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
Benefits of downpipes upgrades for the BMW N55 engine include:
- 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
Best BMW N55 Downpipe Mods
Below are a couple of our favorite downpipes for the BMW N55 engine. This is far from an exhaustive list. The N55 is an older engine with tons of proven options around. However, the below list are some of the N55 downpipes we believe offer the best balance of price, quality, fitment, and performance.
1) VRSF N55 Catless Downpipe Upgrade
VRSF's catless downpipe for the N55 is the best bang for the buck option on the market at just over $250. The pipe is made of 304 stainless steel, and can be finished with a high-temp ceramic coating for an additional $70.
We basically run solely VRSF products on our BMW's - as do many others. They're quality parts for a great price. It's hard to argue against the N55 downpipe upgrades from VRSF.
Buy Here: N55 VRSF Catless Downpipes (F-Chassis)
2) N55 VRSF High-Flow Catted DP Upgrade
Again, VRSF has our favorite high-flow downpipes on the market. At under $500, we believe these are the best combination of price and quality on the market, with most high-flow options running $650+.
Buy Here: N55 VRSF High-Flow Downpipe (F-chassis)
3) Masata N55 F-Series Catless Downpipes
Masata offers a fantastic downpipe for F-Series N55 engines. Their downpipe is manufactured of 304 stainless steel and integrates directly with the OEM turbo and exhaust system.
Since the VRSF products are difficult to find in the UK and Europe, Masata is an awesome choice for anyone looking for a catless option overseas. While Masata is still a great product for US folks, shipping overseas makes it a bit less affordable compared to other options.
Overall, the Masata downpipe is a great product and highly recommended for our European enthusiasts.
Buy Here: Masata N55 F-Series Catless Downpipe
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