Exhaust

Written by BAMF70

One of the first modifications made to most Mustangs is exhaust work. The two major changes that result from exhaust work are more rwhp/rwtq (more power) and sound changes.

Before we get into specifics let’s set the table a bit here and break down the stock Mustang exhaust system. Exhaust exits the heads and flows into the exhaust manifolds (also called headers). From there, the exhaust is pushed through the catalytic converters in the factory H pipe. After leaving the H pipe, the exhaust flows through the stock mufflers and finally out the tips. The pipe that connects to the H pipe and terminates with at the tips is commonly referred to as a “cat back”. All pieces of the stock exhaust system are Stainless Steel and are 2.25” in diameter.

A stock Mustang has a restrictive exhaust from the factory. Upgrading any or all of the pieces mentioned above can add RWHP/RWTQ and change the sound of your car.

Stock Mid Pipe with 4 restrictive Catalytic Converters.

Cat Back

http://www.mustangworld.com/ourpics/fcar/maci1.htm

The most common upgrade is the “cat back” portion of the exhaust. This means replacing only the muffler's back to the tips, leaving the Catalytic converters in place. There are two main routes to choose from when looking to upgrade your cat back. The first choice is a complete replacement of the stock cat back with an after market cat back. The second choice is to purchase some after market mufflers, have a shop cut out the factory mufflers, and weld in the after market ones you purchased. Let’s examine each of these options closely.

Replacing the entire cat back can be costly. Complete cat back systems can range in price from $200 to $700, dependant on the manufacturer and where purchased. The cheaper end of the pricing scale usually means the system is made from aluminized steel, where a more expensive system is typically made from stainless steel. A pipe made from stainless steel will not rust, where an aluminized pipe will eventually corrode. Many cat back manufacturers are using stainless steel mufflers in their aluminized cat backs. Replacing the factory cat back with an after market unit will usually increase the diameter of the cat back piping from 2.25” to 2.5”. The stock H pipe will bolt right up to the after market cat back with no modifications. Replacing the entire cat back is something the average Mustang owner can do in his/her driveway in an afternoon.

Replacing the just the mufflers is not as costly as replacing the entire cat back. Mufflers themselves are not as expensive as a complete cat back, but there is a labor cost with them, as you will need to pay to have the old ones cut off and the new ones welded in. Welding the mufflers in will leave the stock 2.25” piping in place. When Stainless steel is welded, the weld itself is likely to rust over time. This may eventually cause problems down the road.

Pro/Con's Replacing Cat-backs vs. Mufflers

Now that the 2 choices have been explained let’s just quickly outline the Pros and Cons of replacing the cat back vs. replacing just the mufflers.

Pros of complete replacement Cons of complete replacement

1. Lager diameter piping 1. More expensive to purchase

2. No welding required 2. More time (labor) to install

3. Additional HP (depending on the cat back) 3. May cause extra noise

4. Exhaust note changes (Rattles, vibrations etc)

5. Install can be done by owner

Pros of replacing just the mufflers Cons of replacing just the mufflers

1. Less costly to purchase than a cat back 1. Original piping is retained

2. Additional HP (depending on the muffler used) 2. Welding is required

3. Exhaust note changes 3. Not likely to cause extra noise

Mufflers

Chambered vs. Straight through Mufflers

Now that the choices for the cat back portion of the exhaust have been explained, let’s look at the differences in cat backs/mufflers closely. There are two (2) choices for mufflers, chambered or straight through. While every manufacturer builds to their own specifications, the general rule is that chambered mufflers do not free up as much HP as a straight through design.

This is the part of exhaust systems that personal preference really starts to take effect. Most people that use a chambered muffler choose to use an H pipe as their mid pipe, while most people who use a straight through muffler choose to use an X pipe as their mid pipe. This is where the sound difference really shows up. The mid-pipe selection will be covered in detail in the next section. The best advice is to listen to as many different exhaust combinations as possible before making your decision. While it is always best to listen in person, sometimes that is not possible, so it is recommended that a visit to http://www.mustangexhaust.com be made. This site has numerous sound clips of H pipe and X pipes with the many different cat backs and headers currently available for the Mustang.

For reference, here is a list of chambered and straight through manufacturers.

Chambered Straight through

Flowmaster Bassani

Hooker Borla

MAC Magnaflow

This list is not all-inclusive, but covers the majority of manufacturers.

Mid Pipe

Mid Pipe installation:

http://www.corral.net/Tech/Exhaust/magnaflow/magnaflow

After the cat back, the next piece of exhaust that is most frequently changed is the factory mid pipe. All Mustangs come from the factory with an H pipe utilizing 2.25” pipe as the mid pipe. Depending on the year of car you have, your stock mid pipe might have 4 or even 6 catalytic converters.

