Header Design

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 performance.

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.

Muffler Types

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

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.

Catalytic Converters

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 converter.

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 potential.

Exhaust Tips

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

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 purposes.

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 problem.