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Discussion Starter #1 (Edited)
The SRT-4 exhausts system unique. For this reason we decided to add a thread to help owners understand what components make up the exhaust system, and the effects of replacing those components with aftermarket components.


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.
 

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ye real nice good job now i have a problem if any1 can help

well here is what i got i have a 3 inch bended pipe welded on to my stock exsuast but i have been undecided my cousin works for magna flow and he says he can get a 2.5 catback for 300 installed but i dont like the idea of 3 inch to a 2.5 inch so i have thought about 3 inch side pipe since i get the piping so cheap from my cousin or have some one make it or i can buy a 3 inch cat back but now im leaning towards the magno flow 2.5 and geting a electrical cut out valve and welding that all on to my down pipe
what do u people think is the best idea

im also wondering if its possible to do a 4 inch people do 4 inch down pipes on sti`s well atleast thats what i heard just wondering is its possible to do one on a srt 4 and if its possible to do a side pipe or cat back

( YOU KNOW THE BIGGER THE BETTER !!! ) lol
 

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Discussion Starter #4
On the SRT with a stock turbo, 4 inch will not show any gains over a 3 inch .....it's simply not needed.

My personal opinion on cutouts is somewhat jaded. I've tested 3 different brands of electric cutouts....and all of them leaked. Maybe not right away, and maybe not a lot, but they still leaked. As a fabricator, didn't feel comfortable offering a product that has potential leakage issues...especially when the leakage is right under the driver.

Also, the environment e-cutouts operate in is harsh. Harsh environments and moving/electrical parts isn't the best of situations. There have been several failures of the cutouts reported here on the forums. Granted, the cutouts were normally covered under warrenty, but the point is to have a part that doesn't fail in the first place. Nothing like being at the strip 40 mi from the house and have the cutout fail wide open.......then have to drive it home that way...

My suggestion is to buy/build a side exit exhaust. It will give you as much hp as a dp with cutout (or damn close to it) and get the exhaust out from under the car. Just be sure to put a resonator in the system to make the noise levels livable. It'll be simple,reliable and inexpensive. Can't ask for much more.

The saying "bigger is better" is only true to a point. You have to look at the perameters your car will be running under, and pick a system that will operate the best under those conditions at the price you can afford.

I've seen a 3 inch dp+ modified Mopar cat back combo make the same hp as a full 3 inch systems at 270 hp (stage 2 hp levels). Now, the 3inch dp+modified Mopar system is cheaper than most 3 inch systems, which allows the customer to buy a 3 inch O2 housing. The 3 inch O2 housing+3inch dp+modified Mopar cat back combo will now outflow the 3 inch system...in other words....you get more hp per dollar from the 3 inch O2 housing+3 inch dp+modified Mopar exhaust.

Real life comparison:

3 inch O2 housing+3 inch dp+modified Mopar exhaust made 24 hp more than stock with stock boost levels.....cost $715 (MAXX Fab web site)

3 in dual tip turbo backs typically make anywhere from 10-15 hp and cost the same ($700 from the Modern web site)

Also take into consideration the MAXX Fab dp/modified mopar system has never had a failure (broken hanger/tubing) wheres the Modern/HP systems have.

Now where size does matter....typically above 320-360 hp. A 3 inch system will start to show small gains over a 2.5 inch system at those hp levels. So if your going big turbo later on, stick with a 3 inch system.

Bottom line....pick a system that will help you reach your ultimate goal for the car. You'll spend less money and be happier in the long run.

Feel free to emali me with any questions you mught have.
 

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well if i wanted to have a side exhaust custom made at a shop, what exactly would i have to tell the person, so that its similiar to the maxxfab. what diameter as well? 3"/2.5"??
 

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For a newb... the #13 post on this thread is also useful... http://www.srtforums.com/forums/showthread.php?t=105941

It shows the inlet and outlet of the stock O2 housing, which was my only question after reading the write-up.. All other questions in my head were addressed!

Here's a picture of the inlet side of the O2 housing:


The largest part of the opening is 2.75 inches, the narrowest is about 2 inches. It's ported to match the opening in the exhaust manifold. The black line you see around the opening is 3 inches in diameter (as a reference).

Here's a pic of the exit side:


The opening is only about 2 1/8 inches in diameter. The black line you see is 3 inch in diameter (as a reference).[
 

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whats the deal with the whole no cat thing?

i live in Ohio, do you know if you can pass an echeck without a cat?

i have a friend that has no cat and he said his cpu is staged so no check engine lights show, but idk...

thanks...
 

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Discussion Starter #9
If you get a ddp without a cat, and have a stock pcm, you may or may not get cel. If you do get one, you can do the "anti fouler mod" which should eliminate the cel due to not having a cat.

If you have stage 1,2 or 3, you won't get a cel as the pcm ignores the downstream O2 sensor reading as far as exhaust readings. The pcm will look to make sure the downstrem sensor is connected however.
 
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