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Difference between the N51 Engine & N52 Engine

If you are thinking about tuning your BMW E90 328i it is important to know which engine variation you have. The reason this is a big deal is because some engines came with a 3-stage intake manifold (N51) and some came with a 1 stage manifold which is more restrictive (N52). If you have an N52 Engine, upgrading to a 3-stage manifold will add an additional pathway directing more air to make more power at high RPM so you can get the most out of a performance tune.
The easiest way to know what engine your car is equipped with, is to check the emissions label under the hood. The N51 will say “SULEV”, while the N52 will have “ULEV”
Another way to find out is to enter the last 7 digits of your VIN into Real OEM.

 BMW matched the ULEV requirements when they started shipping the N52, making it road legal in a number of states that insist on these emissions standards.

Some states decided to make SULEV the lowest acceptable standard. This put BMW in a position where they had to modify the original N52 and create what is now known as the N51 engine.

 Vehicles with N51 and those with N52 are nearly identical, However, there are several key areas where these engines differ.

Low Emission Vehicle (LEV) Standards

These California emission standards, which applied through model year 2003, were expressed using the following emission categories:

Tier 1
Transitional Low Emission Vehicles (TLEV)
Low Emission Vehicles (LEV)
Ultra Low Emission Vehicles (ULEV)
Super Ultra Low Emission Vehicles (SULEV)
Zero Emission Vehicles (ZEV)

    Ultra Low Emission Vehicle (LEV) Standards

    ULEVs release emissions that are 50 percent cleaner than the current average year's models. ULEVs take the LEV, Low Emission Vehicle, standard a step further but don't yet qualify for Super-Ultra Low Emissions Vehicle (SULEV) status. Examples of these Ultra-Low Emissions Vehicles have begun cropping up more and more frequently starting with 2007's Honda Odyssey minivan, the 2007 Chevrolet Malibu Maxx and the 2007 Hyundai Accent.

    Super Ultra Low Emission Vehicle (LEV) Standards

    SULEVs are 90 percent cleaner than the current average year's models, emitting substantially lower levels of hydrocarbons, carbon monoxide, nitrous oxides and particulate matter than conventional vehicles. Some PZEVs fall into this category by default. For instance, if you buy a Toyota Prius in California and fuel it up, it's considered a Partially Zero Emissions Vehicle (PZEV), however, if you drive east and fuel it up over the next 2,500 miles it's considered an SULEV since California's low sulfur gas formulations are not available everywhere.
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