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I've been reading these emails between you and Rusty and John with great
interest. I am really confused now.
I have a boost gauge made by VDO. It reads both suction 0-30hg and boost
0-+25hg. The center scale (0) is at the 1 o'clock position or there about.
I thought that this is what I need to monitor the turbo boost level, but
now, I don't have a clue.
I'm not sure what this gauge is going to show me.
Steve Brooks
Cozy MKIV #1071
13BT
-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
Behalf Of Marvin Kaye
Sent: Wednesday, November 19, 2003 9:18 AM
To: Rotary motors in aircraft
e.g. roughly speaking boost PSI = (MAP - height thousands)-30/2
MAP = 42 @ SL = 6PSI
MAP = 48 @ 10,000 = 4PSI
Does this makes sense, or am I still missing something?
<p>
Your formula only works at sea level when ambient pressure is at 30"hg. At
altitude ambient pressure decreases about 1"/1kft. (That number isn't
linear, the decrease diminishes with each thousand feet, but it's ok to use
that number for our purposes here...) So, if your MAP at 10kft was 48"
where ambient pressure is about 20"hg you'd have 28" or 14psi of boost, not
4.
Ergo, your formula should actually be: boost PSI = (MAP - ambient
press)/2.
Both MAP and ambient pressure are in inches of mercury. If you run some
other numbers you can quickly see how a manifold vacuum gauge works... let's
say you're at 2000' msl where the ambient pressure is about 28"hg and you
pull the power to initiate a descent... your MAP gauge might read 15"hg...
(15-28)/2 says you have 6.5psi of suction going on inside the manifold and
you're not making any boost at all. I don't know if they even make a boost
gauge that shows manifold vacuum (suction) as well as boost (positive
pressure), but if they do the zero psi point would have to be referenced to
ambient pressure in order for it to display properly. BTW, in reality, 1psi
= 14.695" hg, but that's close enough to 2psi = 30"hg for purposes of our
discussion.
<Marv>
<Marv>
>> Homepage: http://www.flyrotary.com/
>> Archive: http://lancaironline.net/lists/flyrotary/List.html
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