For a while there has been more buzz about the term "Alpha N". I have seen various descriptions and interpretations of what most believe this is about. Without going into great depths about the technical aspects, I thought about writing a brief description of what it is and the impact it has on the E46 M3.
What is Alpha N?
Alpha N is the term used by the DME manufacturers to describe a mode of operation. Most German vechicles are designed with an Air Flow Sensor (aka MAF) to provide optimal driveability and clean emmissions control. Removing the MAF on most BMWs forces the DME to operate in Alpha N mode. Alpha N mode is not something that an aftermarket tuner has created. Alpha N mode is often regarded as a built-in safety mode for the engine should the MAF fail.
How does Alpha N work?
Most German vehicles DMEs use inputs from 1)Coolant temp sensor 2)Crank sensor 3) MAF and 4) O2 sensors to control the clean and efficient operation of the motor. Should the DME loose one or more of these 4 critical sensors, the car will likely run poorly. If the MAF (a very critical input) were to fail, the Siemens MSS54 DME for the S54 M3 drops into Alpha N mode and uses the throttle position sensor to calculate the Mass of the air entering the combustion chamber. This is also know as "Speed Density Mode" ie get-you-home mode and the Check Engine Light will usually light up. The car may run well for the time being, but the S54 was designed to be run with an MAF for emmission and driveability reasons.
So what is the big deal if I want to pull out my MAF and just let the car run in Alpha N?
There are some consequences such as a check engine light, not as accurate fuel trimming and MAF calculations and increased emissions. Fuel Trimming is a major function of the DMEs fuel management system. It represents the actual addition or subtraction from the actual fuel mixture being setup by the DME.
Removing accurate fuel trimming and allowing fuel trimming to be calculated off the throttle position sensor instead of the MAF can have side effects on driveability characteristics such as throttle response, smooth part throttle and tip-in onto acceleration. In order to achieve factory-like driveability, fuel trimming based off MAF data is essential - which is why BMW use a MAF. Adding forced induction does not make it a neccessity to remove the MAF - just look at all the thousands of non-S54 BMW supercharger kits currently in use that all keep their MAFs and the dozens (close to 50 sold in 3 months) of VF-S54 supercharger owners.
The code in the DME can be deleted so that the check engine light does not come on. The code in the DME can also be changed so that the DME does not use its fuel trimming features etc. However all of the forementioned requires altering emmissions and OBD2 programming characteristics of the DME and this is not taken too well by the authorities if you plan to drive your car on public roads. Additionally and possibly more importantly to the performance enthusiast, the functionality and intelligence of the DME is reduced and limited if sections of code are deleted from the DME.
If you are uncertain about Alpha-N ask your BMW service advisor if they could have a technician explain it to you briefly the next time you are the dealer. Or better still maybe there some BMW techs reading who can chime in :thumbsup:
All in all, the aftermarket FI systems for BMWs are extremly challenging to create if one chooses to retain the original parameters of the car. Running the S54 in Alpha-N may work well in certain circumstances, but VF-Engineering S54 supercharger systems are tuned by GIAC to retain the use of the Bosch MAF sensor
for optimal driveability and for the purpose of becoming emmissions legal.