Downpipes vs Headers
Most enthusiasts know that increasing exhaust flow can help increase horsepower. This is typically why exhaust modifications are so popular. An upgraded downpipe or header can accomplish increased flow but, which upgrade provides the most power gains? Let’s talk downpipes vs headers.
We’ve previously touched on the different components of the exhaust system with our “Cat-Back, Turboback, Axle-Back: A Comprehensive Guide” post. One part of the exhaust system not covered was the header. What is a header and what is its purpose?
A header is the portion of the exhaust system that attaches directly to the head or heads. Its job is to evacuate exhaust gasses exiting the exhaust valve from the cylinder head and into the exhaust system.
On engine configurations with two cylinder heads, like a wide V (LS v8 engines) or horizontally opposed (boxer engines like in a Subaru or Porsche) the header collects from two different places. For V configurations, there are usually two sets of headers that join later in the exhaust system. On the boxer engine, especially for Subaru, most headers will come as a single unit even though it collects from different cylinder heads. With inline engine configurations the header is more condensed as it only collects from one side. Vehicles equipped with a turbocharger(s) have headers or a turbo manifold. Despite which unit your vehicle is equipped with, the function is the same.
Since the focus of this piece compares headers vs downpipes, and we know from our “Different Types of Downpipes Explained” post that downpipes are specific to turbocharged vehicles, the remainder of our comparison will be limited to turbocharged vehicles.
As we follow the flow of the exhaust out of the cylinder head, it travels into the header. From there, turbocharged EJ Subarus have an uppipe. The uppipe flows the spent exhaust gasses into the turbocharger. Most OEs have a decent header (and uppipe if applicable) with minimal restrictions that work well enough on the stock turbo. However, through less restriction with increased diameter and smoother bends, along with fine tuned runner lengths, a good header can offer some decent gains.
After the turbocharger, air flows into the downpipe.
The spent gasses travel out of the turbine housing outlet into the downpipe. Side note: this part of the exhaust is usually called a downpipe because it’s a pipe that travels downward. As turbo configurations change, the name might also change.
The 2015+ WRX has a low, front-mounted turbo, where a “J-pipe” is the first part of the exhaust connected to the turbine housing outlet. Whatever you call it, a downpipe is the first portion of the exhaust, post turbo, and is typically the most restricted portion on factory turbocharged vehicles. Upgrading this component, paired with proper tuning, is typically the best bang for your buck modification (although it does vary by platform).
TYPES OF DOWNPIPES
From the above, we know the purpose of an upgraded downpipe is to increase performance by allowing increased flow. Turbulence is one main factor. If the gasses are all stagnating and/or running into protrusions or running into each other, it creates more backpressure than a well-designed system. The more laminar (smooth and straight) the gasses flow, the more the system can flow for a given pipe diameter. Steep angles and abrupt pipe diameter changes should be avoided.
Some of the common downpipe designs include: flanges with a simple pipe, bell mouths, divorced wastegate, and split bell mouths. You also have castings and formed pipes to choose from. Which one works best is also determined by quite a few different factors and how well they are designed and manufactured.
The downpipe and remaining parts of the exhaust system can also be made from different materials. Material changes typically do not have any significant affect on performance but can have other advantages. Check out the “Titanium vs Stainless Steel: What’s the Difference?” post for more details.
Downpipe vs Header Power Gains
Power gains will vary greatly with both of these modifications depending on the vehicle and what step in the upgrade path you find yourself.
Also keep in mind that both of these modifications will require tuning. How far that tuning pushes the engine once modified with one of these parts creates an additional variable making an unbiased direct comparison difficult. Downpipe modifications are covered within the scope of the Off The Shelf Maps on the Accessport. Headers for most applications will require custom tuning.
For most cases, in factory form, the downpipe will be a greater restrictor than the header. With that in mind, a downpipe upgrade would typically be recommended first as it would yield a greater potential benefit. However, there are likely some situations where this may not be the case.
If you’ve got questions on the upgrade path for your COBB supported vehicle, we’ve got the answers! Give us a call at 866-922-3059 or email firstname.lastname@example.org and we’ll be happy to help.