Stock SRT-4 Exhaust System
The first part of the exhaust system is the exhaust manifold/turbo assembly (in grey). The SRT-4 exhaust manifold is unique in that the turbine housing for the turbo is cast into the exhaust manifold. In most turbo cars the turbine housing is bolted to the turbo itself, and that turbo assembly is then bolted to the exhaust manifold. Casting the turbine housing in with the exhaust manifold (as done on the SRT) simplifies manfacture/assy which helps reduce cost. It also makes for a smaller "package" so the turbo/manifold can fit in a smaller area, and allows for quicker light off of the catalytic converter which aids in reducing emissions.
The next component is the O2 housing (in blue). The O2 housing is connected to the exhaust manifold with 4 studs/nuts and or bolts, directs flow from the exhaust manifold to the down pipe/catalytic converter assembly and houses the upstream O2 sensor. Connected to the O2 housing is a black "Y" bracket that braces the O2 housing to the engine block. The "Y" bracket transfers exhaust system movement/stresses to the engine block.
The upstream O2 sensor (on the passenger side of the O2 housing) tells the engines computer what the air/fuel mixture is coming out of the engine. Based on this info (and other info from other sensors) the computer alters fuel delivery to deliver anything from best fuel economy mixture to maximum safe power.
The next component is the flex/down pipe/catalytic converter assembly (in yellow). The flex/dp/cat assembly connects to the O2 housing with 3 studs/nuts and 1 bolt with a captive nut on the fwd side. Inside the converter are two bricks of honeycomb substrate coated with chemicals that when heated, react with the exhaust gasses and reduce the harmful emissions coming out of the engine.
On the drivers side of the cat is the downstream O2 sensor. The engines computer uses the signal from the downstream O2 sensor to monitor the emissions output of the cat. If the cat is removed, the downstream sensor readings change, and the computer will illuminate the "check engine light" or CEL. This will only happen on cars not equipped with Stage 1,2 or 3 upgrades. The CEL will not illuminate on cars equipped with Stage 1,2 or 3 if the cat is removed.
The rest of the exhaust system would be considered the "cat back" part of the exhaust (in green), and is connected to the dp/cat assembly with a slip joint. The first part of the cat back has two resonators which help tune the sound of the exhaust. The tubing on the stock cat back is 2.25 inches in diameter.
Popular SRT-4 Exhaust Modifications
One of the most popular exhaust modifications is to replace the cat back section of the exhaust with an aftermarket cat back. HP gains at the wheels of 5-10 hp are often seen depending on the system, other modifications on the car, and how it is driven. The main improvement in aftermarket cat backs is lager diameter tubing that is less restrictive to exhaust flow. Most cat back exhaust systems can be installed with simple hand tools, are a great first modification.
Another modification is to replace the dp/cat with an aftermarket down pipe with or without cat. Aftermarket dps without cats can show a 5-7 whp gain over the stock dp when connected to the stock cat back, and 10-15 hp when combined with a quality cat back (DP + cat back = 10-15 hp). Keep in mind it is illegal to remove the cat from your exhaust system and most dps are sold as a "for off road use only" application (although this doesn't seem to bother most people).
A variation of dp is the dp with a "cutout". A cutout is a section if pipe welded onto the down pipe at an angle. At the end of the pipe is a block off plate. When more performance is desired (at the drag strip for example), the block off plate is removed. This allows the exhaust to exit at a less restrictive point, freeing up the exhaust flow and creating more power. Electric cutouts are avail (they replace the block off plate) that allow you to open/close the opening with a switch. Electric cutouts can leak (usually just a little bit). Keep this in mind when choosing this modification for your car.
A "turbo back" exhaust system would be one that replaces all components form the O2 housing back (everything in yellow and green in the above diagram). These systems are typically good for 7-15 hp depending on the system, other mods on the car, and how the car is driven. Exhaust tubing diameters of 3 inch are common, and will support the 400+ hp levels of aftermarket turbo kits. Tubing diameters of 2.5 inch are also seen, and will support up to 320 hp. After that, the 3 inch systems will show small gains at first, then bigger gains as hp climbs. Dyno tests have shown that at the 270 hp level, 2.5 inch and 3 inch exhaust systems make the same hp and torque.
One more popular modification is to replace the stock O2 housing (in blue) with an aftermarket 3 inch diameter O2 housing. This can result in a 5-7 whp gain at stock boost levels and 7-10 whp at 18-19 psi. This modification is not for the faint of heart as the location of the O2 housing makes it time consuming to replace. Replacing the O2 housing and installing a turbo back exhaust has shown a 25 hp and 25 tq gain over the stock exhaust system
Noise, Tone, Resonance Changes by Adding an Aftermarket Exhaust Component
Adding a dp /or cat back usually magnify the existing exhaust characteristics without making the car too much louder. This may be a great choice if you want a little more hp and aggressive sound slightly louder than stock.
A dp with cutout when opened up is extremely loud and is the loudest of all the modifications. When closed they are are more quiet becuase the exhaust is traveling through the rest of the the exhaust system.
Single tip systems are typically the quietest of the turbo backs and have the least amount of resonance in the cabin. Dual tip systems are typically the loudest (unless equipped with a muffler), and have increased droning inside the cabin of the car. Either type of system when equipped with a muffler usually offers a deeper, smoother tone and reduced droning depending on the muffler used.
Dump style systems (an exhaust that dumps the exhaust toward the ground under the car) typically make the most hp, but have problems with droning inside the car. The sound waves exit the exhaust, hit the ground, and then bounce back up into the underside of the car causing the drone. resonators can be used to help reduce the drone, but it is still a significant characteristic of this style of system.
Side exit exhaust systems usually point the exhaust out from the side of the car, usually just in front of the rear driver’s side wheel. These systems offer great exhaust flow and almost no droning in the car. They can be loud (due to the lack of tubing and or resonators), but can be toned down using a quality resonator. These systems can be deceptive because while they typically sound quiet in the car, they can be significantly louder than stock outside the car, especially at wide open throttle. Driven normally though, sound levels of a resonator equipped system are usually not an issue.