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


Here’s a comparison of the inlet opening to each of the two intake plenums.  The silicone boot on the left is from the MS stage 1 intake, and the right one is obviously the opening to our new unit from Castro Motorsport.  Hellooo?  ooo…   ooo…   ooo…    ooo…  (Get it?  It's the echo.  Nevermind.)

When we’d first installed the new intake plenum, the large K&N cone filter was going to fit.  So, without my wife seeing, I ran inside and grabbed a small dessert plate, along with my kids' vitamins bottle, and mocked up the size filter I could fit.  I needed a filter that would be this small in stature, yet still have the large 6-in opening.  I then hopped onto the K&N website and found the filter I needed, which we ordered through

Given my measurements, I'd found a K&N filter that would fit fine without any modification.  It’s got 3.5 times the surface area of the stock panel, and is K&N part number RC-2960HD.  It's only 5-in long and still has the 6-in inlet. 

But, what can I say–because the RC-2960HD filter is still 40% smaller than the big RC-5183, I couldn’t resist figuring out a way to make it work.  The look of the larger filter, plus the extra flow, was too enticing to miss out on, and in my stubbornness I made it fit.  I’ll explain how I did this below.

First, I needed to make sure the filter would fit “up” into the plenum about another full inch than what the filter was originally allowing.  Since I didn’t want to cut the plenum—because then the opening might turn out too large for any of my 6-in filters—I took a razor knife and shaved off the inside rubber lip completely.  I made it pretty thin, too.

Make sure to check out our vidoes on Page 10 showing you the new intake sounds!

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