Percent Horsepower (Dynon EFIS) accuracy question

[llaux]

As I've been working to break in my engine (the IO-390-EXP119 variant), I've been watching the Dynon EFIS' 'percent horsepower' widget and then comparing it to the performance chart for this engine in the 'Aircraft Power' iPad app and also the performance data chart from Lycoming Installation and Maintenance Manual for the IO-390-C series manual (the engine variant Lycoming tole me was closest proxy to the EXP119).

I've set up the Dynon with the correct engine information and I notice that my percent horsepower shown on the EFIS while in flight is about 4% higher than the performance chart suggests.

I'm wondering what others have experienced here.

As a follow up question, the Lycoming manual says to break in the engine above 65% cruise power or more (and I also am aware that we're supposed to run the engine a lot harder than this during break in) and the manual also says to stay below 8,000 feet density altitude because above that, the engine won't generate enough horsepower. Well, from my own empirical testing, I've found that (for example) at 8,500 density altitude, at 2660 RPM, I get 24.1" MAP and the EFIS shows 83% HP. Plug this into the Aircraft Power app and the chart data says I'm getting 173HP or 80.5% HP.

Am I understanding this incorrectly, or is the engine in fact producing plenty of % HP above the break in threshold even at this altitude? Or, is there some other reason Lycoming wants the engine operated below 8,000 during break in?

Thanks in advance!

 

[lr172]

As I've been working to break in my engine (the IO-390-EXP119 variant), I've been watching the Dynon EFIS' 'percent horsepower' widget and then comparing it to the performance chart for this engine in the 'Aircraft Power' iPad app and also the performance data chart from Lycoming Installation and Maintenance Manual for the IO-390-C series manual (the engine variant Lycoming tole me was closest proxy to the EXP119).

I've set up the Dynon with the correct engine information and I notice that my percent horsepower shown on the EFIS while in flight is about 4% higher than the performance chart suggests.

I'm wondering what others have experienced here.

As a follow up question, the Lycoming manual says to break in the engine above 65% cruise power or more (and I also am aware that we're supposed to run the engine a lot harder than this during break in) and the manual also says to stay below 8,000 feet density altitude because above that, the engine won't generate enough horsepower. Well, from my own empirical testing, I've found that (for example) at 8,500 density altitude, at 2660 RPM, I get 24.1" MAP and the EFIS shows 83% HP. Plug this into the Aircraft Power app and the chart data says I'm getting 173HP or 80.5% HP.

Am I understanding this incorrectly, or is the engine in fact producing plenty of % HP above the break in threshold even at this altitude? Or, is there some other reason Lycoming wants the engine operated below 8,000 during break in?

Thanks in advance!

The basic power charts use pressure altitude, not density altitude. WHile increased temps reduce the quantity of O2 in a given parcel of air, and therefore the max potential HP output, most engines can't monitor this, so it is not included in the HP calcs and instead use standard temp. Though the right side of the big chart allows you to adjust output based upon temp if you go that far. Further, HP will only match the Lyc charts if the same mixture is used as lyc used when creating the charts, which is best power mixture IIRC. Anything richer or leaner than that will drop HP output. Though they usually also include one for best economy, which means peak EGT to Lyc. Also, the Lyc charts are all standardized to sea level and most don't fly at SL, therefore further adjustment is required; also found on the right side of big chart. Dynon likely takes mixture into account in the HP calc, via EGT, hence the difference.

At 8000', the atmospehere has approx 75% of the O2 found at SL for any given cubic parcel size. THerefore, in theory, you can't make more than 75% HP above 8K, hence the recommendation for break in. IMHO, you should run it hard during break in, but whether that is 73% vs 76% HP is likely not relevant as these are generalized recommendations. We ultimately care about high combustion pressure for break in, as this provides more pressure on the rings against the cyl wall, and HP is not the best way to correlate that. Low RPM/High MAP creates more cyl pressure than High RPM/Low MAP at the same HP level. Therefore, dialing down the RPM on a CS prop HELPS break in. At least up to the point is goes too far and creates other issues (i.e. lugging)

 

[Carl Froehlich]

It just so happens I did a full power run at 8500’ today to collect engine data. Density altitude was 9680’, displayed MP was 22.7”, corrected MP was 22.3”, and the Dynon EMS displayed 76% power.

