-performance
mufflers available today have what is called a straight-through design. These
mufflers quiet the exhaust by absorbing high-frequency vibrations in
heat-resistant packing, usually consisting of stainless-steel mesh and
heat-resistant ceramic fibers.
They typically have an inner core that is straight-through with no baffling
at all, much like a straight pipe with many small holes in it. The pipe is
louvered or perforated when it passes inside the muffler’s shell, allowing sound
energy to pass through the holes but leaving the exhaust gas flow unimpeded. You
can see straight through these types of mufflers. The louvered or perforated
core is usually wrapped with either fiberglass wadding (hence the old-school
term, Glass Pack) or, in the better mufflers, stainless-steel mesh backed by
ceramic fiber to help further absorb the sound.
On straight-through mufflers, the longer the muffler and the bigger the can,
the quieter it is. The length usually has no effect on backpressure, just noise
output. These absorption type mufflers work in the same manner as the silencers
used on guns. If a silencer had baffles that impeded bullet travel, you would
definitely have problems! The same is for a perforated core absorption muffler,
straight through, no baffles, no restriction, and no backpressure.
It is best to avoid straight-through mufflers that have a louvered core. Many
old-school glass packs suffer from this design. Some spiffy polished stainless
and big tip mufflers on the market also have these. The louvers generate quite a
bit of backpressure because they stick into the exhaust stream and create
considerable turbulence. Even though these mufflers are a straight-through
design, they can have more backpressure than a stock muffler.
MagnaFlow has a line of universal high performance mufflers
in many different shapes, diameters and lengths. The muffler and pre-silencer we
got from MagnaFlow are made from high-quality polished 304 stainless and feature
a perforated core wrapped with stainless mesh and ceramic wool. Note how the
perforated core is straight with no obstructions to the flow.
When buying a straight-through muffler, look for one with a perforated core
if you are interested in producing more power. A good, properly sized,
perforated-core straight-through muffler will add only about 1 to 2 psi of
backpressure to your exhaust system. Mufflers like the Walker Ultra Flow,
Thermal, A’pexi, Borla, Edelbrock or MagnaFlow are examples of good,
low-backpressure mufflers with an absorption design. Many Pre-made exhausts like
A’pexi, Tanabe, Greddy, Borla, MagnaFlow, Thermal or HKS also have mufflers of
the free-flowing absorption design.
An old-school type performance muffler, which is still very common in speed
shops, that has seen better days is the Turbo Muffler. This is a less
restrictive version of a stock-like reverse flow muffler. In the old days, these
were well flowing mufflers, but now the new-jack perforated core, straight
through absorption types, has superseded them, Many old-school domestic shops
will try to sell you one of these as a hot set-up, but they should be avoided,
just like the louvered core glass-pack.
A disadvantage to the straight-through muffler is that it is often louder
than a reverse-flow muffler. Usually a straight-through muffler needs a small
sub muffler or resonator to keep the exhaust quiet. A resonator is usually a
small, perforated core glass pack placed somewhere in-between the catalytic
converter and the main muffler. Like the main muffler, the longer the resonator,
the better a sub muffler will be for noise reduction. A Walker Magnum Glass-Pack
is a good muffler to use as a resonator. Almost all of the pre-made performance
exhausts feature resonators.
Some good performance mufflers are only available with a semi-universal
2.5-inch inner diameter. If you have a smaller engine that requires a 2-inch
pipe, it is still OK to use a main muffler with a slightly bigger inside
diameter. This larger step up in diameter at the very end of the exhaust system
won’t hurt performance and sometimes can help it slightly.
When designing your own custom exhaust, it is important to remember to make
it as quiet as possible. Loud might be cool to you, but remember that a too loud
exhaust is perhaps the number one harassment ticket given to performance
enthusiasts by your friendly law enforcement officer. Don’t ask how we know
this.”
So you can see that this is more of a science that you probably have imagined. The last thing that hot rodders want to do is to attract unnecessary attention to their cars from the local police. The goal is to balance out the desire to have the extra ponies under the hood and have an enjoyable ride that can facilitate conversation without raising your voice. This preference is going to vary from hot rodder to hot rodder but local government will give you the local MAXIMUM db allowed. For me, my exhaust was already picked out and installed on my car when I bought it. Thankfully, it is a good system that harnesses my engines true sound via an H-pipe and two flowmaster mufflers. My desire was to have a good enough system that did not have to be un-bolted when I arrived at the local dragstrip. Seeing that I drive my mustang to and from that racetrack I wanted a car that used its everyday exhaust and yet this exhaust would provide the best horsepower for my budget. Seeing that my budget was zero then this system has to work. Are there less restrictive and quieter systems? Certainly so but with a price. However, my system is only slightly louder than my personal preference and I can easily carry on normal conversation while cruising and have never attracted the local authorities with its db’s. I suppose what makes hot rodding so enjoyable is the science of the entire hobby. For example what heads, cam, carb, intake, pistons and headers will produce 400hp in a small 302 c.i. motor like mine. This was a lot of fun doing the research on cylinder head flow when planning my engine build. Here is another link that discusses the exact science of muffling high performance engines. So the next time you think that all hot rodders are simply attaching loud mufflers to their exhaust just to be heard and thus fulfilling some desire to be seen, try considering that there could possibly be a scientific experiment taking place before your eyes!
