The International Cessna^® 170 Association

lowNslow wrote:I don't have a copy of the operators manual to check but I believe, for those of us with C-145 engines, it still shows 45psi as max.

You can download a copy at the Members Only page. While the specifications page originally offered different specs for the two engines, subsequent manuals clarified that both engines have the same operating specifications (also confirmed by the TCDS to be the same.)

Bruce,

I also have a MP gauge (I think we've discussed it before), and like you, I don't pay a whole lot of attention to it on takeoff. (When I get my alternator installed this weekend and fly again, I'll try to remember to do just that).

The reason I say that 2" seems like a lot of loss, is that the picture below was made no lower than 5500msl at full throttle. I know there is some parallax looking at the MP gauge at that angle (not to mention that the gauge has only roughly been calibrated against atmospheric pressure on the ground), but it's showing around 24". Rule of thumb is 1" HG loss per 1000 ft msl, so max available pressure would be in the neighborhood of 24.5" at that altitude. In cruise with wide open throttle the MP gauge makes a crude altimeter.

My MP pickup point is at the base of the right intake manifold near the sump, where the primer would be if the manifold were on the other side. If the pickup point was on the cylinder at the intake port, then the indicated MP might be lower.

Update: I found this spreadsheet of data I took on the first flight with the new engine. The entries with an RPM of 2600 were full throttle.

N9149A wrote:Wouldn't the pitch of the prop and RPM of the engine can with it change what MP is achieved? And wouldn't changing the pitch of the prop produce a different MP for a given RPM?

For a given engine configuration at a constant pressure altitude, the only things that affect MP would be throttle position and flow velocity. Higher RPM = higher flow velocity = more pressure DROP = lower MP, but it's a relatively slight effect.

Miles

Miles

You are getting reading consistent with what I would expect fro my 170. My MP gauge is plumbed as yours is Miles.

You have also confirmed what I thought about prop pitch. Given two different pitch props at the same throttle setting on the same engine the flatter pitch prop would display a lower MP because the rpm would be higher.

N9149A wrote:Miles

...You have also confirmed what I thought about prop pitch. Given two different pitch props at the same throttle setting on the same engine the flatter pitch prop would display a lower MP because the rpm would be higher.

The larger effect is that a flatter pitch prop (less load on the engine) requires less MP to turn the same RPM in the first place. It's essentially running in a lower gear. Pressure drop in the induction system is a secondary effect.

Miles

cessna170bdriver wrote:

N9149A wrote:Miles

...You have also confirmed what I thought about prop pitch. Given two different pitch props at the same throttle setting on the same engine the flatter pitch prop would display a lower MP because the rpm would be higher.

The larger effect is that a flatter pitch prop (less load on the engine) requires less MP to turn the same RPM in the first place. It's essentially running in a lower gear. Pressure drop in the induction system is a secondary effect.

Miles

Manifold Pressure is the obverse of Manifold Vacuum.

"Manifold pressure", to be technically correct, is really "MAP" or manifold ABSOLUTE pressure. This is he pressure in inches of mercury column from an absolute vacuum which would be zero. That's what our aircraft
instruments read. The instrument at rest will read local ambient barometric pressure.

Some manifold pressure instruments, such as are used on turbo-diesel engines, simply read boost above ambient pressure since the air going into the intake manifold is not modulated by a throttling device. There is, therefore, never a vacuum on the intake system and all pressure is ambient or greater. Ambient reads at zero.

How about next time somebody goes flying, post your static rpm and manifold pressure along with indicated altitude. (0-300 and c-145 and type of prop) We can figure this out once and for all. lol

blueldr wrote:"...Some manifold pressure instruments, such as are used on turbo-diesel engines, simply read boost above ambient pressure since the air going into the intake manifold is not modulated by a throttling device. There is, therefore, never a vacuum on the intake system and all pressure is ambient or greater. Ambient reads at zero.

You are speaking of the boosted system, of course. In our normally aspirated engines we do have manifold vacuum. It can even be used in some emergency-gyro systems.

blueldr wrote:"Manifold pressure", to be technically correct, is really "MAP" or manifold ABSOLUTE pressure.

Are you saying BL, that George is not technically correct? I'm sitting here agape.

It certainly is true, George, that there is a system to use engine intake manifold pressure to operate vacuum driven gyros in an emergency such as vacuum pump failure. However, if one flys out west where the MEA may be quite high, perhaps 12,000 MSL or even more, that emergency system would leave a lot to be desired in those circumstances. To effectively drive the gyros, you would be required to reduce the throttle setting to at least four inches below the ambient pressure.
If you're flying at 12,000 MSL, the standard pressure is 19 in.hg. When you reduce the throttle setting to 15 in.hg.,or less, in order to get the necessary differential to keep your gyros working, most of the airplanes that I'm familiar with will take on the characteristics of a sharply pointed cobblestone, pointy end down.
It is then 2,000 ft. to granite.
This system would probably be very economically and operationally desireable in the mid America flatlands and the eastern mountains, but it sure isn't my cup of tea out here in the west.

Ok went flying today and took one or two more looks at my MP gauge. Of course I forgot all the different configurations to take readings but this is what I noted.

