Project E46 M3: Part 11 – Castro Motorsports air box installed and dyno tested!


Using a CSL-style airbox also allows us to ditch the panel filter in favor of a conical unit, and in this case a very big one.  In fact, the surface area of the square-shaped, drop-in replacement we previously used from Macht Schnell and EAS measures 47 in2.  The K&N unit that we sourced from here has a massive 6-in inlet, and its surface area measures 237 in2, which is a whopping five times greater than stock.  Granted, that doesn’t necessarily mean it will outflow it by five times because there are other variables to consider.  Still, this is the largest round/tapered conical air filter that K&N’s offers, measuring 6 inches at the inlet by 7.5 inches long, and 9 inches in diameter.  It weighs almost 3-lb.  It’s huge!

Prior to removing the stock airbox we must get a new baseline since it’s been some time since this car hit the dyno.  During the AEM EMS test in Part 6, we left off with 327whp.  Today we’re down 9whp, so it’s a good thing we took this new baseline.  The reasons for the lesser power today could be a variety of things, but what's worth noting is the fact that we’re back to testing with a stock alternator pulley, since we removed the VAC race pulley in Part 8.  We’re also testing with our lighter D-Force wheels versus putting back our heavy stockers.  In my testing experience on inertial dynos, I’ve seen many times where the car will register a couple of ponies less after installing lighter wheels, even though it should be theoretically quicker on the road.  This has also been especially true when testing turbocharged vehicles, and even moreso when the overall tire diameter is reduced, which reduces the overall load on the engine and turbo.

Alright, let’s get to the intake plenum removal.  First, and please excuse my girly hands, we start by removing the air filter box.  It’s very simple, just pry up the two tabs with the screwdriver.  It’s so easy my 6-year-old can do it, and she's a girl (can I even say that anymore?).

The box and the HFM (which stands for “hot film mass”, and is also referred to as the air flow meter) come off together, exposing the factory air filter.  This is the unit that came with our Macht Schnell upgrade from EAS in Part 1, and it proved to be a good bang for the buck.  If you’re still rocking the factory filter, you’ll find that piece of junk is orange.

Lift the air filter off and at the bottom you’ll find a bolt holding the box together.  Just remove that and your box should come free.  You should be able to pull the snorkel attached to it at the front without removing the entire air diverter from the nose of the car. 

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