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Percent Power: Percent of what?

S

Stewie

Well Known Member
Hi all:
I am using the beginning of Phase I to break in my (I)O320. I have researched break-in methods on this site and others, and the rule of thumb is to run the engine hard, preferably >65-75% power. My Skyview gives me a percent power number, but it is simply based on whatever I have programmed into the computer as "full power". As I understand it, using percent power is basically a proxy number for measuring the cylinder pressure that helps seat the rings; but given this proxy, how do I know what "full power" is and if it accurately reflects the cylinder pressures I am really after? The engine is an O320D1A that has been converted to Airflow Performance FI. Pistons are 10:1, and the cylinders were ported and flowed by LyCon. The engine was run-in for 2.4 hours at LyCon. Full power on the dyno was 193HP@2700RPM. So it seems I have a variety of options for determining 100% power:
a) 193HP(corrected), per dyno testing
b) the published power chart for an O320D1A
c) something else
I'd greatly appreciate anyone's insight on this. I'm still studying/learning how these engines operate, so please be patient if I've overlooked the obvious here.
Eric
 
I would go with (a). If that's what the engine develops at 2700, then that is 100% power.
 
In my opinion the best way to figure percentage of power would be to figure out your fuel flow at full sea level power, 50 deg. LOP then any cruise fuel flow would be a percentage of that, assuming you're running LOP. The only time you're really looking at that number is to arrive to the best RPM to get 75% or thereabouts at wide-open throttle at a given cruise altitude. Generally I think using a factory chart for the basis of a look-up table in your EFIS is useless especially in this case since the engine has been modified.
 
There must be a way to calculate this ...

I am so overwhelmed with efforts to maximize the speed of the RV-6A that jeanine and I built and first flew in 2004 that I have just recently gotten to the engine. I too would like to be able to determine what the horsepower of the engine really is. I'm sure it is there and I will find the way but it is elusive. Obviously in the glass panel determination there is a formula for the calculation and the variables are sensible by the various probes/transducers in the system and the number you put in as the 100% value is part of the calculation. What you and I want to know is the real maximum horsepower of our engine even if it is a theoretical number or more important to me, THRUST which takes into account the propeller. Right now all I have is the meaningless 180 that comes from lycoming. I can take that as a baseline at 2700 rpm at sea level bur when I make a change I want to be able to measure SOMETHING that will give me a comparable. Right now all I have is my testing at 6.000 ft density altitude using the 3-leg USAR handicap procedure and post process the data using the NTPS spreadsheet for the 3-leg method. That shows a 5 kt gain for the Work Red, Marilyn and I did over the year end. Until I find the direct horsepower determination method (and I don't have time to look right now) I guess I will have to be satisfied with that for now. It is not technically satisfying. In your case it seems to me that you are stuck trying to rationalize a goop 100% number some where between the original Lycoming number and the Lycon number. If you have a good baseline maximum speed from before and a current maximum speed under the same conditions I have seen ratios thrown around that should be useable to derive the horsepower gain from the speed gain but I have not verified the accuracy of that method. Once you get that number you can add it to the Lycoming number and it seems to me you would have a reasonable estimate. I would not just accept the Lycon number as 100% power HP.

Bob Axsom
 
You guys sound like a group of PHD's trying to anilyze a cave man-(these engines we are flying behind). They are terribly inefficient, ancient in their technology, and no two are the same. The only good things about them is they are relatively reliable, and they produce rated power (or nearly so) at 2700 RPM. Trying to make precise calculations and making that standardized is futile in my opinion. As long as you are comfortable with your operation and you are not damaging the engine, what more can you hope for? Bob has a reason to maximize thrust when racing and that only relates to speed. Fuel burn doesn't matter, unless range is an issue when racing. For the rest of us (or me if I'm racing - only raced once, can't wait to do it again), we want to maximize speed v.s. fuel efficiency, without doing damage to our antique engine. I talked to an engineer at Lycoming about these matters and he told me that as long as my power setting is at or below 65% as they recommend for long term reliability - based on MP/RPM for my engine - it doesn't matter what I do with the fuel mixture. It can't hurt the engine, and the only difference is fuel flow and TAS. If I go LOP on all four cylinders with my engine (carbed) it gets so rough, I'm not comfortable with it. I'm not hurting my engine, but it's not even close to smooth, and I've lost at least 5 knots of speed. I fly mine to maximize - engine reliability/speed/fuel burn - in that order. Where I end up is probably not that much different than where everyone else is, except maybe Bob when he's out there smokin the course.
 
