Manifold Pressure Gauge

How to keep the Cessna 170 flying and airworthy.

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wa4jr
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Manifold Pressure Gauge

Post by wa4jr »

In the quest for more panel space in my '54 170, I wonder if the manifold pressure gauge can be taken out :?: I find it strange that an aircraft with a fixed-pitch prop has a MP gauge, and since I have no plans for a constant speed conversion, and do not use the MP gauge for any reference, I would like to take it out and free up a nice hole for something useful. Would the 170 be illegal in the eyes of the FAA if the MP gauge were to disappear :?:

Hey George, how do I change my user name to my aircraft N number. I have tried to change it via the profile page, but no luck. :( Do I have to register as a new user to change my user name :?:
John, 2734C in Summit Point, WV
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GAHorn
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Post by GAHorn »

Ha! We are all so different, yet the same. We all want our airplanes personalized in some way, yet we all love 170's. What a crowd, we are!
I just ADDED a mp gauge! (I consider it a great instrument, and usefull for all sorts of things, like engine problem diagnosis and carb-ice detection.
Anyway, the MP gauge is NOT a required item on the Type Certificate Data Sheet, therefore it may be removed, and update your equipment list and wt & bal (if the change is anything other than "negligible").
(PS-is your's an original 2-1/4" gauge? Want to donate it to my collection?) -gh
hbcroft
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Post by hbcroft »

GH

Can you elaborate a little more on the purpose of MP gauge in a fixed-pitch prop aircraft. I looked at a C 170 (O-300) the other day and it had a MP gauge and my first reaction was similar to the person who originated this thread!

Thanks

Bruno
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GAHorn
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Post by GAHorn »

A stuck (or broken or severely leaking) valve shows up on a manifold pressure gauge as a wildly fluctuating gauge needle. As an intake valve sticks or leaks, the compression stroke forces air-pressure back into the manifold system showing a wild momentary increase followed by a fall back to normal pressures. If it's an exhaust valve, the pressures in the exhaust system back up and register through the intake manifold again when that intake valve opens for the next intake stroke. (It's just less pronounced.)
Carburetor ice will show up in flight as a slight loss of selected manifold pressure, before it shows up as rpm loss. (I personally prefer it to a carb-temp gauge because the temp gauge only shows that conditions may allow carb-ice, if sufficient moisture is present. A loss of manifold pressure shows a restriction is actually developing.) A blocked or collapsed air filter will show up as equal or greater MP when carb heat or alternate air is selected (and of course, as less than normal MP on takeoff. The 210 that recently crashed that precipitated the recent AD note on Brackett Filter installations would have seen very low MP as a warning that things weren't right in the intake system, and depending upon when that indication first appeared, might have been a valuable warning to an alert, well-informed pilot.)
Less than normal MP on takeoff can be a warning that a carb temp butterfly valve or control cabel is broken/floating or other intake restriction (or ice again!), or as a reminder to lean for high altitude, as can less than normal static MP. When checking static MP, remember to subtract field elevation from local barometric pressure. (I.E., If you're at Denver and the local pressure is 29.92 (or slightly less than 30 for example) then you should subtract the 5K' elevation from 30", and your MP gauge before engine start should read slightly less than 25", and takeoff power should read no less than 24.9".))
If cruising along, and you seem to suffer a slight loss of rpm, but observe a slight increase in MP, then it's not carb ice, but a loss of engine power due to ignition problems (timing advance has changed) or a too lean mixture. If it happens in accompaniment with a slight roughness, then it's sparkplug or ignition lead problems.
I consider a Manifold Pressure gauge a valuable cockpit instrument. So did Cessna. Looking toward the back of most Owner's Manuals for the 170, it can be seen that it was offered as an optional instrument from the factory.
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Bruce Fenstermacher
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Post by Bruce Fenstermacher »

I love my MP. I have a climb prop and so I run a higher RPM at cruise than I normally might which is between 2550 and 2600 RPM. My MP at 1500 ft at this RPM range is about 21 inches which means the engine is not working all that hard even though the RPMs are up. I actually set my power setting using the MP at about 21 inches and the RPM will settle down to the 25-2600 RPM range. As I go into a slight climb or dive the RPM will go up and down but the MP stays the same. Once you have one and use it you'll like it I think.
CAUTION - My forum posts may be worth what you paid for them!

Bruce Fenstermacher, Past President, TIC170A
Email: brucefenster at gmail.com
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wa4jr
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Post by wa4jr »

NEVER stop learning :!: Posts like this make me really glad I followed this group over from the previous roost :D

No George, I my MP gauge appears to be an original 3.125" instrument by Cessna mounted below and to the left of the right control tube entry point. And no, I don't think I will add it to your collection :cry: Now that I know that it can be a good diagnostic instrument....and I quake in fear of a valve problem after hearing all the horror stories about the O-300 8O ....I will be keeping it in the panel :wink: I suppose I need to go up and record MP gauge readings in different phases of flight, so that I will know what is "normal" in preparation for one or more valves to go on "strike" :(

My combined engine oil temp and pressure gauge will indeed have to be one of the 2.25" Westach combined units. Too bad the combined oil temp/pressure gauge by Westach does not appear to be FAA/PMA approved. Or are they? Spruce just says they are "ideal for the homebuilder".
John, 2734C in Summit Point, WV
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mit
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Post by mit »

wa4jr wrote:NEVER stop learning :!: Posts like this make me really glad I followed this group over from the previous roost :D

No George, I my MP gauge appears to be an original 3.125" instrument by Cessna mounted below and to the left of the right control tube entry point. And no, I don't think I will add it to your collection :cry: Now that I know that it can be a good diagnostic instrument....and I quake in fear of a valve problem after hearing all the horror stories about the O-300 8O ....I will be keeping it in the panel :wink: I suppose I need to go up and record MP gauge readings in different phases of flight, so that I will know what is "normal" in preparation for one or more valves to go on "strike" :(

My combined engine oil temp and pressure gauge will indeed have to be one of the 2.25" Westach combined units. Too bad the combined oil temp/pressure gauge by Westach does not appear to be FAA/PMA approved. Or are they? Spruce just says they are "ideal for the homebuilder".
John

There is no normal, it will always vary with the barometric press how ever you do get use to expecting simular readings. I have one in my airplane, and really like it! I have seen carb ice with it and if your tach goes T.U. it gives you an idea of what going on too.
Tim
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GAHorn
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Post by GAHorn »

If your tach goes belly up, a MP gauge is the only way you'll be able to detect a loss of power due to carb ice until it's way too late. (Well, maybe one of those ice-man ice detectors would work, but it's limited in it's usefulness except for that one particular job. The MP is much more useful, all-around.)
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