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Calculating Horsepower, % power

vlittle

Well Known Member
I've spent quite a bit of time going through the Lycoming power curves.

If I had a constant speed prop, I could work out HP directly from MAP, RPM and altitude.

Unfortunately, I have a FP prop, and I'm finding it difficult to come up with a simple horsepower table...

Basically, what RPM setting for a particular density altitude will give me 75% power (or 65 or 55)? Engine is a 160 HP O-320, standard Sensenich prop (RV-9A).

I did manage to determine HP from RPM and fuel flow for two mixture points, but that's an indirect measure. [As a note, my engine monitor also uses fuel flow to determine %HP, which is only accurate at best power mixture and not LOP or above 75% HP].

I know this is not a simple question, but has any develop a horsepower table, or a formula for my engine/prop combination? I can spend hours flight testing, or just borrow the work of someone else who is smarter than me. I could 'borrow' one from an existing aircraft (Piper), but that's quite a bit different than an RV.

Vern Little
9A
 
Map gauges are for everyone

Vern get a MAP gauge. You don't need a constant speed prop to put one in. A you can see a MAP would solve your problem. I suppose you could install a MAP sensor temporarily and go up and fly all kinds of conditions (alt/temp) and RPMs, coming up with curves based on RPM/altitude alone, but why? Once you have a MAP gauge you'll not want to go back. It's a pet peeve of mine, MAP gauges are for everyone. Van's gauge is only $72. Add a few bucks in hose and fittings you're good to go. (You might find one on eBay for $10.)

I believe airplane manufactures come up with the power tables for their fixed pitch planes not Lyc. You could not use those tables even if the engine is the same for many reasons, like the props different and you're going 60 mph faster. You're RV makes more power (MAP) than a factory plane with the same engine because you have higher MAP (higher RAM air and free flowing intake).
 
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No Map

Vern, you don't need a MAP gauge, that's obsolete technology if you have a F/P prop & already have an engine monitor. You would just be calculating manually from the MAP/RPM setting chart what the monitor is already telling you-- the % HP for a best power mixture setting. The HP at a given MAP/RPM setting at a given density altitude can vary 10-15% depending on lean-rich mixture setting. When you start leaning, the RPM will gradually decrease & you will end up with a different power setting. So-- at low altitudes, set the % power you want to cruise at with the engine monitor, at best power mixture, note the RPM, then advance the throttle to get 7-8% more power, then lean until RPM drops back to the original setting. At high altitudes where you are already full throttle, just lean for about 100RPM drop & figure you are about 7-8% power less than what the monitor shows. Either way you should be LOP which is where you want to be. You can experiment to get it just right.

BOB
 
% Power

I do have a MAP guage on my engine monitor.

After much figgerin', I have decided to use the rule of thumb for my O320 at standard temperatures...

MAP+RPM/100=47 for 75%, =44 for 65%, =41 for 55%.

I'll cross check that with fuel flows for best pwr/econ as required, as derived from the Lycoming manual (actually for an IO-320, but very close to the O-320).

The fuel flows for 75%/65%/55% differ by 1 gph..

Fuel flow between best power/best econ differ by 1.3 gph.

What really was messing me up was that I was getting more than rated horsepower at altitude (8500). Then I realized that I must be getting a MAP boost from ram air that increased my HP (about 5%). My data wasn't fitting on the Lyc curves. So now, I ignore the altitude and use MAP. This is another good reason to have MAP... it's possible to stress your engine if you use altitude for percent HP calculations rather than MAP.

Bob, I don't quite agree with your method of increasing HP, then leaning to the target HP. It is possible to exceed 75% HP and enter the 'red zone' while doing this. It's OK if the initial setting is 75% or below, and you're leaning to a lower percentage. I wouldn't try to get to 75% by coming in from a higher power setting. Perhaps it would work if I gave the mixture the 'big pull', then enrichened to come up to the target power.

As an aside, I have two versions of the Lyc engine manual. In both, figure 3.6 shows the power curves for the O320 B,D. They have the same date and revision number, but are different! My newer manual has a more legible graph, but with errors in the fuel flow curves and temperature lapse rate compared with the older version. If Lycoming can't get this right, how can they make crankshafts reliable? Oh yeah...