This objective of the data run was to measure MP to determine if there was any ram air effect. After a lot of data correction the answer is MP was ~0.4” above atmospheric.

Carl

 

[llaux]

The basic power charts use pressure altitude, not density altitude. WHile increased temps reduce the quantity of O2 in a given parcel of air, and therefore the max potential HP output, most engines can't monitor this, so it is not included in the HP calcs and instead use standard temp. Though the right side of the big chart allows you to adjust output based upon temp if you go that far. Further, HP will only match the Lyc charts if the same mixture is used as lyc used when creating the charts, which is best power mixture IIRC. Anything richer or leaner than that will drop HP output. Though they usually also include one for best economy, which means peak EGT to Lyc. Also, the Lyc charts are all standardized to sea level and most don't fly at SL, therefore further adjustment is required; also found on the right side of big chart. Dynon likely takes mixture into account in the HP calc, via EGT, hence the difference.

At 8000', the atmospehere has approx 75% of the O2 found at SL for any given cubic parcel size. THerefore, in theory, you can't make more than 75% HP above 8K, hence the recommendation for break in. IMHO, you should run it hard during break in, but whether that is 73% vs 76% HP is likely not relevant as these are generalized recommendations. We ultimately care about high combustion pressure for break in, as this provides more pressure on the rings against the cyl wall, and HP is not the best way to correlate that. Low RPM/High MAP creates more cyl pressure than High RPM/Low MAP at the same HP level. Therefore, dialing down the RPM on a CS prop HELPS break in. At least up to the point is goes too far and creates other issues (i.e. lugging)

Larry, thanks for the quick and detailed response. So just to confirm that I'm understanding correctly:

A) I should defer to the Dynon % HP reading because it's taking more factors into account than the charts alone, and is therefore more accurate of the %HP that my engine is actually producing?

B) As far as safely breaking in the engine, it's okay to operate above the 8,000 MSL (or 8,000 density alt) level as long as indicated percent horsepower is above Lycoming's 65% threshold (this assumes bullet point A is true)?

C) Further, I could enhance the effectiveness of break in by running the engine oversquare and doing so would do more to actually cause the break in cylinder // ring friction even at a high altitude/high density altitude?

 

[lr172]

As carl shows in the previous post, anything above 8000' is going to net less than 75% power. Nothing wrong with using the Dynon for HP %age. If you desire to stay above 75% then stay below 8K. Just know that while 80* ROP at 8K will net 75% pwr, if you lean it, it will be less than 75% pwr. Lower RPMs with higher MAPs will be beneficial for break in, so suggest 22 or 23 for cruise instead of 24 or 25 and this will be true at any altitude, though the higher you go, the less MAP you can get, so don't over do it. The more you move to the low RPM/High MAP area, the lower the detonation margin goes (the extra pressure that is helping to seat the rings is also eroding your detonation margin), so don't do this in your high power climb phase. Perfectly safe in the 75% cruise pwr regime.

 

[rv6ejguy]

This topic has been discussed multiple times before.

We don't know what math the EFIS is applying here for one thing but we do know that it isn't taking into account humidity or ignition timing. It may also not be accounting for AFR or baro correction for reduced exhaust back pressure with altitude increases. Each of these can affect hp by several % at the extremes.

I wouldn't rely on the accuracy here to be even within a few percent given these factors.

 

[BobTurner]

This topic has been discussed multiple times before.

We don't know what math the EFIS is applying here for one thing but we do know that it isn't taking into account humidity or ignition timing. It may also not be accounting for AFR or baro correction for reduced exhaust back pressure with altitude increases. Each of these can affect hp by several % at the extremes.

I wouldn't rely on the accuracy here to be even within a few percent given these factors.

+1. I know of at least one efis that doesn’t have mixture data in the calculation. Its ‘power’ number is way off when lean of peak. Of course the same is true of standard charts.

