BMW N55 Upgraded Downpipe
The exhaust downpipe is the first part of the exhaust system on the N55 engine. It bolts-up straight to the turbocharger and passes air from the turbo to the cat-back portion of the full exhaust system.
The OEM downpipes have catalytic converters on them, for emissions purposes. However, the catalytic converters are extremely restrictive and significantly reduce the amount of air you can push out of the engine. This restriction creates back-pressure in the exhaust system. Back-pressure forces air backwards, towards the turbo, reducing turbo spool. The end result is a less-efficient turbocharger, restricting power and performance.
On a stock N55, the OEM downpipe are sufficient for the amount of air the engine is sucking in. But, when you add a performance intake to the engine, or a tune, the downpipe creates even more of a power restriction.
The Importance of the Downpipe on Tuned or Modded N55’s
On turbo engines, you have two types of back-pressure: pre-turbo and post-turbo back-pressure. You want higher pre-turbo pressure because it reduces the workload of the turbo and increases efficiency. Inversely, you want lower post-turbo pressure as the drop in pressure from pre- to post-turbo increases turbo spool. The increased spool from low post-turbo pressure increases the efficiency and power output even further.
Because the N55 stock downpipe has catalytic converters, it cannot push air out fast enough on tuned cars. The more air you are sucking into an engine, the more air that has to leave. With an intake or tune, you are sucking air in faster than the stock downpipe can release it. The end result is a lot of post-turbo back-pressure, and a less efficient turbo with slower spool.
For tuned and modded N55’s, upgraded downpipes are one of the best mods. But, what about catless vs. high-flow catted downpipes?
Catless vs. High-Flow Catted Downpipes
The only difference between these two options, as you can imagine, is a catalytic converter. Catless options will completely remove the catalytic converter while high-flow options will use an upgraded or higher performance cat that is less restrictive than the OEM system.
There are really two factors to consider when choosing high-flow or catless: performance, and legality/emissions.
Less restrictive is better. Obviously, catless systems are going to be less restrictive, therefore offering the best performance benefit. While high-flow is a significant improvement to OEM, it still does not match catless. For people looking to aggressively tune their N55, we recommend catless. If you are just trying to send it with an intake/tune/downpipes, then a high-flow option should suffice.
Additionally, as you could expect, high-flow options are going to be more expensive because they include the catalytic converter, which isn’t a cheap addition.
A catless downpipe is technically illegal as it violated emissions laws. Although there are “fixes” that can hide the engine codes and trick emissions tests, generally, you will fail emissions tests with catless downpipes. With high-flow options, you will pass emissions testing and are in compliance with the law.
This will matter most for people who are in California and other states with very strict emissions laws. We run catless and simply use a “DP fix” to tune out the code and trick emissions tests. To each their own their opinions here.
Performance Benefits of N55 Downpipes
- 20-25whp and wtq on tuned N55’s with catless DP’s
- 10-15whp and wtq on tuned N55’s with high-flow DP’s
- Great sound improvement without being too loud
- Improved turbo spool and increased turbo efficiency
Our Favorite N55 Catless Downpipe: VRSF
VRSF’s catless downpipe for the N55 is the best bang for the buck option on the market at just $250. The pipe is made of 304 stainless steel, and can be finished with a high-temp ceramic coating for an additional $70.
Our Favorite High-Flow Catted N55 Downpipe: VRSF
Again, VRSF has our favorite high-flow downpipes on the market. At $400, these are the best combination of price and quality on the market, with most high-flow options running $650+.
Summary on N55 Downpipes
Catless downpipes are the least restrictive and offer the most performance benefit, at approx. 25whp for tuned vehicles. However, catless downpipes do create emissions testing issues and are technically against emissions laws.
High-flow catted downpipes are more restrictive than catless, but still offer 30%-40% more flow than OEM. Unless you are shooting for huge horsepower numbers, a high-flow system is perfectly capable of handling the increased air flow from an intake or tune. On the downside, these are more expensive