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75% power at 10500 density altitude?

alpinelakespilot2000

Well Known Member
In preparing for an EGT lean range test on a recent cross country, I noticed that at 8700MSL/10700 density altitude, and still climbing, my Dynon was indicating 74-75% power. This was WOT, full rich, on my O-320. However, I thought about 8000 density altitude is generally where normally aspirated engines cannot produce above 75%. Is my understanding incorrect or is my Dynon?
Thanks.
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both or neither

Your MAP shows 21.2 which is 73% of 29.0. My reading of manufacturer charts reveals that they rate 100% at 29, not 29.92.

But your RPM is 95.5% of the usual 2700 for maximum power.

73% times 95.5% would be 69.7%.

However, most such systems add HP for additional altitude, maybe 10HP for that altitude.

160*69.7% is 111.5 but if you add 10 HP as a "what if" you get 121 and that is 75.6%.

It that a correct number? I don't know.

If I wanted a good estimate of actual HP, assuming an engine in good condition, I would use fuel flow and assumed SFC for best power (after leaning for best power, of course). A rough estimate is .5 pounds per HP per hour and 6 pounds per gallon.

Using 10.2 GPH would mean 61.2 pounds which would give you approximately 122 HP or a little more than 75%. Since you were full rich (not a good idea at that altitude) your SFC would be higher and your HP would be proportionally lower. Try the test at best power. Also try it LOP and use around 0.40 for the SFC. If you just lean to rough and then add fuel until no longer rough your SFC will be in the .45 region, assuming a healthy Lyco.

This method was obtained from a conversation with one of the "Advanced Pilot" instructors but any error is mine, not his.

The usual rule of thumb for 75% at 8000 feet is for spam cans with lousy induction systems. Your MAP is the same as ambient for 9200', for example.

So there is a long answer with a shrug. Sorry.
 
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Take any power readings from your glass with a grain of salt. These are rough figures, certainly not accurate to within 1%.

HP will vary with rpm, ignition timing, AFR, MAP, humidity, CHT, oil temperature, exhaust back pressure (altitude again), alternator load and some other more minor factors. Since the computer does not measure some of these parameters and estimates others, the figure cannot be precise.
 
What did you set for your HP rating?
If your engine has electronic ignition, free flowing exhaust, etc. it will put out more than 160 hp.
 
ditto what Ross said!

Take any power readings from your glass with a grain of salt. These are rough figures, certainly not accurate to within 1%.

HP will vary with rpm, ignition timing, AFR, MAP, humidity, CHT, oil temperature, exhaust back pressure (altitude again), alternator load and some other more minor factors. Since the computer does not measure some of these parameters and estimates others, the figure cannot be precise.

A very wise answer from a very knowledgeable guy.
 
I have a similar setup and have configured the DYNON with an engine red line of 2600 RPM so I get an audible warning for my Sensenich prop which has a 2600 red line. This works great however it does cause the percent power reading to be a few percentage points higher than actual.
 
The ram air effect does give some boost but I agree the numbers seem a bit odd.

I understand Dynon factors in fuel flow into the percent power calculation and this is confirmed by the ROP tag against the figure.

My suggestion is to check the accuracy of each of the elements in the power calculation - Tacho, MAP, OAT, Altitude and fuel flow.

MAP can be checked on the ground by comparing against the Altimeter subscal minus the pressure drop due to airport elevation.

Lets hope altitude is accurate but can you cross check it with something else - ex a GTX327 will display the flight level - adjust for local qnh.

OAT, 16C at 8700' is hot - explains the high DA but is this true?

Fuel flow seems high to me - I am lothe to make a comparison with my carbed O-320 RV-6 because I cruise at 60% with 25 lph fuel flow at about 148 kts TAS and I dont think the difference between the RV-9 and RV-6 is that great. Have you checked the fuel consumption against what you see at the fuel bowser - this may take a few tankfuls but would help to verify the fuel flow figures.

I suspect your power is being over estimated because the fuel flow is high.
 
I would suspect his fuel flow is high because he is running FULL RICH. I suggest leaning to best power and seeing what the fuel flow, % power, CHT temps, etc. will be then.
 