 

[RV8JD]

Dynon %Power

Dynon updated its Lycoming and Continental %Power calculations with SW version v16.0.1 (02/01/2021).

Prior to first flying with the SW that included the new %Power calculations (v16.0.1), I manually changed the "Engine Type" from the "LYCOMING - DEPRECATED" (previous calculations) choice to the new "LYCOMING" choice, to take advantage of the new percent power calculations.

I noticed that the % Power numbers were higher than what I was used to seeing at a given power setting. To explore this, several times in-flight I went back and forth between the "LYCOMING - DEPRECATED" to the new "LYCOMING" choice at the same power settings. I found when ROP the newer % Power calculations gave roughly 3% to 4% higher readings (e.g., instead of 70% it would give 73% or 74% Power). I did the same comparison LOP a couple of times and the %Power numbers were the same between the two "Engine Type" choices. Note that Dynon does try to do LOP calculations correctly (see article linked below).

My impression is that the new ROP % Power calculations indicate too high, and the previous %Power calculations are closer to the truth, but I need to do a little homework first to be sure. It would be helpful if Dynon Support would describe the improvements that were made to the %Power calculations and explain why the think they are more accurate.

Note that the current SW (v16.4.4) still allows choosing "LYCOMING - DEPRECATED" if one desires to continue to use the older %Power algorithms (I do).

For others that may not have seen it, years ago Dynon published the article below on how they calculate Percent Power. Unfortunately, several figures no longer show in the article.

https://preflight.dynonavionics.com/2014/02/did-you-know-percent-power-dynon-way.html

I'd like to see Dynon update the article and restore the missing figures and explain how they now calculate % Power, or at least explain the differences between "Lycoming - Deprecated" and the new "Lycoming" choice.

(I posted these comments on the Dynon Forum back in 2021.)

 

[Carl Froehlich]

SNIP

For others that may not have seen it, years ago Dynon published the article below on how they calculate Percent Power. Unfortunately, several figures no longer show in the article.

https://preflight.dynonavionics.com/2014/02/did-you-know-percent-power-dynon-way.html

I do mostly cross country flying of late, and this is always LOP. I can report that when the SkyView says I’m LOP the engine is within 0.2 gph of actual LOP fuel flow.

Note - I swapped out a couple of injectors to get the GAMI spread down to 0.0 to 0.1 GPH. I suspect the tighter your GAMI spread the better the system accuracy on identifying when you go LOP.

Carl

 

[rv6ejguy]

Yup Bob and that's what these should be thought of as a rough guide rather than an exact percentage.

Many people assume digital displays are dead on accurate. They're only as accurate as the variables considered and the calcs involved.

There are multiple variables affecting hp, some minor, some quite significant.

 

[sailvi767]

I have noted that many RV OAT probes are often way off on temperature. That in turn is going to cause an incorrect density altitude reading. Worth checking against the forecast temp. They all seem to read high in flight.

 

[llaux]

As carl shows in the previous post, anything above 8000' is going to net less than 75% power. Nothing wrong with using the Dynon for HP %age. If you desire to stay above 75% then stay below 8K. Just know that while 80* ROP at 8K will net 75% pwr, if you lean it, it will be less than 75% pwr. Lower RPMs with higher MAPs will be beneficial for break in, so suggest 22 or 23 for cruise instead of 24 or 25 and this will be true at any altitude, though the higher you go, the less MAP you can get, so don't over do it. The more you move to the low RPM/High MAP area, the lower the detonation margin goes (the extra pressure that is helping to seat the rings is also eroding your detonation margin), so don't do this in your high power climb phase. Perfectly safe in the 75% cruise pwr regime.

Lots of good discussion here on the thread and it was helpful to review the Dynon webpage that was shared where they discuss how their computer handles percent power calculations.

