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What Does an Upgraded Downpipe Do?

Typically, the first step in improving a vehicle’s performance is an ECU tune. This software upgraded provides significant gains in horsepower and torque on an otherwise completely stock car. After this, the most bang for the buck upgrade on turbocharged vehicles is a high-flow downpipe paired with a stage 2 software reflash.

This package is viewed as “stage 2” on the upgrade paths of many Volkswagen/Audi vehicles. While BMW, Mercedes, and other brands follow different upgrade paths, a downpipe and accompanying software provides significant power increases for all turbocharged vehicles.

Why do downpipes provide such significant gains? What makes them such a potent upgrade when compared to other hardware components like cat-back exhausts and intake systems? Read on to find out!

What is a downpipe?

Downpipes are sections of tubing that connect the exhaust side of a turbocharger with the start of the vehicle’s exhaust system, allowing spent exhaust gas to exit the engine. Downpipes also contain catalytic converters, reducing the emission of harmful gasses during operation. With its position immediately after the turbo, the efficiency of a downpipe directly impacts turbo spool and performance.  

k04_turbo_and_downpipe
A downpipe mated to the hot side of a turbo

Downpipes move air

Factory downpipes are meant to meet emissions standards and provide enough air movement to meet a stock vehicle’s performance requirements. Multiple high-cell catalytic converters, small pipe diameter, and restrictive angles all contribute to low flow rates on stock downpipes.

On most applications, the stock downpipe flows efficiently enough to handle a Stage 1 software upgrade and basic bolt-on hardware like an intake system. Anything beyond this, the key to unlocking significantly more power begins with uncorking the exhaust system, starting with replacing the restrictive stock downpipe with a higher flowing aftermarket one.

Stock_and_APR_downpipe_comparison
Stock vs APR downpipe, notice the less restrictive bends in the APR hardware on top
Stock vs Integrated Engineering downpipe diameter comparison

A higher flowing downpipe will need upgraded software to take full advantage of the higher flow rates. Without a tune, an upgraded downpipe provides modest gains that are not worth the price. With software, up to 100 horsepower and torque over stock can be seen on smaller engines like the 2.0T EA888. Larger engines like the 4.0T EA824 can see up to 150 horsepower and over 230 pounds of torque over stock. These numbers depend on fuel grade and other modifications/variables.

Types of downpipes

Upgraded downpipes come in two forms: High-flow catted and cat-less. The only difference between them is the inclusion or exclusion of a catalytic converter.

Advantages of a cat-less downpipe: 

The biggest advantage of a cat-less downpipe is unrestricted exhaust flow. With no cat, the downpipe is a straight tube allowing exhaust gasses to exit the turbo at the highest possible rate.

Disadvantages of a cat-less downpipe: 

The most obvious disadvantage is the environmental impact of unrestricted exhaust flow. With a cat-less downpipe, it is highly unlikely that a car will pass emissions testing even in states with lenient rules.

Advantages of catted downpipes:

A catted downpipe provides a significantly higher rate of flow over stock hardware, but retains catalytic converters, better protecting the environment and increasing the chances of passing yearly emissions tests.

High cell count factory catalytic converter

In practice, the difference in power gains between a catted and cat-less downpipe is negligible. In most street applications, a downpipe with high flow cats will be the best option for both performance and environmental impact.  

High-flow aftermarket catalytic converter

What changes besides horsepower? 

The most obvious change with an upgraded downpipe is improved performance, but several other things can change.

Sound

Larger diameter piping and less restrictive catalytic converters not only allow more air to flow, but it also allows more sound to flow as well. While not nearly as loud as an exhaust system, a performance downpipe will provide more engine noise while driving, adding to the overall experience.

Improved engine bay appearance

This is highly dependent on the vehicle and positioning of the downpipe, but in some instances, the area where the downpipe meets the turbo can be seen when the hood is up. Depending on the brand of the downpipe, the polished metal will stand out, adding to the overall appearance of your engine bay.

An aftermarket downpipe can improve the appearance of your engine bay

Potential downsides to an ungraded downpipe

There are some potential negative effects of removing the factory downpipe and catalytic converters from your vehicle.

Check engine light:

A vehicle’s ECU is looking for very specific emissions numbers at the post-catalytic converter oxygen sensor. An upgraded downpipe, regardless of whether it has cats or not, will skew these numbers. This can cause the vehicle to throw a check engine light for catalytic converter efficiency. The check engine light can be combatted in several ways.

Tuning:

The best and most effective solution is with proper tuning. A software upgrade will change the parameters the ECU is looking for at the post-cat oxygen sensor, preventing a check engine light from coming on.

Oxygen sensor spacer:

Another solution is to use an O2 sensor spacer. An O2 spacer moves the sensor out of the direct flow of the exhaust, tricking the ECU into seeing the emissions numbers it is looking for. O2 sensor spacers are not a guaranteed fix for a check engine light, and will not always work.

Failing emissions testing: 

The biggest potential issue with upgrading any part of your vehicle’s exhaust system is failing emissions tests. This issue is particularly pertinent to upgraded downpipes, as they change or eliminate the catalytic converters. When upgrading to a performance downpipe, you take the risk of your vehicle failing emissions testing.

Conclusion

Upgraded downpipes provide great increases in horsepower and torque on turbocharged vehicles by allowing exhaust gasses to flow easier as they leave the turbo. Paired with the appropriate software, a downpipe can provide significant gains and make a vehicle more fun and engaging to drive. Keeping the potential downsides in mind, upgrading to a higher flowing downpipe is a great option for those who want more out of their car without moving into Stage 3/upgraded turbo territory.

Ready to upgrade your downpipe? Alex’s Autohaus is Utah’s premiere European performance shop. Our service advisors will help you find the right downpipe and software for your Volkswagen, Audi, BMW, Mini, or Mercedes. Give us a call or schedule an appointment online!

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