Performance Exhaust System Basics
Torque is determined by header design and all of the power band tuning is
done between the head and the final collector, where the last of the primary
or secondary pipes is joined to a single exhaust pipe.
4-1 headers typically loose a lot of low end and mid range torque by their design.
The 4-1 design will show a single peak in power improvement with a sharp decrease
in power both above and below the single peak, when compared to the stock power
level. Most header makers using this design will tune for peak horsepower at
or above the engine's maximum speed, loosing power throught he lower half of the
power curve for a very small gain at maximum engine speed. However, the exact
point in the RPM range at which that peak occurs is determined by the diameter
and length of the primary pipes, and some makers will use longer and smaller
diameter primary pipes which will result in a small peak in power in the mid to
low RPM range with power loss at both very low RPM and across the upper half of
the range of engine opperating speed.
Tri-Y headers produce even power across the entire RPM range with very
noticeable improvement in mid and low range torque. This is because the Tri-Y
design produces two seperate peaks in power gain which taper off more gradually
below the first peak and above the second peak, most often not showing any
dip below the stock power level. Where those peaks occur in the range of engine
speed is determined by the diameter and length of both the primary pipes and also
the secondary pipes. Most makers of headers with this design will
tune for the first peak in power in the lower half of the engine's opperating speed
and the second peak in the upper half of the engine's opperating speed, for an even
and consistent improvement in power across the entire range of engine speed which
will in all cases out perform the 4-1 design for both street and race use.
The main reason that 4-1 headers are so common is the fact that they are less
difficult and less expensive to manufacture and, therefore, provide greater
profit margins for the manufacturer and the retailer.
Pipe Diameter and Bend Type
For non-turbo engines under 2.4 liter in displacement and turbocharged
engines under 2.2 liter displacement and producing less than 400 HP, 2
1/2 inch mandrel bent exhaust pipe size is optimum for high performance.
2 1/4 inch pipe can limit air flow once the engine buildup gets serious
and the owner has bolted on more than header, intake, and pulley.
Many people will claim that 2 1/2 looses torque, but this is not true.
As discussed earlier, all of the tuning for torque is done between the head
and the final collector, in the header. After the final collector, the only
thing determined by pipe diameter is how much work the engine has to do to
push the exhaust gas to the back end of the car and out the tail pipe, and
the less work the engine has to do, the more power it can produce.
The people advocating 2 1/4 inch diameter exhaust systems and complaining
about low end power loss are also using 4-1 headers. This isn't unique to
Storms, it is a serious misconception among the Cavalier/Sunfire crowd as
well. These people are compensating for poor header design by strangling
down the exhaust pipe diameter at the expense of mid and upper RPM engine
All of this has been specific to mandrel bent pipe. Squeeze bent pipe
looses 1/3 of its interior cross sectional area in a right angle bend.
There are three right angle bends in the exhaust system of both the Storm
and the I-Mark.
Mandrel pipe bending keeps a consistent inside diameter of the pipe through
right angle and even U bends.
2 1/2 inch diameter squeeze bent pipe will flow the equivalent of 1.6 inch
diameter pipe at each of those squeeze bends. The entire exhaust system will
flow only as much as the tightest constriction in flow, meaning that all of
the 2 1/2 inch pipe has been a waste, the total system will flow the same as
a 1.6 inch diameter mandrel bent exhaust system.
To build a 2 1/2 inch diameter system using squeeze bent pipe, every bend
would have to be made using 3 1/2 inch diameter pipe to compensate for the
inside space lost from the squeeze bend. This means that the straight
sections of pipe would be 2 1/2 inch pipe, then expanded to 3 1/2 inch bends
for each and every curve, and then turned back down to 2 1/2 inch pipe for
the straight pipe until the next bend.
For maximum flow, a straight through muffler works best.
Conventional reflection mufflers use U bends within the muffler to reflect
the flow and sound back and forth in a S curve through the body of the
muffler. This is the most restrictive muffler design, but provide the most
sound volume reduction.
Chambered mufflers have angled walls within the muffler body to reflect the
sound and flow from side to side. These are less restrictive, but not optimum
and provide less sound volume reduction than the conventional reflection
muffler type. It is easy to see the restrictions within these mufflers as
there will be areas of heat discoloration on the side of the muffler anywhere
there is a restriction in flow. This is usually at the first V wall separating
flow and the first reflection back toward the middle of the muffler.
Straight through mufflers offer the highest flow, because they have no
restriction or impediment to flow through the muffler, but they offer the
least reduction in sound volume. These are usually made up of a perforated
core, wrapped with steel cloth, and packed with a sound absorbent material.
The idea is that the sound is absorbed and deadened by passing through the
perforations into the deadening material. Avoid designs with louvers, they
stick out into the air flow and inhibit flow through the muffler. One
important advantage of the straight through muffler is that they produce
little or no back pressure to reduce performance and, because of this, two
of this type muffler can be used in series to reduce sound volume more without
sacrificing any measurable amount of performance. This is discussed further
under the heading of Resonators.
Resonators are designed to reduce harshness and volume of the exhaust sound
prior to the muffler. These are typically an additional muffler which is
placed directly behind the catalytic converter. For maximum flow and performance,
these should be eliminated. In most cases, resonators are the same restrictive
design as conventional reflection type mufflers. From a performance perspective,
they serve one purpose, to cause back pressure and decrease power output.