One thing that I'm still confused about after reading the Dynon post, is how to reconcile that the engine manufacturer says you can't produce more than 75% HP above 8,000' (which seems to be based on the proportion of air in the atmosphere at that altitude), but Dynon says the following (and also Dynon's comment seems to check out when you change the variables in the performance charts):
"while it may seem counter-intuitive, it requires less work to do this the higher you go, since the engine has less atmosphere to work against. Given the same MAP and RPM, the higher you are, the more power the engine produces. You can see this on the chart as about a 25% difference in percent power as you go from sea level to 14,000 feet."

For example, using the performance chart (and accounting for OAT difference from standard // taking into account density altitude effect) if MP = 24" and RPM = 2600 and OAT = 70*F, at a Press. Alt = 6,200 then HP% = 78%. IF I change only Press Alt to = 4,000, then HP% falls to 76%.

 

[rv6ejguy]

One thing that I'm still confused about after reading the Dynon post, is how to reconcile that the engine manufacturer says you can't produce more than 75% HP above 8,000' (which seems to be based on the proportion of air in the atmosphere at that altitude), but Dynon says the following (and also Dynon's comment seems to check out when you change the variables in the performance charts):
"while it may seem counter-intuitive, it requires less work to do this the higher you go, since the engine has less atmosphere to work against. Given the same MAP and RPM, the higher you are, the more power the engine produces. You can see this on the chart as about a 25% difference in percent power as you go from sea level to 14,000 feet."

Dynon is correct. Many people fail to appreciate in how the reduced exhaust back pressure affects power as altitude increases. On our EFI, the standard ECU baro correction is 1.5% per 1000 feet so that's 21% at 14,000 feet. Dave Anders verified this in flight testing. Power doesn't drop off as much as many people think on a naturally aspirated engine. It's only proportional to MAP at a given altitude and rpm.

The same thing applies to turbine engines where delta across the turbine increases with altitude- one of the reasons why BSFC is better up high with turbines.

 

[llaux]

So, to sum it up here (and keeping in mind Larry's point that percent HP isn't the best proxy for cylinder pressure as far as break in is concerned), one can safely break in an engine above 8,000 FT MSL/Press. Alt/Density Alt because of the point made by Dynon and further explained by Ross?

 

[jabarr]

Seems odd that you can get over 24 inches MAP at 8500 ft. That's nearly 2 inches of ram pressure. That would sure run up the %power number.

 

[lr172]

Seems odd that you can get over 24 inches MAP at 8500 ft. That's nearly 2 inches of ram pressure. That would sure run up the %power number.

You can't. But the point Ross is making is that it is not just about RPM and MAP. I believe Ross is saying that reduced backpressure is increasing power more than the lower MAP is reducing power. This is not surprising as I believe that the Lyc charts are all based upon dyno runs near SL that are then mathematically corrected for air density. Pretty sure they are not dragging the Dyno up to Leadville to determine the exact %age of power produced, so not surprising that Dave found the %age pwr calcs to be theoretical vs actual.

 

[lr172]

So, to sum it up here (and keeping in mind Larry's point that percent HP isn't the best proxy for cylinder pressure as far as break in is concerned), one can safely break in an engine above 8,000 FT MSL/Press. Alt/Density Alt because of the point made by Dynon and further explained by Ross?

I am not sure we can assume that. Ross is saying the HP % can hold in much longer than the reducing MAP would lead us to believe way up there in altitude and I believe that. HOWEVER, when it comes to break in/ring seating, we really only care about combustion pressure. I am speculating that cyl pressure will drop with low MAP (vaguely recall seeing charts showing that), even though the power output isn't dropping that much due to reduced exhaust back pressure. There are other things like mixture and timing advance that also impact comb pressure.

There are many variables in play here and outside of a lab, we cannot measure combustion pressure, so we have to infer it from other variables and don't believe anyone has done much study on that outside of the GAMI guys. Not a scientist and have no combustion pressure instruments or a dyno, so can't really say if combustion pressure is dropping too low above 8K. Not knowing all the details leaves me thinking that staying low, with a ROP mixture at higher MAPs is wise for best breakin. Then there are the guys that say the break in is done within 2 minutes; which I don't believe BTW. We could debate all day and still have no real answers.

 

[jabarr]

My point is that the OP stated the he gets 24.1 MAP at 8500 ft. No wonder his percent power is so high. I'm suggesting his MAP reading is incorrect.

