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Ultimate Downpipes Guide [2022]

Ultimate Downpipes Guide [2022]

Ultimate Downpipes Guide [2022]

By John

Thu Oct 13 2022


VRSF 3″ Cast Stainless Steel Catless Downpipes N54 V2 2007 – 2010 BMW 335i / 2008 – 2012 BMW 135i(https://www.vr-speed.com/vrsf-3-stainless-steel-catless-downpipes-n54-07-10-bmw-335i-08-10-bmw-135i.html/ref/365/)

Downpipes are a section of the exhaust system that connects the exhaust manifold or the turbocharger to the rest of the exhaust. Some downpipes have integrated catalytic converters, while others are between the exhaust manifold or the turbo and the catalytic converter.

Some people use the word “downpipe” to describe a part of the exhaust system of naturally aspirated engines; however, the term downpipe is commonly associated with turbocharged engines.

The best terms to describe the tube that connects the exhaust manifold with the rest of the exhaust system on naturally aspirated engines are “front pipe” or “head pipe”. Naturally aspirated engines with headers don’t have front pipes, but turbocharged engines with “header-like” turbo manifolds require a downpipe.

Downpipes and Naturally Aspirated Engines

Although the word downpipe is not the most popularly accepted word to be used in naturally aspirated engines, there are some cases where “front pipe” doesn’t describe the part as well as “downpipe” does. For example, many users and performance parts manufacturers of the non-turbo Mitsubishi 3000GT, Nissan 300ZX, and Toyota Supra call downpipe to the piping connecting the pre-cat part of the exhaust with the catalytic converter. Most models have the catalytic converter integrated into this tubing like many turbocharged engines.

Vehicle manufacturers seem not to care much about this part; they use it as a simple connection and/or catalytic converter bearer. There isn’t much room for improvement for this piece; the best way to see a significant improvement is to replace the exhaust manifold with a header when is possible. Another way to see some positive results is to replace the catalytic converter with a high-flow cat.

High-Performance Downpipes / Frontpipes For Naturally Aspirated Engines

High-performance downpipes for naturally aspirated engines are usually made of stainless steel. They are lighter and less restrictive than the OEMs. They have a larger diameter than the stock ones and smoother curves to provide a smoother flow of the exhaust gases and reduce turbulence. The power gains are small, but they help prevent a bottleneck effect when combined with a high-flow cat and a high-performance cat-back exhaust system. They make the engine slightly louder, but don’t expect a significant improvement. Besides, high-performance downpipes are necessary on upgraded naturally aspirated engines, mostly when the intake airflow has been highly increased.

Downpipes in Turbocharged Engines

Downpipes are essential parts of turbocharged engines’ exhaust systems, connecting the turbocharger’s hot side to the rest of the exhaust. Most stock downpipes contain the catalytic converter because cats reach their peak efficiency when they are hot, and the downpipe is the hottest part of the exhaust system after the exhaust manifold. However, on rare occasions, stock downpipes are catless, and they are installed just before the cat.

Stock Turbo Downpipes

Stock AUDI TT Downpipe

Vehicle manufacturers have to consider thousands of variables, including manufacturing costs, emissions laws, durability, low fuel consumption, and other factors that result in parts that meet the requirements for standard engines but have room for improvement. Even sportscars’ stock downpipes are highly improvable.

High-Performance Downpipes

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It’s pretty well known among car and tuning enthusiasts that swapping the stock downpipe with a high-performance one is one of the easiest and best modifications for turbocharged cars in terms of value for money.

If you check some dyno charts, you will see that upgraded downpipes unleash serious horsepower and torque. Besides, high-flow downpipes rise peak boost levels from one or two PSI on mildly tuned engines.

Downpipes are attached straight to the turbochargers; hence improving their flow immediately impacts the engines’ performance. High-flow downpipes increase the turbo and the engine efficiency as the increased flow reduces the exhaust back pressure, which helps the turbine’s propellers to expel exhaust gases faster, reducing the turbo’s spool-up time, commonly known as turbo lag.

