Re: KR> Cockpit vacuum gauge

[Jeff Scott via KRnet]

 
 

Jeff said,

> "I don't think the manifold pressure gauge [vacuum gauge] is going to
tell you much other than your throttle setting for a given altitude."

It won't even do that if you fly wide open throttle - which if above 8K I
assume everyone does.

Steve Bennett had a chart that showed what information could be derived
from a vaccuum gauge. I just went looking for it but couldn't find it.
It's out there on the internet - behavior of the needle indicating
various possible engine problems. Steve once recommended I put one in so
I did . . . but I've never found the gauge useful at all. The way I fly,
unless I've just taken off and am climbing, it always reads zero.

Mike
KSEE
---

Actually, a manifold pressure gauge will show absolute pressure, which changes 
with altitude even at full throttle. (30 in manifold pressure at sea level vs 
20 in manifold pressure at 10,000').  That allows you to calculate percent 
power.  FWIW, I am almost always flying above 8000' since the airport is at 
7200', but I rarely cruise at full throttle.

I suspect the initial question was asked based on the vacuum gauge 
interpretations on this web site: 
.  First off, the 
interpretations on that web site are pretty lose interpretations.  Secondly, 
the things they are interpreting via a vacuum gauge would be detected much 
earlier during any type of regular engine maintenance (differential compression 
test, mag drop test on run-up, and mag timing check).  The third point is that 
a manifold pressure gauge is usually installed with a restricted port, and a 
fairly long 1/8" copper line, so any of the vacuum interpretations from that 
web site would be pretty well buffered out of the manifold pressure gauge 
readings.  That is by design in order to prevent having a significant vacuum 
leak into the intake manifold should the line to the manifold pressure gauge 
fail.

-Jeff Scott
Los Alamos, NM

 

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KR> Cockpit vacuum gauge

[Mike Stirewalt via KRnet]

Jeff said, 

> "I don't think the manifold pressure gauge [vacuum gauge] is going to
tell you much other than your throttle setting for a given altitude."

It won't even do that if you fly wide open throttle - which if above 8K I
assume everyone does.  

Steve Bennett had a chart that showed what information could be derived
from a vaccuum gauge.  I just went looking for it but couldn't find it. 
It's out there on the internet - behavior of the needle indicating
various possible engine problems.  Steve once recommended I put one in so
I did . . . but I've never found the gauge useful at all.  The way I fly,
unless I've just taken off and am climbing, it always reads zero.  

Mike
KSEE


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Re: KR> cockpit vacuum gauge

[Jeff Scott via KRnet]

 
I have a manifold pressure gauge in my KR. It's nice for fine tuning throttle 
settings and calculating percent power, but it's only there as a personal 
preference and is completely unnecessary without a constant speed prop.  Can't 
say that I would be able to diagnose valve issues or engine health with it.  I 
find EGTs and a static compression differential test to be a much better 
indicator of what's going on with the engine.  

1 EGT running 50 - 100° higher than normal is an indication that a spark plug 
is not firing.  All EGTs running 50-100° higher than normal is an indication 
that a mag may have failed.  Just pulling the engine through as part of your 
preflight should indicate if you have a valve or cylinder issue as you'll feel 
the soft cylinder and will hear it leaking.  I've seen a lot of different 
failures in aircraft engines, but short of launching a cylinder off the case, I 
don't think the manifold pressure gauge [vacuum gauge] is going to tell you 
much other than your throttle setting for a given altitude.

-Jeff Scott
Los Alamos, NM


--
Subject: KR> cockpit vacuum gauge
Anyone running a vacuum gauge in their panel. Is it not an indicator of engine 
health, valves etc

Craig


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KR> cockpit vacuum gauge

[Craig Williams via KRnet]

Anyone running a vacuum gauge in their panel.  Is it not an indicator of engine 
health, valves etc

Craig

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