Vern
 
% Power

Hi Vern

This is the best I could do with a temporary MP gauge installed, probably not as accurate as your engine monitor.

POWER CHART RPM VS ALTITUDE
% PWR ALT. RPM Gal/Hr
50% 3000 2050 7 USG
5000 2100 26.5 Ltr
7000 2150
9000 2200
11000 2250
60% 3000 2200 8.3 USG
5000 2250 31.5 Ltr
7000 2300
9000 2350
11000 2400
70% 3000 2325 9.7 USG
5000 2375 36.7 Ltr
7000 2425
9000 2500
160 H.P Lyc & Sen 70X79" Prop


Sorry I can't get the formatting in the reply post.

George
 
Thanks, George. This is the chart that I remember from flying with you.

What procedure did you use to generate this chart? It's very hard to determine HP in flight by reading the charts, so I suppose you did all of the number crunching on the ground.

Since I have MAP, I can probably make it a bit simpler by just using MAP and RPM, cross-checking with fuel flow. The fuel flow takes a lot of guess work out of this, because the BSFC varies only with mixture and RPM (slightly).

Vern
 
Vern,

----What really was messing me up was that I was getting more than rated horsepower at altitude (8500). Then I realized that I must be getting a MAP boost from ram air that increased my HP (about 5%). My data wasn't fitting on the Lyc curves. So now, I ignore the altitude and use MAP. This is another good reason to have MAP... it's possible to stress your engine if you use altitude for percent HP calculations rather than MAP.----

There may well be a slight ram air increase but be sure to use DENSITY ALTITUDE not just pressure altitude.

----Bob, I don't quite agree with your method of increasing HP, then leaning to the target HP. It is possible to exceed 75% HP and enter the 'red zone' while doing this. It's OK if the initial setting is 75% or below, and you're leaning to a lower percentage. I wouldn't try to get to 75% by coming in from a higher power setting. Perhaps it would work if I gave the mixture the 'big pull', then enrichened to come up to the target power.----

If you want 75%, set the power, then lean to the proper setting, then advance the throttle to recover the lost RPM. Since Lyc recommends no more than 65% for max engine life, I seldom cruise above that power setting. Also, remember that your engine was certificated 50 yrs ago using 91/96 fuel. Using 100LL the detonation margins would be somewhat increased. With a F/P prop, I doubt it would be possible to make this engine detonate as long as CHT is kept to a normal range.

----As an aside, I have two versions of the Lyc engine manual. In both, figure 3.6 shows the power curves for the O320 B,D. They have the same date and revision number, but are different! My newer manual has a more legible graph, but with errors in the fuel flow curves and temperature lapse rate compared with the older version. If Lycoming can't get this right, how can they make crankshafts reliable? Oh yeah...----

Lots of inconsistencies in the Lyc data, for example, they say not to lean below 75%, but recommend leaning for TO above 5,000ft. I have no problem with this, I think they are just trying to be conservative & sometimes step on their own foot.

BOB
 
Vern
I have an excel file that has hp vs fuel flow for a 160 hp lycoming. Seems to work quite well compared to other charts I have come across. I also have a large table with rpm, map, temprature, %hp, and altitude. Let me know and I'll send them to you.
 
N916K said:
Vern
I have an excel file that has hp vs fuel flow for a 160 hp lycoming. Seems to work quite well compared to other charts I have come across. I also have a large table with rpm, map, temprature, %hp, and altitude. Let me know and I'll send them to you.

Cam, please post a link or send it to me rv-9a 'at' telus.net. I'm collecting all these spreadsheets and I will calibrate against actual flight data.

Just a note: there are (at least) two different power charts for 160 HP Lycomings. One set (B, D) has 164 HP at 2700 rpm, the other 160 HP at 2700 rpm (D2J).

Then, of course, my engine is not a Lycoming.... Superior crankcase/sump + ECI cylinders. I think the Superior sump and AeroSport's tuning makes the engine breath a bit better, so the Lycoming charts may (at best) be a guideline that needs flight testing to calibrate. George McNutt (and others) tend to cruise at a maximum 70% cruise, leaving gobs of margin for power setting and leaning. I think this is the way to go rather than running 'to the limit'.