Besides, high-flow downpipes bring other advantages to turbocharged engines, such as a reduction of the EGT (Exhaust Gas Temperature), which helps to reduce the engine’s thermal load and helps the pistons to evacuate exhaust gases swiftly (clean the cylinder), allowing them to suck up more fresh and clean air on the intake stage of the combustion cycle, producing stronger combustion and improving the engine performance even more. Last but not least, on stock and mildly tuned engines, the improved exhaust flow relieves mechanical stress from the engine internals and turbo, extending its durability and improving the MPG.

In the case of engines with medium to extreme modifications, high-performance downpipes are essential to take the most out of the engine and minimize the risk of breakdowns. Heavily tuned engines are fitted with big turbos that simultaneously produce an incredible boost and heat. That’s why these engines need large downpipes, usually custom-made and capable of dissipating the heat and gases from the combustion chamber.

High-performance downpipes also enhance the engine sound, making it sound deeper and more aggressive. The lower back pressure, the smoother bends, and the materials used to make them allow sound to travel more freely inside them.

Catted Vs. Non-Catted High-Performance Downpipes

If your stock downpipe is catted (includes the catalytic converter), you will have to consider your local laws, your environmental awareness, and your vehicle’s purpose (street legal or off-road), before choosing a high-flow catalytic converter for your vehicle.

There are two types of aftermarket downpipes, catted and non-catted. The names are pretty self-explanatory, but there are some considerations to be made:

Catted High-Performance Downpipes

Removing the catalytic converter from the vehicle is illegal in most countries and states. Even in places where removing it isn’t illegal, you will probably have to pass an emission test which is unlikely to pass without a proper catalytic converter.

You will find catalytic converters of all prices and qualities. You should check your local regulations and choose a high-flow catalytic converter complying with the law. Be careful of suspiciously cheap catted downpipes; it’s probably that their catalytic converters - if they actually have one- won’t do a good job.

Top-notch high-performance cats have metallic honeycombs instead of the traditional, heavy, and restrictive ceramic honeycombs used on most stock cats. Metallic catalytic converters also dissipate heat better, warm up faster, are smaller, shock resistant, and have more catalytic capacity than their ceramic counterparts. 200-300 cell metallic cats are usually enough to meet pollution regulations of most US States and European Euro4+ norms. You can check the product specs sheet to see if the product meets your local regulations. If you don’t find them, it’s always best to contact the manufacturer or distributor to find out. These products are pretty expensive, and you are spending extra bucks upgrading your vehicle without breaking any law.

There are catted high-performance downpipes with such a great design that they have 400-cell metallic cats and better flow than some de-catted downpipes. The only problem is their price; they cost more than twice the average price of 300-cel downpipes.

You can read more about high-flow catalytic converters here.

De-Catted High-Performance Downpipes and Test Pipes

These high-performance downpipes are meant for off-road use only because, as their name suggests, they don’t have a catalytic converter.

Most cheap de-catted downpipes have poor construction and design, so the increased flow and power gain are due to the catalytic converter’s removal. Although these downpipes noticeably increase the engine’s performance, they don’t unleash as much power as a well-built, catted, high-performance downpipe. The term “test pipe” is often used as a synonym for catless downpipe, and while test pipes are basically that, they were first used to check the catalytic converter’s airflow. A mechanic would install a test pipe instead of the catalytic converter, compare the engine behavior, and check whether the cat was clogged or not. Some people started using test pipes on their engines to trick the authorities, but nowadays, test pipes don’t fool anyone; they are as illegal as removing the catalytic converter.

There are many high-quality catless downpipes available on the market. They provide the highest increases in horsepower and torque and make engines sound awesome. The problem is that they aren’t street-legal in many parts of the world. However, if you are building a track day car, a drag race vehicle, or intend to use your vehicle for off-road use, you should go for a high-quality catless downpipe.

