C-FAH Q

C-FAH Q

Well Known Member
Here is a question thats been irking me for a week or two.
Should your manifold pressure gauge and the atmospheric pressure read the same before you start your engine? Is there some inherent pressure difference between a cold engine and outside air? Mine never do seem to be the same, maybe I need to make an adjustment on my Advanced EFIS?
Take a look at your mp and altimeter before start up and see if they match. I would think they should read the same, but inquiring minds want to be satisfied.
 
The reading on the MP gauge without the engine operating should show the current ambient barometric pressure wherever you are. I believe that would be different than what is shown in the Kollsman window unless you happen to be located at sea level.
Good Luck,
Mahlon
?The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."
 
Same-Same

Set your altimeter to the known field elevation. Your manifold pressure should match what is in the Kollsman window (engine off). If they don't, one or the other is incorrect.

Don
 
cfiidon said:
Set your altimeter to the known field elevation. Your manifold pressure should match what is in the Kollsman window (engine off). If they don't, one or the other is incorrect.

Don
Click to expand...

ONLY at sea level. Engine-off MP should read lower by roughly 1 inch per thousand feet of elevation. Now, if your altimeter has the range, set it to sea level (zero feet), then the MP gauge should match the Kollsman window reading.
 
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Too funny this just came up ...

I powered up my GRT systems for the first time a week ago, and noticed that local baro was 29.86 while my MAP gauge read 29.2. I was concerned 'cause I too thought they should match. Since my elevation is around 720', the "1" per 1000ft rule" gives me just the answer I was looking for ! Thanks much. :)
 
Your MP gauge is reading the absolute pressure at your current altitude and temperature. What are you using to compare this to? If it is the Altimeter setting, you will get the wrong pressure since this is an adjustment to compensate for altitiude, temp etc. If you dial your altimeter to 0 feet and see what the setting is in the Kollsman window, it should match your engine MP reading.
 
When comparing settings, I use the pressure reading given from the AWOS and i also have a Kestral 4500 weather meter. The Kestral and the AWOS are always very close, mp gauge....not so much.
 
Yes, if the engine is stopped, the MP should read the same as the atmospheric pressure. But, understand that the altimeter setting you put in the altimeter is not atmospheric pressure. For example, if you are at the airport at Leadville, CO, at 9934 ft elevation, and you have the rare day where the pressure matches the standard atmosphere, you'd set 29.92 in the altimeter setting to get it to read 9934 ft. The actual atmospheric pressure is 20.63 in HG, which is long way from 29.92.

If you want to find the atmospheric pressure, set the altimeter to 29.92, and read off the pressure altitude. Use the Digital Dutch Pressure Calculator to find the pressure that corresponds to that pressure altitude. Put the pressure altitude in the "Altitude" box in the "Input" section. Select "ft" for the units. Select "inches of mercury" as the units in for "Pressure" in the "Output" section. This is what your MP gauge should read.

Note that there are other pressure vs pressure altitude calculators on the internet, and some of the ones that float to the top in Google don't give the right answers. If it doesn't give 20.63 in HG for 9934 ft pressure altitude don't trust it.
 
Thanks Kevin,
That seems like an accurate test. Will check it out today. Now, if this crappy weather here in sw Ontario moves away, I can start flying again.






Kevin Horton said:
Yes, if the engine is stopped, the MP should read the same as the atmospheric pressure. But, understand that the altimeter setting you put in the altimeter is not atmospheric pressure. For example, if you are at the airport at Leadville, CO, at 9934 ft elevation, and you have the rare day where the pressure matches the standard atmosphere, you'd set 29.92 in the altimeter setting to get it to read 9934 ft. The actual atmospheric pressure is 20.63 in HG, which is long way from 29.92.

If you want to find the atmospheric pressure, set the altimeter to 29.92, and read off the pressure altitude. Use the Digital Dutch Pressure Calculator to find the pressure that corresponds to that pressure altitude. Put the pressure altitude in the "Altitude" box in the "Input" section. Select "ft" for the units. Select "inches of mercury" as the units in for "Pressure" in the "Output" section. This is what your MP gauge should read.

Note that there are other pressure vs pressure altitude calculators on the internet, and some of the ones that float to the top in Google don't give the right answers. If it doesn't give 20.63 in HG for 9934 ft pressure altitude don't trust it.
Click to expand...
 
Kevin Horton said:
If you want to find the atmospheric pressure, set the altimeter to 29.92, and read off the pressure altitude. Use the Digital Dutch Pressure Calculator to find the pressure that corresponds to that pressure altitude. Put the pressure altitude in the "Altitude" box in the "Input" section. Select "ft" for the units. Select "inches of mercury" as the units in for "Pressure" in the "Output" section. This is what your MP gauge should read.
Click to expand...
Cool explanation, Kevin! Does anyone know if there is a downloadable calculator like this anywhere? It looks like you have to be in the airplane when doing the calculations and I don't have web access at my hanger. I could, however, just run a downloaded program on my laptop.
 
I created an Excel spreadsheet years ago to do this. Unfortunately I can't easily upload it to my server from work. I'll get it online tonight and post a link to it.

Or, that Digital Dutch page has a Table option. You can use it to create a table of pressure vs pressure altitude for an altitude range of interest - e.g. every 100 ft from 500 ft below your field elevation to 500 ft above. Select the "Table Wizard" link on the left side of the Digital Dutch Calculator page.
 
Thanks Kevin,
I went out to the airport and tried the calculator, turns out everything is right on.
 
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