REFLECTOR:MORE ON COMBUSTION AIR AND COOLING

[Jim Agnew]

If you are getting 2" of MP you are not getting it from ram
pressure and the scoop.  At 200MPH you can get about 3/4"
of MP.  If you are using downdraft cooling (e.g., plenum)
and you are picking up the air for combustion from inside
the lower cowl you will probably find that it is at a
slightly lower pressure than outside due to the vacuum
caused by the prop.  Your increase in MP was probably more
due to additional airflow in the lower cowl.  

I have updraft cooling and even at idle the upper cowl is
at a negative pressure.  I found this out since I have a
small rear facing vent (about 3 sq in) to vent the upper
cowl after shutdown and yarn tufts show airflow into the
vent even with the prop sucking air past the vent.  I
assume at higher airspeeds this may change but you will
always have a pressure drop across the engine.

On the Reflector are pictures of my air filter in the wing
with air coming from the underside of the wing and
delivered to the intake via large hose.


Jim


Jim
--- RJohn15183@aol.com wrote:
> >>And how about combustion air?  I don’t like the idea
> of diverting cooling air from the top nacas through a
> duct.<<
> 
> At first I not getting quite as much manifold pressure as
> I should. I ended up building a little scoop for intake
> air underneath. The ram air gave me another two inches of
> manifold pressure at altitude. A poor man's super charger
> as it were! 
> 
> On the other hand it was pretty ugly breaking up the
> clean lines of the fuselage BUT you had to be on your
> hands and knees to see it down there on the belly so what
> the heck? :-)
> 
> Ugly in exchange for go faster. Hmmmmm. I'll take that
> deal! :-)
> 
> DM Rob
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=====
James F. Agnew
Jim_Agnew_2@Yahoo.Com
Tampa, FL
Velocity 173 Elite Aircraft Completed & Flying