The problem with engineers (like me), is that we need to understand everything before we trust anything.

Thanks, Vern
 
keep it simple

vlittle said:
I do have a MAP gauge on my engine monitor.

If I had a constant speed prop, I could work out HP directly from MAP, RPM and altitude. Vern Little 9A
From your original post it was not clear with the mention of constant speed prop/ fixed prop, I got confused? Sorry. You make a good point about ignoring ALT on numerical look-up table and using MAP. That is a work around. However with a proper chart that is not necessary.

What you need is a generalized nomagram (sorry no 320 data):

No matter the MAP/RPM/ALT/TEMP, the chart can be interpolated for any condition, which you probably know, but it should not be necessary to ignore altitude with the this chart.

The nomogram curves, as you know, are found in Lycoming's operating manuals, which BTW I suggest everyone get for their engine. Non Std temp is adjusted as noted on the nomogram.

Vern you could go up, fly wide open. Measure MAP, RPM, ALT, OAT, TAS and FF (if you have it). On the ground using the nomogram to come up with your own tables. Repeat for other power settings. Lean as you normally would (power/econ/lop). I think you will find in the end it correlates well with other data as Mr. McNutt's tabulated data gives you. Since you have a MAP gauge you can get closer.

BOBM said:
Vern, you don't need a MAP gauge, that's obsolete technology if you have a F/P prop & already have an engine monitor.
I'm collecting all these spreadsheets and I will calibrate against actual flight data. The problem with engineers (like me), is that we need to understand everything before we trust anything. BOB
What is obsolete Bob, analog gauges or reading the parameter of manifold pressure? I agree electronic engine monitors are awesome, better than OLD analog gauges, but even a fancy engine monitor needs a MAP sensor to calculate %HP. If you don't have a fancy engine monitor with %HP read out, you have two choices, use a chart or load a spread sheet into a PDA to run it.

The spreed sheet is too much trouble to use in the cockpit. I loaded it into my iPaq PDA for use in the plane, but for the few standard power settings and conditions I operate at, it's too much trouble. My GRT EIS4000 engine monitor does not have %HP display. :eek: I guess it's obsolete. :rolleyes:

If I need to, I like to keep it old school and simple with a little look up pwr chart I made. I do admire the fancy engine monitors, but if you don't have one, it's no big deal to use a look up table.

Vern as you said, a MAP gauge is needed, even for a fixed prop RV, which I agree with 100%. With out a MAP gauge of some kind (fancy or mechanical) you're in the dark.
 
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I don't understand all of the fuss and hair splitting over calculating HP. Is there some fear that if you run at 70% HP and not 65%HP your engine life is going to go to heck?

All of these measurements and spreadsheets make assumptions. If you don't have all the variables you are introducing errors. Are you getting humidity measurements? What mixture? MP isn't enough since the engine cares about MP AND pressure at the exhaust valve (e.g. 30in at 10K is better than 30 in at MSL)? How accurate are your temp sensors (accuracy not resolution...)? How accurate are all of the engine parameters? Reading Kevin Horton's Python script on calculating this stuff is enlightening, even if it ONLY gets you to a standard lycoming engine.

You can measure many of the parameters to fairly good precision BUT since you've (probably) never dyno'ed your engine you don't know what your max output is. If you did do you know how it's changed over time? If you've made any modifications (e.g. 4-1 exhaust, porting, electronic ignition) the Lyc. charts are out the window.

If you really want HP and %HP on a flying airplane, I think the only way to get it directly with minimum error is with strain gages (or some other torque/deflection measurement) on the motor mount.

I'm putting my flame suit on now :)
 
Hp calculations can be estimated quite closely when ROP by using mass air flow. (FF, in the commonly used mixtures, is not a variable.) FF differences are essentially ineffectual to Hp when ROP, unless one is VERY rich. You can attain mass airflow without a MP gauge, but it is much easier with one. The ROP Hp equation is:

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

When LOP, mass airflow has no effect on Hp. ONLY FF has any effect. The equation when LOP is based on the BSFC(min) of the engine. For the 8.5:1 CR engines that would be:

14.9 * FF in GPH = Hp produced.