Mel and Bob, thanks for your input. I will use 193HP.

Again, I'm not really interested in HP except as a proxy measurement for indicating that the cylinder pressures are adequate to seat the rings. Some engineer at Lycoming (or elsewhere) has determined that for baseline engines at 2700RPM, 75% of the rated horsepower does the trick. But if an engine is high compression, or has other modifications that affect the horsepower available at 2700RPM, I wonder if that also therefore directly affects the cylinder pressures? I thought there might be another formula for determining break-in cylinder pressure visavis MAP, RPM, fuel flow, etc. For me it is mostly academic - I'm doing all I can to keep %power over 70% (home airport is 4848'), but out of curiosity I would like to know.

Taylor's ICE Theory book is on my Amazon to-buy list. Perhaps I'll be able to come back to this thread and post a comment once I read it.
 
Power

I would use the dyno sheet numbers. Just make sure they are "corrected" (adjusted for standard atmosphere)numbers. As installed in your aircraft the engine has different intake, exhaust, & accessory configurations than on the dyno and this will change the power output ,usually less but sometimes more, depending on weather the change helps or hurts power output. In an ideal world the engine on the dyno would be equipted just as it would be installed on the aircraft but........ good luck Russ
 
We have an 0- 320 and use this formula which seems to work well for us.

POWER SETTING PERCENTAGE
RPM X 100 + MP (INCHES)
60=100%
57=95%
51=85%
48=75%
45=65%
42=60%
39=55%
36=50%
33=45%
30=40%
 
In my opinion, you should be using Lycoming book rated, not actual measured maximum horsepower to determine the 65% level for longevity, break-in and detonation margin.

This means that the EFIS could display greater than 100% power at times due to the high compression pistons and ram-air effects.

A thought experiment works this way: Let's assume that you have 14:1 pistons, which is absurd, but will juice the power to 203 hp from an 8.7:1 180 hp engine. If you run this engine at 65% of 203, you'll get 132 horsepower.

This represents 73% of the rated horsepower, well above Lycoming's recommendation of 65%. You'll probably have detonation problems with these pistons and destroy the engine.

....So convert the Lycoming ratings to an absolute horsepower and use it for break in, regardless of what your engine dynos at.
 
Your first sentence sums it up pretty well.
I am using the beginning of Phase I to break in my (I)O320. I have researched break-in methods on this site and others, and the rule of thumb is to run the engine hard, preferably >65-75% power.
Click to expand...
It's a rule of thumb, not very precise or scientific and for break in, more power is better. Surely you can trust your Dynon to be in the upper ball park of 75%
of whatever they base their percent power on.
After having your engine run for more than 2 hours on the Dyno, your rings have probably been seated. I am sure Lycon provided you with a good test protocol to follow for the rest of the break in procedure.
I'd go with (a) and program your Dynon with the numbers from the Dyno Test.
You now own a Lycon 0-320 not a Lycoming O-320 and their charts are useless
for your specific engine.
 
The original question was concerning break in of a new engine. What is wrong with using lycomings power curve chart.
For an O-320 B or D it gives 160hp as 100%
75% of 160 is 120hp and that is shown at 2450rpm.
For the purposes of breaking in surely that would be a good number to use.
 
I'm using the Lycoming manual

I can't speak to his particular engine, but I have an Aerosport IO-375 with 9 to 1 pistons, 205 HP. Aerosport sent me the Lycoming operators manual to use with it, and the break in instructions from both Aerosport and the Lycoming manual state specifically to use 75% power during the break in period. I also called Aerosport to find the specific model in the Lycoming manual to use. So I will be going with the 2450 RPM for the break in.
 
Last edited:
re "percent power: percent of what?"

Seems to me the most important value to determine here is when it is safe to lean? You do not want to induce detonation.

My rule of thumb of limiting to 75% of rated power is 23" manifold pressure. Pull the throttle back after take off or climb at WOT until MP is 23" and lean. (A key element for determining power is atmospheric pressure in the intake manifold. Temperature is also a factor but 23" MP probably is safe at any temperature in determining 75% power, it certainly is in normal summer OATs.)

