After much thought, I decided to revisit the intake system.
Here is my first design, which I flew almost 150 hours. It gave me 0.8 inches of ram effect at 7650' msl. I obtain this number by recording the manifold pressure before startup (home field being 7650' msl), then flying south a few miles where I can fly at 7650' msl with wide open throttle, and recording the m.p. again. The difference is the ram effect.
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I used a coffee can for a mold for a fiberglass can. It is a little crude. The important part is that the end telescopes so that I can get the cowl on. Inside is a K&N filter # RU-0640. It has an effective filter area of 60 square inches.
K&N has a design guide on their website for filter area required for a minimal pressure drop.
Area=(displacement X max RPM) / 20,839 I am not sure where they came up with that constant :o)
This yields a required filter area for a 360 cu. in. engine at 2700 rpm of 46.6 sq. in.
For a K&N round filter, find the filter area by multiplying the diameter by PI, then by the length minus 3/4". (The ends of the filter don't flow much air).
I was never happy with this system, because the air coming in the snorkel ran headlong into the solid end of the filter, and then had to turn and go around to go through the filter. David Howe invented a system of smoothing this airflow with a cone on the front of the filter, and a can around the filter shaped to guide the air through. I am sure this helped the situation, but I kept remembering the early Mooney system of bypassing the filter at cruise altitudes. In a Mooney this would result in an inch or so of manifold pressure gain.
Here is my new system. With the silicone rubber tube on each end of the filter, it is flexible enough to get the cowl on.
There is a 3" butterfly valve in the forward end of the filter. When closed, the engine gets warm air from under the cowl, through the filter. When open, the engine gets cool unfiltered outside ram air. Using the above test method, I get about 1.3" manifold pressure gain at 7500 msl and cruise speed. During the Sun 100 race last spring, at 500 ft msl, I saw 31" mp. This was about 1.8" of gain, at 190 KIAS.
This system also works as an automatic alternate air system, since anything that might block the intake is no worse than closing the valve. Also, any rain water that blows in while parked will simply drain out through the filter, so no worries about hydraulic lock either.
I am trying to work with a local machine shop to produce the valves in quantity. They take me about 3 hours each to make by hand. I have several out to adventurous builders for beta testing.
Update -- 5/28/05
Rod Bower of Visalia, CA has agreed to produce a kit for this system. He will supply a valve, filter, and adapters. Contact him at --
Rod Bower, n180rb "at" aol.com, or his cell phone, 559-972-6291.
Update -- 9/21/05 Rod has taken the ball and is running full steam ahead !!! I have made a new page for his stuff
After some more careful testing, my original system produces about 0.75" of ram gain. So, it must be that some of the intake airflow is escaping as back-flow through the air filter. I am going to replace the filter with an Al tube to see what the ultimate gain is for my cowl and intake placement.
Update -- 1/11/06 See my new page 2