Walter
 
Walter Atkinson said:
Hp calculations can be estimated quite closely when ROP by using mass air flow. (FF, in the commonly used mixtures, is not a variable.) FF differences are essentially ineffectual to Hp when ROP, unless one is VERY rich. You can attain mass airflow without a MP gauge, but it is much easier with one. The ROP Hp equation is:

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

When LOP, mass airflow has no effect on Hp. ONLY FF has any effect. The equation when LOP is based on the BSFC(min) of the engine. For the 8.5:1 CR engines that would be:

14.9 * FF in GPH = Hp produced.

Walter


Walter, this is extremely bizzarre.... I derived this exact formula from the Lycoming power tables... it's where I developed my 'rule of 47'.

Maybe you got it from me? I hope not, because that would make it very suspect, indeed! Where did you find it/derive it? I had used 2600 RPMmax and 28.3" MAPmax.

Thanks, Vern
 
Question

Walter Atkinson said:
The ROP Hp equation is:

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

Walter

Walter, in the ROP fomula above, what do you mean by the Max values? The top of the green arc? Or the max attainable at a given altitude. I played with the top of the green arc values and that doesn't look right assuming the formula is correct.

As an example. My 2004 C182 will make 23 square at 7000'. Top of the green arc for rpm is 2400, top of the green arc for MP is approx 24-25" (thats approx cuz I don't have my POH in front of me, and the G1000 doesn't have any gradiations and I can't make the simulator go below 27.5" which is still outside the green arc.)

But, if I plug in those numbers, I get over 100% power...I wish :)
 
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vlittle said:
Maybe you got it from me? I hope not, because that would make it very suspect, indeed! Where did you find it/derive it? I had used 2600 RPMmax and 28.3" MAPmax.

Thanks, Vern

Vern, I guess my version of excel doesn't calc correctly... If I understand the formula correctly, if you used 2600 and Max RPM and we'll just say 2300 as your current RPM and you used 28.3 as MAX MP, and we'll again say 23 as your current MP. That formula gives a number of 111.05 and that's suppose to be %HP, not actual HP. So either I messed up, or something else is messed up???
 
I'll bet, Walter can you confirm

Walter Atkinson said:
The ROP Hp equation is:

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

When LOP, mass airflow has no effect on Hp. ONLY FF has any effect. The equation when LOP is based on the BSFC(min) of the engine. For the 8.5:1 CR engines that would be:

14.9 * FF in GPH = Hp produced.

Walter

OK, after all of that, I'll bet I just figured it out.... There is a missing set of parens. The formula, for those of you like me, who just copied it into excel, should be as follows...

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

Note extra parens colored for emphasis.
 
Walter Atkinson said:
Hp calculations can be estimated quite closely when ROP by using mass air flow. (FF, in the commonly used mixtures, is not a variable.) FF differences are essentially ineffectual to Hp when ROP, unless one is VERY rich. You can attain mass airflow without a MP gauge, but it is much easier with one. The ROP Hp equation is:

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

When LOP, mass airflow has no effect on Hp. ONLY FF has any effect. The equation when LOP is based on the BSFC(min) of the engine. For the 8.5:1 CR engines that would be:

14.9 * FF in GPH = Hp produced.

Walter


Walter, one more thing... I used max economy fuel flow from the Lycoming manual to make a table of economy gph at particular power settings. Your formula gives me higher power than that... is your formula just for lean of peak, or is it also valid near peak EGT?
 
File to play with

I built a quick excel file that takes 3 variables (they are highlighted in the upper left corner and are in light blue).

Plug those in for your engine and the other 2 tables will generate appropriately.

Excel File
 
Alan, I'm still not sure about FF %HP, it doesn't agree with the Lyc manual, or my understanding is flawed.

