Inlet Blog

Cowl Inlet Size Upgrade


The original air inlet for the engine was something I knew would be something that we could improve.  For starters, it was small.  Or at least it looked small.  2.5″ diameter.  Now I ran a lot of math regarding inlet size, but I can’t say I ever figured out what the “best” inlet size should have been.  I do know that 2.5″ diameter, or 4.9 square inches of area was enough to feed the engine as much as it was breathing in, but not much “boost”.  So with a lot of head scratching, I ended up with 3″ diameter, or 7 square inches, because that it the size of pipe I had to make a mold with.. 🙂

I took a section of 3″ diameter, 1/8″ thick aluminum tube.  This would be perfect to make a mold for the inlet.  I had a good friend machine 1/8″ off the diameter of the tube, or 1/16″ of the wall thickness on about half of the tube.  The goal was to make a “step” in the part so you could attach a 3″ scat tube with ease.  The fiberglass around this part of the mold would build up to exactly 3 inches.  Another benefit of this part is the inside of the part (outside of the mold) was very smooth.  I think this is critical to maintain good flow inside the intake.

Once that part was completed, the next step was to bond it into the cowl.  I used a digital level to align the intake with the level point of the aircraft.  In theory the airplane should be level in cruise flight, so I wanted the intake to be parallel with the relative wind.  Once I found the level point I used a hot glue gun to place the part in the cowl.  This was just temporary so I could go back with fiberglass and bond it in place.

Another key point to an intake is the lip of the intake.  Square sharp edges are bad.  Smooth round edges are good.  To achieve this edge I had to make the inlet thicker, thereby giving me some material to sand down to a rounded edge.  I mixed up some micro and laid it on thick around the inlet.  Once hard I was able to sand the inlet down to a nice smooth round shape with a rounded edge leading into the engine.  Not too shabby,  I was proud of this part.

  1. Kevin

    Do you guys build and sell these lower cowls ?
    And if so…… Price?

    • admin

      No, I do not have molds for a cowl.. I have been threatening to make molds for an angle valve cowl, but the demand hasn’t been high enough to offset the costs. See for parallel valve cowls.


  2. Danny

    What did you gain on this?

    • admin

      Well, good question, but i can’t say for sure. The previous setup was about 2 feet of 2.5″ scat tube. So, pretty easy to gain efficiency over that. In all I gained 1.5″ MP, but I attribute most of that gain to the increase in inlet size to 3″, and the horrible scat tube. I am working on a new airbox now that I should have done soon, with a few differwnt ideas.