As mentioned above, all after market mid pipes utilize 2.5” piping and can be either aluminized steel or Stainless Steel. Like the after market cat back, an after market mid pipe will connect to the exhaust manifolds and ANY cat back without the need for any modifications. These pipes do not require welding to attach the mid pipe to the exhaust manifolds or to the cat back. Depending on the type of exhaust manifold you decide to use (stock, ¾ length or long tube) you will need to purchase the matching size H pipe or X pipe in order for everything to bolt up correctly.

Let’s start with the difference between a catted mid pipe and an off road mid pipe. Typically, a catted mid pipe is designed with 1 catalytic converter on each side of the pipe, this replaces the 2 or 3 cats found on each side of the stock H pipe. The cats that are used in after market mid pipes are what they call high flow (less restrictive than the stock cats are). These pipes are typically street legal (check your state’s pollution control device laws BEFORE purchasing any mid pipe for your car) and will allow you to pass emissions tests.

An o/r mid pipe, as the name implies, is not intended for street use, as there are no catalytic converters on the pipe. This type of pipe is illegal in all 50 states (except for track use). These pipes will free up the most possible HP because there are no restrictions (cats) in the pipe. These pipes will also cause even a stock car to become considerably louder. The sound tends to be hollow, and you will typically smell unburned fuel in the exhaust gas much more so than a pipe that has catalytic converters.

H Pipe vs. X Pipe

X Pipe Installation:

http://www.mustangworld.com/ourpics/fcar/hpip1.htm

Now let’s discuss the 2 main designs for mid pipes, H and X type. An H pipe gets its name because there is a piece of pipe that connects each side of the pipe that connects to the header collectors (hence the name H pipe). This pipe is supposed to help equalize the pressure and thereby create a better scavenging effect. An X pipe however, has each side of the pipe meet in an X type junction. Again, the idea is to equalize pressure and create better scavenging. As a general rule of thumb, the H pipes allow for more low-end torque and slightly less HP than X pipes do. In addition, the sound note is quite different. The X pipe has a “NASCAR” sound while the H pipe is more “muscle car” sounding. As mentioned in the cat back section, the mid-pipe choice seems to be a personal sound preference and most people who use chambered mufflers opt for the H pipe. Where as the straight through users tend to choose the X pipe. Typically, the X pipe/straight through muffler combination yields higher HP and less torque than the H pipe/chambered muffler design.

Headers

Header installation:

http://www.fnsweet.com/garage/lt_install/index.shtml

Headers are the final piece of the exhaust puzzle. The stock exhaust manifolds on a 4.6 GT are much better than the exhaust manifolds that came stock on the older 5.0 GT, but there is still plenty of room left for improvement. There are 3 choices when considering after market headers, stock replacement (Shortys), mid length and long tube (LTs). Each of these headers requires a certain length mid pipe be used so that the exhaust can be bolted together correctly. Let’s examine the different headers in more depth. After market headers can be coated (Jet Hot and ceramic are the most common) or non-coated. The coating helps to protect the headers from rusting and the helps to keep most of the heat in the header instead of allowing it to escape into the engine compartment. The degree to which the heat stays in the header is dependent on which coating is used.

Let’s start with stock replacement (shortys). These headers are a direct replacement for the stock manifolds. Typically they are better flowing and therefore help to free up more HP than the stock manifolds and allow the engine to produce more torque. These headers are usually the least expensive to purchase and take the least time to install. With these headers, there is no need to purchase a different mid pipe, as the current mid pipe will bolt directly to them. Installation typically runs approximately 6 hours for these headers. With the correct tools, these headers can be installed by the average Mustang owner in their garage/driveway.

The next type of header is mid length. These headers are a mix between shortys and long tubes. The only company that manufactures these headers is Bassani. The install time takes around 6 hours. Mid length headers either need Bassani’s matching mid pipe or have a standard mid pipe cut and welded to fit by a muffler shop. Mid length headers produce similar (but not exact) HP and TQ numbers as long tubes while allowing ground clearance with a lowered vehicle.

The last choice for headers is long tubes (LTs). These headers typically flow much better than any other header available and free the most HP possible. They also allow for significantly more torque to be produced by the engine. These headers are slightly more expensive than shortys to purchase and take approximately 8 hours to install. When LTs are used, a special “shorty” mid pipe is required to bolt the exhaust together correctly. This increases the initial cost as the mid pipe must be installed at the same time as the LTs. While it is possible for the average Mustang owner to install LTs in their garage/driveway it is significantly more difficult to do. For this reason it is recommended that they be installed by an experienced shop.