The drawback to this is increased exhaust sound volume. As described earlier,
straight through mufflers cause little or no back pressure and adding an
additional straight through muffler to an exhaust system will not decrease
performance measurably. However, adding a straight through muffler will help
reduce exhaust sound volume. This is a very effective use for a straight
through muffler without sacrificing performance.
Hollowing out a catalytic converter or replacing it with a test pipe or
straight section of pipe is a violation of Federal Law punishable by fines
in excess of $25,000 and/or jail time for both the vehicle owner and the
person doing the work.
Removing a catalytic converter will result, at most, in 5 HP in power
improvement. However, hollowing out a catalytic converter will cause
resonance problems. The symptom is most often seen as a hesitation,
popping, or backfire at or about 3,200-3,500 RPM. Upon hard acceleration,
the engine will appear to cut out for a split second as RPMs drop and a
split second later recover and continue past 3,200-3,500 RPM. Additional
symptoms are a loud and continual popping sound when the vehicle is
allowed to decelerate under load and a whooshing sound that is delayed
from or following the engine speed. This is due to the open chamber
within the hollowed out catalytic converter case. It loads up and
releases pressure in the exhaust system that acts almost as a capacitor.
The open chamber allows the formation of standing waves within the
chamber which reflects back toward the engine and causes the hesitation
and miss in the RPM range.
The symptoms become more pronounced as the exhaust system is made freer
flowing, meaning that hollowing out a cat with the stock exhaust may cause
mild annoyance, while it becomes a serious problem when a header and
performance exhaust system are installed, mimicking a ignition miss or
backfire. This is probably the best reason not to hollow out a catalytic
If a vehicle is to be used off of the road and a catalytic converter is
not required, the correct and proper way to handle this is to install a
test pipe or straight section of pipe in the place of the catalytic
converter. This will eliminate all of the problems associated with the
open chamber which causes standing waves and hesitation problems.
For street driven vehicles, a high flow catalytic converter that is
correctly sized to match the header and exhaust system is the way to
get the most performance from the vehicle while staying within the
requirements of the law. Once a header and performance exhaust system
are installed, the OEM catalytic converter becomes the point of most
restriction in the exhaust system. OEM catalytic converters typically
have inlet and outlet sizes of 1 3/4 - 2 inches in diameter at most.
Aftermarket headers and exhaust systems are normally 2 1/2 inches in
diameter. Again, the entire system will only flow as well as the tightest
restriction, and regardless of the flow capacity of the header and exhaust
system, a two inch catalytic converter will limit exhaust flow to what will
pass through a two inch diameter hole going into and out of the catalytic
converter. To eliminate this restriction in flow, install a catalytic
converter that is sized to match the header and exhaust system, a 2 1/2
inch diameter catalytic converter for a header with a 2 1/2 inch diameter
collector and a cat back exhaust system with a 2 1/2 inch diameter pipe.
This will allow the header and exhaust system to perform up to their full
Un-resonated exhaust tips do actually serve a purpose other than adornment
of the back end of the car. Enlarging the pipe diameter at the tail pipe
as with the use of a meg style tip reduces the flow speed of the exhaust
gasses and all but eliminates the high pitched sound of the exhaust gas
passing down the pipe. In addition, the larger diameter of the tip shifts
the sound to a longer wave length which results in a lower tone exhaust sound.
This helps to achieve the deeper exhaust note that is more sought after by
North American auto enthusiasts.
Dual exhaust on a four cylinder car, as in one pipe joining cylinder 1 and
4 and another joining cylinder 2 and 3, and then travelling side by side to
the back of the car, passing through separate mufflers, and then ending in
separate tips, is going to cause power loss. To cause an extraction affect,
the secondaries of the 1,4 and 2,3 pipes must join. Not doing this will
cause power loss by lack of extraction at that point.
Two tail pipe exhaust is a waste. It adds only weight. There is no performance
gain from having exhaust pass from one correctly sized pipe into two separate
pipes. In fact, the turbulence of separating the exhaust flow by forking it
into two pipes will cause back pressure and will likely reduce power. In
addition, the added weight of a second pipe that serves no purpose to enhance
power will do nothing but add more metal for the car to drag around. No power
gain and more weight is a waste.
If someone wants to do this for looks and appearance, that is fine. It does
look nice to have two tail pipes coming out, one on each side of the license
plate. However, don't be fooled into thinking that this will enhance performance
and just accept that you are adding weight to the car solely for appearance
Performance Mufflers On OEM Pipe
Several companies sell performance bolt on mufflers for mainstream model vehicles.
These bolt onto the back section of the OEM exhaust system. In addition, many
shops are glad to offer to install a performance muffler onto a OEM or OEM
replacement exhaust system. Installing a performance muffler onto OEM or OEM
replacement exhaust pipe will not improve engine performance noticably and will
at best simply increase exhaust noise. The problem is that the small OEM size
pipe and its squeeze bent construction restrict exhaust flow to the point that
a freer flowing muffler will make no measurable reduction in back pressure.
The muffler itself is only part of the problem, the pipe itself is most of the
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