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Percent Power Calculation

Dugaru

Well Known Member
I've got a GRT Horizon EX and EIS4000. I've input the data for the percentage power lookup table as suggested by GRT here:

https://grtavionics.com//media/Percent-Power-Entries-Lycoming-O320.pdf

And I then get a calculated percent power on my EFIS. Problem is, the percent number I'm getting seems to be extremely optimistic. As just one example, the other day at 24 inches and 2400 RPM at 4500 feet, I was showing 99.9 percent power. If I'm reading the extremely complicated chart in the O-320 manual correctly (big "if".....) my engine is making considerably less power than that. Which also matches with my limited understanding of the physics involved.

Has anyone tweaked this lookup table to get better results?

FWIW, I've got an O-320-D1A, carbureted, with a Hartzell CS.

Thanks!
DMG
 
I was looking at this the other day, looking at the power displayed on my RV-9A's AFS EFI. At 4500 feet, 24/2400 gave me 76% power on my IO-320-D1A with MT C/S prop.
 
It seems to me that a NA engine (also not counting ram air or anything else). you could only get 100% MP at sea level, ~31 inches.

Similarly for RPM. A ratio of RPM/Max RPM.

So for 24"/2400...
24"/31" = 77 % of max MP
2400/2700 = 89% of max RPM
77% * 88% = 69% power
(there are some assumptions here. adjust as needed)

Does the GRT do additional calulations?
 
I have an Hx and a -10 but I think the calculations are the same, and mine works. The obvious answer is to review your table entries carefully, especially the changes with altitude.
And, as this is just an automated version of the table, what you get is for the table’s assumed mixture setting (50 F rich of peak, iirc). If you run other mixtures, it will be off; a little for ROP, quite a bit if LOP.
 
Don't forget the inaccuracies of MAP measurement as an influence on accuracy of the % power reading. I have calibrated mine and found it to be inaccurate by 0.3" which equates to about 3% power error. Now I run the engine up to whatever reading makes 75%, then add 0.3" additional manifold pressure to get the engine to something like 75% actual power output.
 
Don't forget the inaccuracies of MAP measurement as an influence on accuracy of the % power reading. I have calibrated mine and found it to be inaccurate by 0.3" which equates to about 3% power error. .

My simple math says 0.3/24 (24” MP, for example) is a tad over 1%. Am I missing something?
 
I don't believe the EFIS makers expect these to be 100% accurate, it's for reference only. I've seen manuals state this. No two engines are exactly the same, and adding the data from all the different sensors not being exactly the same.
 
I downloaded this app, and used it to calculate power settings to input into my GRT Horizon EFIS. You can select your engine on the setup page, and it uses the Lycoming power charts to calculate power settings. I did calculations for the various altitudes and entered the variation from standard sea level into the power setting page in the set up page for the engine. I entered my calculations from this app, including altitude (DA) variations in the set up menu chart. These settings were a little different from what is published on the GRT website. Works very well. When in flight, I can open the app on my mini iPad and check settings against what I see on the GRT engine page. They always match, which gives me confidence that what I am seeing is fairly accurate.

https://apps.apple.com/us/app/aircraftpower/id718023187
 
So what happens when you don't have a table that is applicable to your engine? High compression, doctored cam, cold air induction, etc. Which direction do you cheat the altitude numbers and by how much?

Ed Holyoke
 
Bob - let me run this past you as a process...
1) set 75% power as displayed on GRT EFIS screen
2) note MAP
3) add 0.3" MAP to make up for known/calibrated MAP sensor error
4) observe GRT-generated % power displayed

When using this technique the outcome observed in Step 4 is often 78%.

I'm happy to use a better method of power setting and measurement so I'm standing by for better ideas of how I might better account for the error of my MAP sensor.
 
There is something else going on with the OPs situation.
The chart provided, compared to the flight data gives 128hp not 99.9% of 160.

The problem is not the chart.
 
Years ago Van's had an article in the RV-ATOR on calculating percent power from your manifold pressure and RPM. Haven't looked for the article but I did have these numbers written down. Take manifold pressure at altitude and flying, say 21" and 2400 rpm. Add 21 and 24, you get 45.
The figures I have are: 42=55%, 45=65%, 48=75%.
 
A few Lycoming power calculation rules of thumb that I’ve collected over the years, probably from this forum site. I’ve made some calculations for my engine, Lyc O-360-A1A (180 hp) :

335A2047-AA53-4D8F-A4B0-9B2531C27056.jpg

Sorry, I can’t figure out how to make the picture appear upright.
 
I had the same problem with my GRT Sport EX. When I put in the power table i forgot to put the rated HP in the legend at the top of the table. Mine had "0" an d it always showed 99.9%. After I went back in and put 180 for HP at the top it works fine.

Hope this helps.
John
 
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