If that method does not suit you, use the dyno numbers to determine HP vrs fuel flow. The dyno data should reflect the fuel flow at 193HP, I will guess it is about 16.4 gph. Whatever it is, percent of max HP can be derived by taking a percentage of it, i.e. 75% of 16.4 is 12.3 gph.

How accurate is an EFIS derived % of power(?) - depends on inputs I suppose. I've never used it. It is a cute number but I wonder if it is accurate.

Beyond all that, knowing how far you can fly on a tank of fuel is useful information. That can easily be determined by setting various fuel flows at various altitudes and noting the TAS. The data can be interpolated for altitudes in between and is reasonably accurate.
 
The thought experiment

Vern got to the worm of doubt/curiosity I have about this with his thought experiment: it seems the rings and cylinders for any particular O320 (or O360, etc.) are essentially the same (please correct me if I'm wrong). If that is true, then 120HP (75% of 150/160HP) is generally required to seat those rings. Thus, regardless of the full power potential of one's engine, as long as you can produce at least 120HP, that should be sufficient to seat the rings. BUT, if mods to the engine (porting, compression) etc affect BMEP as well as HP, then perhaps more (or less) than 120HP is required. I'll call LyCon and/or Lycoming see if they have any thoughts on the matter. Interesting discussion, thanks for everyone's input!
 
Percent power is just that. It's percent of max power. The absolute HP isn't relevant. If you make an engine more efficient in using the air that it has (which is basically what higher compression pistons do), then that doesn't move where the 75% point occurs. It just means that when you are at 75%, you have more absolute HP, but you still have 25% more power the engine can create in some situations.

When you look at a power chart in detail, it's figuring out how much air goes into the engine. You need RPM, manifold pressure, altitude, and OAT. Altitude and OAT give you the density of the air outside the engine. RPM and MAP give you how much air makes it into the engine and how much work you loose to pumping losses. Ignore OAT and Altitude at your own risk like some EFIS systems do- they change the number by 30%.

The SkyView doesn't follow what you program into it. When ROP, the calculations are based on the full Lycoming power charts, no matter what HP number you give it. The charts return % power, not HP, so the HP number you put it has no relevance. We run the same calculations for a O-320, O-360, or O-540. If you take a Lycoming chart for a 320, 360, 540, even with variants with different compression, and hold them up against one another, you will see they are identical given the same input numbers.
Why? Because % power is the same for the same given RPM, MAP, OAT, and altitude because it's percent, not absolute power. Move from a 360 to a 540 and all that changes is two more cylinders which are producing the same % power as the first 4.

SkyView uses a HP number ONLY to figure out if you are ROP or LOP. The % power charts are for ROP only. The instant you are LOP they are worthless. So we ask for the power of your engine so we can figure out if you are LOP or ROP and both tell you and run correct calculations on both sides of peak.

So what am I trying to say simply? The % power number on your SkyView is accurate. It doesn't matter that your engine is modified unless you changed cam profiles. You can prove this to yourself by going to sea level and doing a full power takeoff at your dyno rated RPM. It will read 100%. It won't read 128% power even though your engine is producing 28% more HP than a standard 320.

However, what you should discuss with your engine builder is if they want you to break in at a HP or a % power. A standard O-320 breaks in at or above 75% power, but that is 112 HP. On your engine 75% power is 145 HP. So, is break in based on absolute HP or % power, given that you haven't changed the engine dramatically?

You cannot use fuel flow to determine % power when rich. You can throw almost a 2:1 ratio of fuel at an engine and still be ROP, so you can be 75% power at 12 GPH or 16 GPH. This is the beauty of the % power number which is far from a "cute" number. It's an accurate number of how much power you are producing, no matter where all your knobs are set. It lets you find the best place to operate your engine. Would you rather produce 65% power with 390 degree CHT's and 11 GPH, or 320 degree CHT's and 8.1 GPH?

--Ian Jordan
Dynon Avionics
 
break-in by the 'factory'....how it's done???