What You Should Consider Before Upgrading Your Downpipe

  • Catted or Catless: This is the first question you should solve first. Leaving environmental awareness and ethics aside, you should check your local laws before buying a high-flow downpipe. Some states charge extremely expensive fines for removing catalytic converters or failing emissions tests, and you may end up with your car impounded.
  • Construction And Design: High-performance downpipes’ main goal is to improve and increase exhaust airflow. It’s important to note that downpipes are not just simple tubes. They must have smooth curves and be built of a smooth material like polished T-304 stainless steel to ensure the best possible airflow. The best downpipes’ main sections are made in one piece; all the bends are carefully done using a mandrel bending process. This ensures that their inner parts don’t have wrinkles, burrs, or any irregularities that could restrict the airflow and prevent any turbulence formation. All pieces like the flanges, O2 sensor port/s, the catalytic converter (on catted-downpipes), etc., should be attached using TIG welding and back purging (a process that shields the inner side of the welded parts with a gas, preventing contamination resulting from the welding process). Besides ensuring the best airflow, high-quality aftermarket downpipes have the best sound.
  • Fitting: If you plan to install the high-flow downpipe in a semi-stock or mildly modified vehicle, it’s always best to make sure you buy a plug-and-play, easy-to-install product. This will allow you to install it without further modifications and save labor costs and lots of troubles such as undesired vibrations and other issues that poorly made products may cause.
  • O2 Sensor/s Bung Location: If you are buying a catted downpipe, make sure that the O2 sensor/s bungs are located in the right place. This will allow the sensors to work properly without triggering the “check engine” light on your dashboard or an emissions cell. If you’re buying a catless downpipe, check that the O2 sensor/s bungs allow you to plug the sensors back. It’s 99% probable that your catless downpipe will trigger a warning light on your dashboard, but at least you will be able to plug your sensors back in. Some cheap aftermarket downpipes have the O2 sensor bungs in the wrong place requiring extra modifications and additional work.

Should I Have My ECU Reflashed When Installing A High-Flow Downpipe?

There is something of a chicken-and-egg situation in this question. Chip tuning and high-flow downpipes complement each other extremely well. A good ECU flashing unleashes serious horsepower to a stock turbo engine, and a combo of an excellent remapping and a high-flow downpipe releases even more horsepower and torque without any risks of engine damage.

If your engine is still stock, you are planning the first mods, and you don’t have immediate plans to replace the turbo or make any major upgrades, then you should install a high-flow downpipe before reflashing your ECU. Doing the mods in this order will help you to get the most out of the chip tuning. This doesn’t mean that you can’t install a high-flow downpipe after reflashing your ECU, you will notice a power increase, but the software won’t be optimized for the enhanced exhaust flow and the lower EGTs.

If you are buying an “off-the-shelf” chip tuning, you will be able to choose from different files, usually called “stages”, according to your vehicle’s tuning level. It’s important that you make sure that you are buying a file compatible with your current or future mods (downpipe, cat back exhaust, cold air intake, etc.). If you choose to go for a custom chip tuning installing the high-flow downpipe first is even more important. The tuner will put your car in a dynamometer to measure the engine's parameters and make a custom file for you. Even though the tuner can estimate how your engine would work with a high-flow downpipe and create a file ready for a future downpipe upgrade, the best outcome is achieved when the tuner can work on your actual engine parameters.

Check Engine Light (CEL)

Some aftermarket downpipes will trigger your check engine light. 02 sensor spacers sold to prevent it don’t usually work, but ECU reflashes efficiently keep the “check engine” light off no matter what kind of downpipe you are using.


High-performance downpipes provide remarkable power gains to turbocharged engines, and the benefits can be significantly larger when combined with a proper ECU software upgrade. Depending on the engine’s characteristics like displacement, turbo size, cylinder head, etc., a high-flow downpipe can increase the engine power from 5 to 15 percent, and a high-flow downpipe plus an ECU remapping can help your engine to produce up to about 35+ percent more power. Besides, downpipes make your engine sound sportier and louder.

Try to get the best high-flow downpipe you can, and remember that construction and design make a huge difference in the results. Last but not least, check your state laws and the downpipe’s specs before upgrading your downpipe. If you plan to use your vehicle off-road, catless high-performance downpipes are for you.

If you are looking to purchase aftermarket downpipes for your S55 equipped F80 M3, F82 M4, or F87 M2C, make sure to checkout our article on the best downpipes.

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