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Date: Tue, 03 Dec 2013 07:01:14 -0500
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From: cwfmd@yahoo.com
Reply-To: cwfmd@yahoo.com
Subject: cabin pressure valve vs manifold pressure drop
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We put a new canopy seal in the IV-P. I fiddled with the cabin pressure val=
ve to try to reduce or locate=A0 a squeal. (My Grainger smoke matches don't=
 light, with reduced partial pressure of Oxygen at altitude) I noticed that=
 pulling the cabin pressure control partially out, reduced the cabin inflow=
, as expected, but also dropped the manifold pressure by about 4 inches wit=
h a loss of about 50 hp (estimated). This was at flight level 230. I had no=
t previously noticed this large effect on manifold pressure, but it could d=
emystify performance changes in previous data. Who knew the cabin pressure =
valve affected the "throttle" setting :) It makes sense because the previou=
s cabin pressure just dumps overboard at the firewall. Wondering why this i=
s set up this way.... Why not just close the valve to the cabin, when not n=
eeded or wanted, and leave the manifold pressure in the manifold??=0A=A0 In=
 general, I noticed others have similar issues with the cost of repair of t=
he Dukes valve. I would rather redesign the system, using the on-board inst=
rumentation and computers that already know the static pressure and the cab=
in pressure. I hot climates I prefer to minimize the heat entering the cabi=
n, and maybe just use 4-5 psi relief valves to distribute air flow. I think=
 my system where all the inter-cooled air routes thru the firewall to under=
 the back seat was designed by Eskimos, who always need max heating.=0A
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<html><body><div style=3D"color:#000; background-color:#fff; font-family:ti=
mes new roman, new york, times, serif;font-size:14pt"><div>We put a new can=
opy seal in the IV-P. I fiddled with the cabin pressure valve to try to red=
uce or locate&nbsp; a squeal. (My Grainger smoke matches don't light, with =
reduced partial pressure of Oxygen at altitude) I noticed that pulling the =
cabin pressure control partially out, reduced the cabin inflow, as expected=
, but also dropped the manifold pressure by about 4 inches with a loss of a=
bout 50 hp (estimated). This was at flight level 230. I had not previously =
noticed this large effect on manifold pressure, but it could demystify perf=
ormance changes in previous data. Who knew the cabin pressure valve affecte=
d the "throttle" setting :) It makes sense because the previous cabin press=
ure just dumps overboard at the firewall. Wondering why this is set up this=
 way.... Why not just close the valve to the cabin, when not needed or
 wanted, and leave the manifold pressure in the manifold??</div><div>&nbsp;=
 In general, I noticed others have similar issues with the cost of repair o=
f the Dukes valve. I would rather redesign the system, using the on-board i=
nstrumentation and computers that already know the static pressure and the =
cabin pressure. I hot climates I prefer to minimize the heat entering the c=
abin, and maybe just use 4-5 psi relief valves to distribute air flow. I th=
ink my system where all the inter-cooled air routes thru the firewall to un=
der the back seat was designed by Eskimos, who always need max heating.<br>=
</div></div></body></html>
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