Perry Mick wrote:

Recently Jon Lauter asked why I was using a 24 psi pressure
cap.  I think we need to discuss this.  It was Tracy who originally
advocated using higher pressure caps for high altitude use.  I
think Ed Anderson is also running a higher pressure cap, while
I'm throwing names out.

The 2nd gen RX-7 uses 13 psi caps, the 3rd gen 16 psi.  I was
running a 16 psi cap initially but changed to 24 psi during my
flight test period.  My coolant pressure is typically between
10 and 15 psi, so I think 13 and 16 psi is a little marginal.
I think I'm also running a little higher temperature/pressure
than normal because I have a relatively small volume of
coolant.  This will change slightly in the future when I install
a real radiator with end tanks.

I would like to hear what others are using and think about
this issue.

--
Perry J. Mick
Duckt N7XR
http://www.ductedfan.com

 Perry is correct.  I use a 21-24 psi radiator cap.  It is not uncommon on
my installation to see 14 - 18 psi coolant pressure on a hot day right after
full power take off.  Also, at higher altitudes you lose the assistance of
atmospheric pressure on the cap making the pressure differential even
greater than when at seal level.  The end results is that a certain amount
of coolant can be lost if your coolant pressure differential exceeds the
capability of your radiator cap.  A slightly higher psi cap seems to prevent
this from happening most of the time.   I would typically lose 1/8" of
coolant level on each flight until I replaced the 16 psi cap with the 21 psi
cap.

Ed Anderson

 
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