What is an EcoBoost Mustang's Downpipe and What Does it Do?

A downpipe is the section of your exhaust that runs from your exhaust manifold to your cat-back. Exclusive to turbocharger cars, downpipes are equivalent to the H-pipe or X-pipe found on GT and '11+ V6 Mustangs, or the Y pipe found on older V6 Mustangs. Downpipes often house a catalytic converter and can be restrictive on stock or modified cars. Because the downpipe is connected directly to the turbo, replacing the factory downpipe with a higher flowing version can result in a dramatic increase in power. They come in two different styles, off-road or catted. The off-road downpipes provide better flow, but will not pass emissions testing.

As you modify your EcoBoost Mustang, you will surely be ramping up your boost. With the increased boost, you’ll have even more exhaust gasses fighting to get out. Even with a completely stock engine, the factory downpipe is restrictive and will rob you of horsepower. Deleting the catalytic converter or replacing it with high flow converter will result in more horsepower over the factory catalytic converter, allowing your modified Mustang to breathe a lot better.

Since the turbocharger is essentially a pump, it is important the pipes entering and exiting the turbocharger are free from restriction. The stock downpipe usually has many bends which creates pressure within the system. By reducing the amount of restriction, the turbocharger can spool up more quickly. A faster spool-up time will increase efficiency in power and fuel economy.

If you have ever installed an off-road downpipe or mid-pipe before, you know that they drastically increase the volume of your Mustang. Without the catalytic converters on the car, you will also notice the smell of raw fuel. Exhaust restriction is reduced even further by using larger diameter piping and mandrel bending. Mandrel bending is a process that prevents crushing of the tube being bent. This means the tube maintains a consistent diameter, even throughout the bends. Consistent tube diameter keeps exhaust flowing at a consistent rate, instead of slowing down, speeding up, or experiencing turbulence due to imperfects in a bend. If you do not mind having a really loud Mustang and excess fuel smell, an off-road downpipe will provide the biggest gains. Keep in mind off-road downpipes are for those who live in states without emissions testing or are planning on building a car that will not be driven on the streets.

For those of you that live in states with emissions testing, you will want to install a larger diameter, catted downpipe. Catted downpipes include a high-flow catalytic converter that flows better than stock while keeping your emissions in check. Not only do these pipes keep the raw fuel smell to a minimum, but the converter can also act as a muffler, taming your exhaust if you live in a quiet area. This will allow you to pick up most of the gains of an off-road downpipe, while still keeping your Mustang emissions compliant. In states with California emissions standards, aftermarket catted downpipes may not be legal. You will need to check with your local testing agency before ordering your downpipe if you need a downpipe that is California compliant. Even if you do not live in an emissions restrictive state, a catted downpipe is perfect for Mustang owners who do not want the increased exhaust volume and fuel smell associated with an off-road downpipe.

Installing an off-road downpipe on your Mustang will give you a gain of around 20-25 rear wheel horsepower if you already have a custom tune. You will gain even more power on a stock Mustang if you load a custom tune while installing your downpipe. For those of you that choose to go the catted route, you can expect to see about 5 RWHP less than an off-road downpipe, since the high flow catalytic converter will still restrict your exhaust a little bit.

Catted Downpipe:

• Live in an emissions restrictive state
• Want to keep your exhaust volume quiet
• Do not want excess fuel smell
• Do not want to risk getting a ticket

Off-road Downpipe:

• Do not live in an emissions restrictive state
• Want a loud exhaust
• Do not mind excess fuel smell
• Are not worried about getting a ticket
• Looking for maximum power gains

Choosing the right downpipe can be a little tricky and it’s best to examine all your choices before making a decision. Exhaust diameter plays a huge in role in how your Mustang will make power, so you will have to take your current horsepower and end goal into consideration. A downpipe with a diameter that is too narrow can act as a huge choke point and will restrict flow on a highly modified car. A mostly stock or lightly modified Mustang can suffer reduced power output from a downpipe that is too large. A downpipe that is oversized on a car can reduce exhaust gas velocity, scavenging, and increase turbo spool times. 

Another factor when considering a downpipe is if you’d like to tune your Mustang or not. Off-road downpipes will require tuning in order to prevent check engine lights or poor performance. Vehicles with catted downpipes can also benefit from tuning that optimizes engine and computer performance for the catted downpipe; however, tuning is not required with most catted downpipes.

When and Why Should I Upgrade My Downpipe?

Changing downpipes can affect the passing of inspection/emissions in certain states. If your state tests for catalytic converters, swapping to an off-road downpipe can be cause for failure to pass inspection. On the other hand, aftermarket catted downpipes can still pass provided one is not in California. In California, changing the downpipe or modifying in any way is prohibited and will not pass inspection.