Stewie said:
Vern got to the worm of doubt/curiosity I have about this with his thought experiment: it seems the rings and cylinders for any particular O320 (or O360, etc.) are essentially the same (please correct me if I'm wrong). If that is true, then 120HP (75% of 150/160HP) is generally required to seat those rings. Thus, regardless of the full power potential of one's engine, as long as you can produce at least 120HP, that should be sufficient to seat the rings. .... I'll call LyCon and/or Lycoming see if they have any thoughts on the matter. Interesting discussion, thanks for everyone's input!
Click to expand...

This also got me thinkin'.....does it require an explosion in the combustion chamber to seat rings? I can imagine that at some factories, they could fully prep an engine, but just spin it without fuel on a test stand, couldn't they? Full pressures would be seen on compression, and you'd have a very clean break-in without all the residues of combustion.

just sayin'.....can it be done this way? seems almost too easy! :)
 
I had an engine overhauled some years back with new millennium cylinders. The guy honed the new cycinders which removed ridges from the manufacture process. You could feel the ridges before honing, they were gone after honing.

He told me the engine is broke in, go fly it like normal. This honing job is what they do if the rings do not get rid of the ridges and glazing occurs, if the engine is not run hard and furious when first started.

I believe a dyno run essentially does the job. My first engine from Barrett used very little oil from the get go - oil consumption was stable - the rings were seated.
 
The short version of a paper I will write someday

I've been spiraling around this issue for a couple of years and I'm still not happy with what I "know". It's a surprisingly difficult subject.

But no less an authority than Walter Atkinson has told me that the LOP ratio between fuel flow and HP is the best real measure of power in an airplane in flight (regardless of altitude, within reason). The SFC is a standard way of expressing this but the Advanced Pilot guys offer an easy calculation which is essentially the inverse of SFC.

In other words, if your SFC is 0.40 pounds per HP per hour and if you are willing to use, for this purpose, 6 pounds per gallon, then the pounds per hour is 6 x gph. Example, 10 gph is 60 pounds. If you divide 60 pounds by 0.40 you get 150 so that is your HP at that moment. The shortcut would be 15 times fuel flow. For the big Conti's, the SFC that Advanced Pilot measured was as low as 0.38 which would make the multiplier 15.8.

My personal opinion, not supported by anyone else so far, is that you could use fuel flow with a different SFC such as "best power" to get the answer for different conditions. Best power is easy to approximate by simply adjusting the mixture at constant throttle and altitude until RPM peaks. An SFC for best power will vary with compression ratio just as the SFC for LOP does. You can see from some data that a value of .5 to .55 works. My Superior's best power when it was tested at the factory was in the low 0.6 range at full power for acceptance testing. Your engine should have a similar number.

My charts that came with my 180 HP Superior IO-360 and some similar charts that I've seen for other engines clearly show an SFC curve. That means that only under certain conditions do you get to the lowest value. For my engine it is around 8 gph where that happens. It tends to vary from engine to engine with compression ratio and other factors to a lesser degree. CR is the big one and the physics of it are the reason.

Walter has responded to my inquiries twice that he ignores the altitude correction that the manufacturer charts show (for this SFC based calculation). I'm not completely convinced, but he and the other Advanced Pilot guys know much more than I do.

What's the simple answer for "what is my true, installed HP"? HP is torque times RPM over 5252. Do a runup at full power on the ground while tied to something really strong. Use analog scales under the wheels and measure the torque. Use a digital tach. Compute your HP. I have not been able to do this in real life to my satisfaction, but it's the only feasible and correct way once the engine is in the airplane. While you are at it, observe fuel flow, MP. Calibrate your power chart for the DA, RPM and MP.

BTW - most "experts" believe that at 8500 feet you cannot exceed 75% if you don't exceed your rated RPM max. I think I can demonstrate that the correct altitude for that limit is much higher, perhaps 10,000 or more. Take special note of any increase in MP as a result of speed because if your MP is, for example, 1 inch higher than ambient then your engine is 2000 feet below the airplane. Many RV pilots have observed this MP increase. I do, often.

My final shot: for a really solid overview of cylinder pressures:
http://www.epi-eng.com/piston_engine_technology/bmep_performance_yardstick.htm
"Note that BMEP is purely theoretical and has nothing to do with actual cylinder pressures. It is simply a tool to evaluate the efficiency of a given engine at producing torque from a given displacement."
 