Here's what I got from the manual (and from my %power spreadsheet)

Best Power Mixture Fuel Flow (gph)
RPM/PWR 2600 2500 2400 2300 2200 MAP+RPM/100
75% 10.2 10.1 9.9 9.8 9.6 47.0
70% 9.7 9.6 9.4 9.3 9.1 45.5
65% 9.2 9.1 8.9 8.8 8.6 44.0
60% 8.7 8.6 8.4 8.3 8.1 42.5
55% 8.2 8.1 7.9 7.8 7.6 41.0
50% 7.8 7.6 7.4 7.2 7.1 38.5

Best Economy Mixture Fuel Flow (gph)
RPM/PWR 2600 2500 2400 2300 2200
75% 8.9 8.8 8.6 8.5 8.3
70% 8.4 8.3 8.1 8.0 7.8
65% 7.9 7.8 7.6 7.5 7.3
60% 7.2 7.1 7.0 6.8 6.7
55% 6.9 6.7 6.6 6.4 6.3
50% 6.5 6.4 6.3 6.1 6.0
 
Whew

Well, you guys lost me a while back. If figuring power settings was that complicated, I would have quit flying years ago. But I think I can understand Walter"s formula for lean of peak which is where I want to be in cruise anyway. Applied to Vern's O-320 160 with F/P & engine monitor it would go like this:

After establishing cruise & LOP, I would simply look at FF.

If it were 8.05gph I would multiply by 14.9 & get 120hp or 75%

If it were 6.98gph I would have 104hp or 65%

If I wanted something else I would adjust the throttle toward that setting.

The altitude, RPM, & MAP don't matter. If Walter is correct, only FF determines the power setting LOP & it will remain constant for a given power setting.

And by the way, if a MAP gauge is necessary in a F/P airplane, somebody should tell the manufacturers, cause I don't think they have installed one in a new airplane in about the last 50 yrs.
 
Bob, sorry for throwing all the numbers at you.

I can't reconcile Walter's LOP percent with Lycoming's best economy numbers.
Walter's fuel flows are lower than Lycoming's and don't vary with RPM.

Perhaps I'm missing something.

The objective with all of this is simply to find out when we can aggressively lean our engines. This can be done to any point below 75% power, hence the magic number we're looking for is the 75% power point.

Then, we can run LOP freely, and set the RPM with the mixture control and get maximum economy.

There is so much interpretation of power charts that it's possible to be off by several percent unless we calibrate our engine installation. This is why we need the HP formula.

Then, using this formula we can create a chart of RPM vs pressure Altitude, (for 75% etc) and MAP is no longer essential. This varies for every airplane/engine/prop.
 
chuck said:
...
If you really want HP and %HP on a flying airplane, I think the only way to get it directly with minimum error is with strain gages (or some other torque/deflection measurement) on the motor mount.

I'm putting my flame suit on now :)
I agree. I have been planning on strain gaging my motor mount and then it's a pretty simple torque times rpm divided by 5252 and voila! HP!
This system will allow observation of hp output degradation as the engine wears also.


-mike
 
I can help you resolve the difference.

Lycoming's numbers are off because they arbitrarily assign peak EGT as best economy for the power charts while their hard data agrees with everyone else's that best economy is found at BSFC(min). Why they arbitrarily assign peak EGT as best economy is a whole nuther thread. My calculations are based on the actual data, not an arbitrary definition. The ones I gave you are in harmony with the known physics.

Walter
 
Walter Atkinson said:
I can help you resolve the difference.

Lycoming's numbers are off because they arbitrarily assign peak EGT as best economy for the power charts while their hard data agrees with everyone else's that best economy is found at BSFC(min). Why they arbitrarily assign peak EGT as best economy is a whole nuther thread. My calculations are based on the actual data, not an arbitrary definition. The ones I gave you are in harmony with the known physics.

Walter

Thanks, Walter. I suppose they use peak EGT because it's a known point based on instrumentation. Lean of peak EGT operation is very engine/installation dependent, so I imagine that they could not/would not specify BSFC there.

So, as I suspected your numbers apply to LOP and not peak operation.
Thanks for that. They'll go into my spreadsheet for flight testing.

"Harmony with the known physics"... when I took thermodynamics at university, I was in harmony with the girl beside me. I don't remember any of the thermodynamic cycles (ask me about electrons only), but I sure remember the girl!

