Paul,
What size prop is this that gives you 5600 rpm now?  Is it still the one off of the O-235 or have you gone back to your original?  What's the total area of the NACA duct (like top view)?
Just wonderin' ... Jim S.

Paul wrote:

Today I finished installing the firesleeve on the remaing fuel lines, made some stand-offs so that the fuel lines would not be right up against the engine, and finished hooking up and securing a few other items. With the top cowling off,  I ran the pumps for a minute, then checked for leaks...none found.  Then I started the engine and ran it for 20 or so seconds and shut it down and reinspected for fuel leaks. None found.  I restarted and taxiied away from the hangar a bit so as to not be as loud for others in and near the hangar and did a full power runup.  I hit 5600 static rpm's. In the past, the best I could hope for was 5200, most of the time settling for 5100 rpm's.  I taxiied back in and installed the top cowling....max rpm's 5100.  Took the top cowling back off....max rpm's 5600.  One more try....5100 with the cowling back on.  It is beginning to become more clear to me, that one of two things are affecting rpm's.  Option one is that with the top cowling on the engine is ingesting heated air and just cannot develop the same power as when it ingests outside unheated air.  Option two is the muffler bearing may be worn.  I'm kinda leaning toward option one.
     I was thinking of installing two small NACA ducts on the bottom of my cowling with two hoses coming up to feed air to my TWM throttlebody. The NACA ducts I purchased from Van's RV (made for cowling ventilation) already have a male outlet already molded in the assembly, so you just slide a hose over it and secure with a clamp. Very simple installation.(Works well for my cabin ventillation) If I recall correctly, they are about the same size as my throttle body bore. I wonder if I will loose much efficiency as the air goes through the corregated hose, and if the Van's RV ventillation NACA ducts will provide a sufficient volume of air. Of course while flying, I would have the advantage of a little bit of ram effect as well as the ability to suck in fresh outside air that has not been heated inside the cowling during taxi/takeoff run.  Any opinions/suggestions would of course be much appreciated.  I will discuss the vapor lock issue in the next post, so as to not complicate responses, etc.  Thanks to all who reply.  Paul Conner

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