DrQuinn Posted March 24, 2018 Report Share Posted March 24, 2018 I started to keep some more detailed stats to see how not even close to the book numbers I am getting. Depending on altitude I typically setup for closest settings under 75%bhp and lean. Usually I am 15 or more mph short of the known unrealistic POH figures. I just started keeping fuel statistics though and I am running at 7-8 gph, certainly way better than the books range of 9 - 9.4. I have a portable tach that gets the actual RPM, leaving either I'm leaning waaaaay too much (which I don't think I am) or the MP gauge is off by an 1 - 1 1/2 ". Does anyone know if this is likely? Would my mechanic be able to verify the meter similar to the tach? Quote Link to comment Share on other sites More sharing options...
Marauder Posted March 24, 2018 Report Share Posted March 24, 2018 I started to keep some more detailed stats to see how not even close to the book numbers I am getting. Depending on altitude I typically setup for closest settings under 75%bhp and lean. Usually I am 15 or more mph short of the known unrealistic POH figures. I just started keeping fuel statistics though and I am running at 7-8 gph, certainly way better than the books range of 9 - 9.4. I have a portable tach that gets the actual RPM, leaving either I'm leaning waaaaay too much (which I don't think I am) or the MP gauge is off by an 1 - 1 1/2 ". Does anyone know if this is likely? Would my mechanic be able to verify the meter similar to the tach? There are a number of factors that come into play. The POH I’m sure was at part a marketing document. Number of antenna, rigging and even condition of the paint will affect some bearing on the true airspeed you can achieve.You certainly can verify your electronic RPM with an external RPM gauge. Your MP should be close to local barometric adjusted for altitude with the engine off. It may not be linear, but I think it is a low failure item. Based on the fuel flows you are talking about, it sounds like you are way off. Do you have an engine analyzer?Sent from my iPad using Tapatalk Pro Quote Link to comment Share on other sites More sharing options...
DrQuinn Posted March 24, 2018 Author Report Share Posted March 24, 2018 (edited) I use an external gauge to set RPM and I have no reason to believe that it is inaccurate; it gets the same readings as my mechanics. I will have to check the manifold pressure reading with engine off next time I'm at the plane. I have a EI ubg-16 but it's not hooked up for fuel flow or MP unfortunately. Edited March 24, 2018 by DrQuinn Quote Link to comment Share on other sites More sharing options...
Igor_U Posted March 24, 2018 Report Share Posted March 24, 2018 If I understood you correctly, you are running your engine at roughly 2gph less then "per book" meaning you are developing that much less power and result should be lower TAS. Without looking at POH or doing the math, seems to me you are at the ballpark... So, yes, MP gage could be reading low. What does it read with the engine of, on the ground? It should read close to Altimeter setting value at See level and (negative) difference would transfer to the reading in flight as well. Quote Link to comment Share on other sites More sharing options...
DrQuinn Posted March 24, 2018 Author Report Share Posted March 24, 2018 4 minutes ago, Igor_U said: If I understood you correctly, you are running your engine at roughly 2gph less then "per book" meaning you are developing that much less power and result should be lower TAS. Without looking at POH or doing the math, seems to me you are at the ballpark... So, yes, MP gage could be reading low. What does it read with the engine of, on the ground? It should read close to Altimeter setting value at See level and (negative) difference would transfer to the reading in flight as well. Thanks for the hint, I will definitely check on that at startup next time. Quote Link to comment Share on other sites More sharing options...
Browncbr1 Posted March 24, 2018 Report Share Posted March 24, 2018 Your MP gauge will read high at low power settings if there is a leak in the 1/8” aluminum line that goes to the gauge, or an actual induction leak. I had a flare break on the bunt reducer fitting where the 1/8” pipe was attached at the baffle and both of my MP gauges showed 19-20” with throttle closed. You can use the “Baro” iPhone app to check gauge accuracy on the ground with engine off. 1 Quote Link to comment Share on other sites More sharing options...
Guest Posted March 24, 2018 Report Share Posted March 24, 2018 You can use a pitot static tester on the MAP gauge, one inch per thousand feet should be close enough to check the gauge. Clarence Quote Link to comment Share on other sites More sharing options...
bradp Posted March 27, 2018 Report Share Posted March 27, 2018 And yes they can fail in high or low readings and in non lob at fashions. Just be cognizant that the entire system is involved in the reading (line, calibration hole, gauge, etc). Quote Link to comment Share on other sites More sharing options...
ShuRugal Posted March 27, 2018 Report Share Posted March 27, 2018 Usually I am 15 or more mph short of the known unrealistic POH figures. POH isn't that unrealistic: My '64C is frequently within 5mph of the POH numbers.Sent from my Pixel using Tapatalk Quote Link to comment Share on other sites More sharing options...
carusoam Posted March 27, 2018 Report Share Posted March 27, 2018 My M20C’s MP gauge started to read higher than normal.... i grabbed the 1/8” vac line while in flight... suddenly, my MP was reading like an altimeter... The soft aluminum line has a calibrated drilled hole in it. A crack developed from vibration, when I grabbed it, the crack turned into a break... Expect a larger number than usual, when the line leaks, or it’s calibrated hole gets torn open... At full throttle, the error is hard to notice... At idle, the error is huge, indicating lots of power that isn’t actually being produced... PP thoughts only, not a mechanic... Best regards, -a- Quote Link to comment Share on other sites More sharing options...
DrQuinn Posted March 29, 2018 Author Report Share Posted March 29, 2018 Thanks for all the responses. To confuse matters I checked it with engine off and it actually under-reads what the pressure was by 1 1/2". My mechanic suggested that he could have it calibrated or that I just play around with different settings over the summer (while keeping an eye glued to the cht's) and keep records so I can find settings my airplane like. Quote Link to comment Share on other sites More sharing options...
carusoam Posted March 30, 2018 Report Share Posted March 30, 2018 Consider getting it calibrated.... It is impossible to set the power very precisely without a proper MP. If you fly IFR, you will have your go to settings for various regimes of the flight. All based on MP and rpm... When things get difficult, like High DA, while loaded to MGToW, you want to know the power you have right now, not a close estimate.... The one day you get carb icing, same thing... you want to know how much power you have, now. calibration or gauge replacement isn’t very expensive. Drilling the calibrated leak hole is a pretty simple task. Requires the right diameter drill bit.... Missing 1.5” of MP is the equivalent of approximately 5% of total available hp... (1.5/29.92 x 100) Where this shows up is on T/O performance and climb performance.... it is better to have real numbers in your hands for those hot and or heavy days.... and damp and cold days.... PP ideas only, not a mechanic. Best regards, -a- Quote Link to comment Share on other sites More sharing options...
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