Great post!

RVs do generate ram air effect, but not 2000' of it in my experience. Sub 1" in MP is about right.

There is a formula which is easily found (not on my iPad ) that calculates HP when ROP and it is MP not altitude that matters. Mass Airflow is all that matters when ROP.

What Walter has given you advice on for LOP power calculations is correct. Data backed fact.

Onto the issue of engine break in best done ABOVE 75% not necessarily at 75% power.

Object is to gain high Mean Effective Pressure, not high peak pressure. The difference is significant.

Best way to achieve this is low level, full bore (WOT) 2500-2700 RPM, and around 80dF LOP.........yes THE BEST WAY is exactly as I wrote. This gives best MEAN EFFECTIVE PRESSURE, lower CHT, lower peak ICP and far less deposits. It does all it should and nothing it should not. EOM

The next best way, and if you can't get the above to work is full bore, full rich.

Simple as that :)

I know from seeing the test stand data which is better, Walter would agree, so would John Deakin, and George Braly. Why? It is not our opinion. It is the data.

Most posts are only opinion. Data has no opinion.
 
Getting the idea now

First off, thanks for your input Ian. I understand better now how the various calculations are being made by SV. Dave's posting also motivated me to do a bit more research, and I found this article by the Gami folks to be enlightening, particularly as concerns BMEP:
http://www.gami.com/articles/bttfpart1.php

This thread is reminiscent of a zen koan ("What is the sound of one hand clapping?"...). Once I truly understand the nature of my question, the answer probably won't matter...:D
 
humbly disagree on a detail

RV10inOz said:
Great post!

RVs do generate ram air effect, but not 2000' of it in my experience. Sub 1" in MP is about right.
...
Most posts are only opinion. Data has no opinion.
Click to expand...

First, I want to express my gratitude to the Advanced Pilot folks and Walter in particular for the very patient help I've been given.

Since I agree on the latter point, here's the data to support my assertion of approximately 1" and/or 2,000' (which is over-stated - oops)

My MP instrument is calibrated by the same instrument as was used for my IFR certs. I do regularly see 1" or even more. This can be observed at a constant altitude (8000 for example) with speed-up. It can also be observed when my readings are compared to the accepted values for ambient.

If my measurements themselves are calibrated and if I can also observe the change as speed increases then I must conclude that my "boost" equals or exceeds 1". I admit that not all cowls and not all intake systems are equal and thus mine may be doing better than others. Mine is SJ for both the snout and the airbox. My engine is unmodified. My air cleaner is K&N from a big Ford which was specified by the James's for that airbox.

I used a crude rule of thumb: 1/2" per thousand feet, so 1" is 2000'. More or less. But, when I look at the atmosphere tables the rule fails: 8000'=22.22, 6800'=23.26". So at typical speed and power testing altitudes, 1" is only worth 1,200'. Still, that's enough to affect your other calculations. These are density altitude so temperature corrections are necessarily baked in.

I'm aware that a net increase of that magnitude, downstream of the airbox, may exceed the theory. My only thought about that is that my snout is very close to the trailing edge of the down blade so we are not talking about the free stream velocity but I can't estimate the increase. I don't have any other explanation. But the data look quite solid to me.
 
It is an interesting thought experiment going on here....but I am not sure what you disagree with, although maybe it is not that important. I assume it was my rough rule of thumb claim of sub1"MP ram air effect?

This will vary all over the place, different cowls air cleaners you name it.

Your set up may well be giving 1" or more, I have never bothered to get too scientific about measuring it, however I do have a photo of a day with QNH1011 or 29.86"Hg, and at 1000' I was showing 29.3"Mp. That equals a loss of only 0.56" which is effectivelyy a 0.5" gain in ram air.

At say 8000' on a QNH1015 day (29.98") I have 22.6"MP compared to 22.2". Of course one decimal place and a few small tolerances mean this could be 0.5 also.

The intersting comparison is when compared to a Bonanza or C210. Similar airspeeds but they are often an inch less in MP due to system losses and less ram air effect.

So basically the filter and system losses are probably worth about 0.5" and mine is typically raming that plus a bonus of 0.5 to 0.75, depending on the day/moon/tide/coffee temp :D


I suspect, but do not know, but you may be doing better than most.
 
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