I am not sure which data is in your D180 but when ROP the power is not fuel flow dependent and it is MASS AIRFLOW that determines HP. Here is a simple formula to use

100-((Max RPM/100-RPM/100)*2.5 + (MaxMP-MP)*3.5) = %HP

When LOP it will be fuel flow (think about why) and that varies with compression ratio. USGPH x 14.9 for standard CR's = HP a bit more for higher, a bit less for lower.

I think 70% at that MP and RPM. I have never found the Dynon to be that far out
 
I would suspect his fuel flow is high because he is running FULL RICH. I suggest leaning to best power and seeing what the fuel flow, % power, CHT temps, etc. will be then.

I agree but perhaps he is reluctant to lean because the engine monitor is teling him he is at 75% power. After all the Lycoming OM says (paraphrasing) to "maintain full rich in cruise above approximately 75% power".
 
Pretty sure Dynon uses MP, RPM, and fuel flow to calculate HP. I'd be curious about the fuel flow contribution to the equation. Does it always treat more fuel as more HP, or does the math recognize that very rich equals less HP?

You were going to do a mixture pull. Did you happen to note how the power calc responded when you leaned to 100-150 ROP with the throttle full forward?
 
I don't know the details of the Dynon algorithm (wish I did), but I recall some time ago on this site someone from Dynon confirmed they did indeed factor in fuel flow. I suspect it is rather more complex than a first order contribution.
As far as engine percent power calculation is concerned about the best in the public domain is Kevin Horton's algorithm which he has available as a spreadsheet. It works very well, albiet for max power conditions only. He essentially interpolates the Lycoming power charts to calculate the power. I think Kevin might be working on tweeking this to include fuel flow.
Since the only detailed power charts available are for the Lycoming engines the spreadsheets are for standard engines only. Adding high compression and different induction systems will obviously screw the calculation.
 
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I can't imagine anyone NOT leaning at 10,500 feet! Just watch the fuel flow reading, from full rich to normal lean. Several Gallons Per Hour delta, with no loss of power. And at 10,500" you would get an INcrease in power as you leaned.

Eh?



I agree but perhaps he is reluctant to lean because the engine monitor is teling him he is at 75% power. After all the Lycoming OM says (paraphrasing) to "maintain full rich in cruise above approximately 75% power".
 
I agree but perhaps he is reluctant to lean because the engine monitor is teling him he is at 75% power. After all the Lycoming OM says (paraphrasing) to "maintain full rich in cruise above approximately 75% power".

Yes, this is partly why I had not yet leaned, right or wrong. I've generally followed the no leaning above 75% rule. It was also full rich because I was about to do the lean range check (though I ended up not doing after all.). Usually I lean out as far as possible once established in cruise at 65% or so.

Will mull over some of the other thoughts and suggestions tomorrow. Gotta get to bed.

Thanks for everyone's contributions.
 
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Yes, this is partly why I had not yet leaned, right or wrong. I've generally followed the no leaning above 75% rule. It was also full rich because I was about to do the lean range check (though I ended up not doing after all.). Usually I lean out as far as possible once established in cruise at 65% or so.

Will mull over some of the other thoughts and suggestions tomorrow. Gotta get to bed.

Thanks for everyone's contributions.
Don't be afraid of leaning! The relatively small displacement normally aspirated engines we fly behind are highly unlikely to be damaged in any leaning scenario. As Mike Bush is commonly discussing, keep the CHT temps below 400 deg and you will not damage anything by leaning. At 10.5K I would propose that even if you purposely attempted to abuse the engine by leaning you could not do so.
 
Don't be afraid of leaning! The relatively small displacement normally aspirated engines we fly behind are highly unlikely to be damaged in any leaning scenario. As Mike Bush is commonly discussing, keep the CHT temps below 400 deg and you will not damage anything by leaning. At 10.5K I would propose that even if you purposely attempted to abuse the engine by leaning you could not do so.

These statements should be qualified.

With EI and unknown ignition timing, coupled with auto fuel instead of 100LL, throw in some higher CR pistons and leaning close to peak, yep, you can get detonation at 75% power or below. Lycoming has published some restricted data to show this from fuel testing on the dyno.
 
This table might be helpful - credit to Kevin Horton. This is max power.