I'm at 500 ft ASL with a stock McCauley 1A170DM7654 prop. At full throttle full rich on climb out at about 50 ft AGLl, about 65 mph the prop rpm is about 2450 and I've got about 27.5" on the MP.

At about 1500 ASL at about 80 to 85 mph full throttle full rich in about a 500 fpm climb(maybe more) I get 2500 rpm and 25"MP.

As I said before I cruise in the area of 2000 ft asl at 2550 to 2600 rpm at 22.5 to 23" MP all day long.

BL...correctomundo....too much altitude and the manifold-vacuum systems lose effectiveness.
Bruce, Your recordings do not mention the accuracy of your gauges. (The 2450rpm/65mph is suspect I believe... and the 7654 prop is not "standard" I don't believe. (I believe the 7653 was the standard prop...?)
And the recordings you made did not include the atmospheric conditions necessary to reach a reasonable conclusion.
Full throttle at 1500 MSL on a standard day would be about 27.5" ... not 22.5 - 23".

OK George let me try again

N9149A wrote:OK. went flying today and took one or two more looks at my MP gauge. Of course I forgot all the different configurations to take readings but this is what I noted.

I was having fun flying today and took note of some readings which may mean nothing of value since I didn't have my test pilot hat on and run through a complete test series. PA as 3014 and the temperature was about 60 though.

N9149A wrote: I'm at 500 ft ASL with a stock McCauley 1A170DM7654 prop.

I'm at 500 ft ASL with a stock McCauley 1A170DM7654 prop. Stock meaning I don't have a Sensenich prop or California twist McCauley. The 54" is slightly more pitch than what might be considered standard or 53 for a McCauley 1A170DM on a Cessna 170A/B with a C-145-2 engine.

N9149A wrote: At full throttle full rich on climb out at about 50 ft AGL, about 65 mph the prop rpm is about 2450 and I've got about 27.5" on the MP.

I was just coming of the runway about about 50ft AGL, 1000ft from departure with 10 degrees of flap. My airspeed was about 65 mph from my uncalibrated known to be wrong airspeed indicator but I believe the reading to be reasonably accurate at this moment. My RPM was about 2450 which is the best I could estimate as the gauge has some bounce and I haven't calibrated it with my optical digital tach that I carry, in at least 4 flights. My MP gauge which can be hard to read because of parallax was at about 27.5. +- .25. I have not recently calibrated the MP gauge against the atmosphere but it was reasonable correct last checked or I would have disregarded it long ago.

N9149A wrote: At about 1500 ASL at about 80 to 85 mph full throttle full rich in about a 500 FPM climb(maybe more) I get 2500 rpm and 25"MP.

At about 1500 ASL at about 80 to 85 mph full throttle full rich in about a 500 FPM climb(maybe more) I get 2500 rpm and 25"MP. You will note I was in a climb so my RPM was not at red line of 2700 RPM or there about. (the gauge bounces remember but I use the average which has proven to be accurate in the past).

N9149A wrote: As I said before I cruise in the area of 2000 ft ASL at 2550 to 2600 rpm at 22.5 to 23" MP all day long.

As I said before I cruise in the area of 2000 ft ASL at 2550 to 2600 rpm at 22.5 to 23" MP all day long. Note I don't cruise at full throttle which is indicated by the MP only being between 22.5 and 23" and the RPM settling at 2550 to 2600. If I was at full throttle in level cruise my RPM would be 2700. I would assume but don't know that my MP would read at least 27.5

If you have a 170 and your readings and performance are smilier to mine then your aircraft and gauges are performing as well as mine or perhaps as badly as mine.

(How did I do this time George. )

Re-wordng the data doesn't alter the accuracy of observations of uncalibrated/unvalidated instrumentation. But ...anecdotally,...I certainly believe your tach is suspect, and possibly your prop-pitch is as well, by simple comparison to other aircraft I've flown that have similar instrumentation.
The description of the 65 mph configuration is certainly much more informative.... Without the knowlege of the flap-setting the earlier msg post certainly looked erroneous to me. The flap-setting info helps to better understand that, considerably.
The 500 fpm climb adds another complexity to the equation... Unless level flight is used for the recorded observations, then it will be virtually impossible to make valid comparisons with other aircraft.

N9149A wrote:...As I said before I cruise in the area of 2000 ft asl at 2550 to 2600 rpm at 22.5 to 23" MP all day long.

I don't know in what context you made that statement before...I don't see it in this thread...but it would also depend upon your weight, atmospherics, and true airspeed as to whether that could be relied upon for comparisons.

I'm not trying to be critical of the efforts to add data....I was attempting to broaden the view we all take of such data. Unless we all participate using identical techniques and standardized configurations ( a standardized flight-test program), then we'll not really develop dependable baselines.

N9149A wrote: At full throttle full rich on climb out at about 50 ft AGL, about 65 mph the prop rpm is about 2450 and I've got about 27.5" on the MP.

Taken into account temps this pretty much matches up with what I get static. (60F vs 80F) So I guess I am ok there. These tachs can be ****y, so it is much more likely the tach is suspect in my case then the manifold pressure gauge or the engine.