 

[lr172]

My point is that the OP stated the he gets 24.1 MAP at 8500 ft. No wonder his percent power is so high. I'm suggesting his MAP reading is incorrect.

Likely true. Though I have seen several post, possibly from Dan, stating they are getting close to an inch of MAP rise from tightly sealed ram air in RVs. Dan is pretty particular with his testing and would trust whatever he found.

 

[llaux]

My point is that the OP stated the he gets 24.1 MAP at 8500 ft. No wonder his percent power is so high. I'm suggesting his MAP reading is incorrect.

It could be off, but perhaps there's another explanation -- the 390EXP119 has induction parts from Airflow Performance and, paired with the fact that the snorkel has quite a large(r) diameter opening (as compared to the standard Bendix throttle body) and perhaps this design allows for greater efficiency that could explain a higher MP than one might expect.

 

[RV8JD]

Adjusting Manifold Pressure in the SkyView EFIS

The traditional quick check of your MP gauge is to set field elevation, note the value in the Kollsman window, and subtract 1" Hg per 1000 feet above sea level. For example, if your field elevation is 200 feet set 200 feet on the altimeter, then subtract 0.2 from the Kollsman. In theory, if the Kollsman said 29.92, the MP gauge should read 29.72.

Like any rule-of-thumb, it's close, but the answer will be as much as 0.1" Hg too high below 6000 feet, and could be worse if your altimeter is out of calibration.

Here's a simple web calculator which will work on your phone, at the airport. Seems to be pretty accurate.

http://www.csgnetwork.com/stationpressurecalc.html

Input the altimeter setting from your best local source (AWOS, ATIS, etc.) and the altitude where your airplane is parked. Tap calculate, and compare "station pressure" to your MP gauge, engine off. The calibration risk is the accuracy of your local FAA equipment.


There is a way to adjust Manifold Pressure (MP) in the SkyView. MP can be adjusted +/- 0.5" Hg.

Choose/Highlight the MP sensor (Pressure Sensor 100434-000) in the Sensor Input Mapping pic below:

And then touch on the Sensor number (100434-000) and pick which adjustment you need.

I needed -0.5" HG, as shown below.

 

[rv6ejguy]

In round figures, you'd get about a 1 inch MAP rise at 8000 feet and 170 KTAS with 100% ram recovery.

 

[Carl Froehlich]

In round figures, you'd get about a 1 inch MAP rise at 8000 feet and 170 KTAS with 100% ram recovery.

In round numbers, at 8000’ you will get ~0.5” MP increase over atmospheric when using the Van’s snorkel (IO-360-M1B, RV-8).

Carl

 

[rv6ejguy]

You can't. But the point Ross is making is that it is not just about RPM and MAP. I believe Ross is saying that reduced backpressure is increasing power more than the lower MAP is reducing power.

The reduced back pressure offsets some of the loss of MAP at 8000 feet but you're still losing power as you climb.

 

[rv6ejguy]

In round numbers, at 8000’ you will get ~0.5” MP increase over atmospheric when using the Van’s snorkel (IO-360-M1B, RV-8).

Carl

Indeed on the RV-10 with Showplanes/ Barrett cold air setup, (horizontal- 80mm throttle body) we see MAP around 0.5 inches over the baro reading from the SDS ECU under these conditions. About 1.7% more hp than you'd have without ram air.

Unless you stop the air, ram recovery will not reach 100%.

 

[llaux]

Per Carl's post above, I double checked the accuracy of the MAP sensor by following the instructions outlined.

Set field elev in the alt and got 29.88 in the Kollsman window. Subtract 1.071" from that (since my field elevation is above 1,000 ft MSL // it's 1,071 MSL) value gets you 28.81.

My MAP sensor was indicating 28.9, so it looks like my MAP sensor is high by 0.09".

So, I would say that this concludes that sensor error does not account for my engine's ability to get a higher than expected manifold pressure at the density altitudes we were discussing in the prior posts. Unless there's something else I'm not thinking about here.