Code:
Lycoming O-320-B and D Series (derived from Lycoming Curve No. 11260A Jan1977)
-----------------------------------------------------------------------------------------------------
| Press. | Std. |     88 HP -- 55% Rated    |    104 HP -- 65% Rated    |    120 HP -- 75% Rated    |
|  Alt.  | Alt. |  Approx. Fuel 7.5 Gal/Hr  |  Approx. Fuel 8.5 Gal/Hr. |   Approx. Fuel 10 Gal/Hr  |
|        |Temp. |                           |                           |                           |
|  Feet  |deg F |     RPM & Man. Press.     |     RPM & Man. Press.     |     RPM & Man. Press.     |
-----------------------------------------------------------------------------------------------------
|        |      | 2100 | 2200 | 2300 | 2400 | 2100 | 2200 | 2300 | 2400 | 2300 | 2400 | 2500 | 2600 |
|       0|    59| 22.0 | 21.4 | 20.6 | 19.8 | 24.4 | 23.8 | 22.9 | 22.1 | 25.3 | 24.4 | 23.7 | 23.1 |
|    1000|    55| 21.6 | 21.0 | 20.3 | 19.5 | 24.0 | 23.4 | 22.6 | 21.8 | 24.9 | 24.0 | 23.4 | 22.8 |
|    2000|    51| 21.3 | 20.6 | 20.0 | 19.2 | 23.6 | 23.0 | 22.3 | 21.5 | 24.5 | 23.7 | 23.1 | 22.5 |
|    3000|    48| 20.9 | 20.3 | 19.7 | 18.9 | 23.3 | 22.6 | 22.0 | 21.2 | 24.2 | 23.4 | 22.9 | 22.3 |
|    4000|    44| 20.6 | 20.0 | 19.4 | 18.7 | 22.9 | 22.3 | 21.7 | 20.9 | 24.0 | 23.1 | 22.6 | 22.0 |
|    5000|    41| 20.3 | 19.7 | 19.1 | 18.4 | 22.6 | 21.9 | 21.4 | 20.6 | 23.6 | 22.8 | 22.4 | 21.8 |
|    6000|    37| 20.0 | 19.4 | 18.8 | 18.1 | 22.2 | 21.6 | 21.1 | 20.4 |  FT  | 22.5 | 22.1 | 21.6 |
|    7000|    34| 19.6 | 19.1 | 18.6 | 17.9 | 21.9 | 21.4 | 20.8 | 20.1 |  FT  | 22.3 | 21.8 | 21.3 |
|    8000|    30| 19.4 | 18.8 | 18.3 | 17.6 |  FT  | 21.1 | 20.6 | 19.9 |  FT  |  FT  |  FT  | 21.1 |
|    9000|    26| 19.1 | 18.6 | 18.0 | 17.4 |  FT  |  FT  | 20.3 | 19.6 |  FT  |  FT  |  FT  |  FT  |
|   10000|    23| 18.8 | 18.3 | 17.8 | 17.2 |  FT  |  FT  |  FT  | 19.4 |  FT  |  FT  |  FT  |  FT  |
|   11000|    19| 18.5 | 18.0 | 17.6 | 17.0 |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |
|   12000|    16| 18.3 | 17.8 | 17.3 | 16.8 |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |
|   13000|    12|  FT  | 17.5 | 17.1 | 16.5 |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |
|   14000|     9|  FT  |  FT  |  FT  | 16.4 |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |
|   15000|     5|  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |  FT  |
-----------------------------------------------------------------------------------------------------

Your airspeed is low because you were still climbing at 500fpm. I previously missed that point.
 
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The FAA sponsored some testing with 100L and other fuels for detonation limits.

Here is a link for searching. http://f10011.eos-intl.net/F10011/OPAC/Search/SimpleSearch.aspx

I think it should work. Let me know if it does not.

There are some good test reports in there. They use 100L as the baseline and several different engines. Some high compression and even turbocharged. I think a 320 is in there somewhere too. I think there are 6 reports when searching for detonation. Note the temperatures of heads, inlet air and note that the cooling airflow is extremely low measured as delta-p across the engine. I downloaded the reports for later study but it does show detonation comes on pretty quickly at some settings.

Detonation is more going to be a function of BMEP than % power. It is calculated in the better written papers.
 
At 10k + altitude your fuel flow seems to be high even at full throttle.

You may need to recheck your K factor for fuel flow, if its off that would affect the indicated %power.

Glenn Wilkinson
 
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