V
 
torque

mlw450802 said:
I agree. I have been planning on strain gaging my motor mount and then it's a pretty simple torque times rpm divided by 5252 and voila! HP!-mike
I think you mean the crankshaft. :D G
 
V:

The issue with Lycoming and peak EGT being best economy is interesting. They define Best Economy in the engineering drawings as everyone else does--at about 40dF LOP (as an average number). Their engineering drawings and curves are properly drawn and are accurate. They then clearly state that the most desirable mixture to operate the engine is at best economy or BSFC(min) and that there is no other option as good where longevity and efficiency are concerned. THEN, they say that *practically speaking* peak EGT is best economy because it is near the edge of the best economy range (true) and that it is impractical to operate leaner (true as they deliver the engine; false if the engine is set up IAW the design criteria and is a conforming engine).

Based on the poor F:A ratios that their injected engines suffer as delivered and the fact that they probably don't know how to operate a carbed engine LOP, that's not an unexpected recommendation from them. Makes sense to me, anyway.

So, their best economy FF numbers are not even in agreement with their own engineering data but are in harmony with their sub-optimal operational capability. It all kinda comes together when one looks at the big picture of why my engine management recommendations do not agree with Lycoming's. The two sets of engineering data are in full agreement.

Walter
 
gmcjetpilot said:
I think you mean the crankshaft. :D G
Instrumenting the crankshaft snout would certainly be a good place for the torque measurement except that it is rotating and the cost for electrically quiet sliprings is probably prohibitive (to me at least).
The torque has to react somewhere and the motor mounts are as good a place as any. As long as everything stays elastic (a really good assumption) you should be able to identify one of the members that responds in a measurable and repeatable manner to the torque created by the prop. Certainly the measurement won't be torque but it should vary linearly with torque and then it's just simple arithmetic to get torque from that. You could consider the motor mount tubes as being elements of a somewhat inefficient (in a measurement sense) "torque cell".

-mike
 
Walter,

For those of us with the low-compression Lyc cylinders (7.0-1), could I trouble you for the multiplier in the LOP equation?

Thanks
BOB
 
"...And by the way, if a MAP gauge is necessary in a F/P airplane, somebody should tell the manufacturers, cause I don't think they have installed one in a new airplane in about the last 50 yrs. "

FWIW, I had a 1995 Waco, Jacobs engine, big wooden prop, that had a MP guage. I found it useful.

Jim Pleasants
RV-7A QB, still in fuselage
 
map,lop,ff and thermodynamics

hello all.
i just spent 15 or so minutes on this thread, and i still don't know what you are all talking about! well, maybe a little bit. But when i get to that point in my building, i will have something to go back to for lop operation. I enjoyed it.
oh, by the way, if the girl was that pretty, and in an engineering course like thermodynamics, i would have spent my energy trying to snag her!!

dennis giammarco
rv7 rudder
 
dennis said:
hello all.
i just spent 15 or so minutes on this thread, and i still don't know what you are all talking about! well, maybe a little bit. But when i get to that point in my building, i will have something to go back to for lop operation. I enjoyed it.
oh, by the way, if the girl was that pretty, and in an engineering course like thermodynamics, i would have spent my energy trying to snag her!!

dennis giammarco
rv7 rudder

Hey Dennis, the objective of this discussion was to find a straightforward way to compute horsepower so that we know when we are at 75% or below, and can lean of peak without damaging the engine.

Simple question, lots of numbers.

By the way, as for the girl... well lets say my three avocations in life are electronics, airplanes and girls. Focusing on electronics and airplanes keeps me out of trouble!
 
One could use 13.6 as the multiplier on a 7:1 CR engine and be pretty close. If you can get me the BSFC(min) for that engine, I can calculate a more accurate answer.

Walter
 
**my three avocations in life are electronics, airplanes and girls. Focusing on electronics and airplanes keeps me out of trouble!**


So there's my problem? I've been multi-tasking too efficiently. <g>
 
Is 75% The Magic Figure?

vlittle said:
The objective with all of this is simply to find out when we can aggressively lean our engines. This can be done to any point below 75% power, hence the magic number we're looking for is the 75% power point.

This may be a dumb question, but I don?t think it has been specifically raised before. Is 75% still the correct reference if an engine has been modified to produce more HP? An original 0-320 with 7.5:1 (I think) compression ratio produced 150 HP. Most 0-320?s now produce 160 HP with 8.5:1 compression. My 0-320 with 9:1 pistons, electronic ignition and superior sump should produce around 170 HP. So, to be conservative, should I be using 75% of 150 HP = 112 HP (what the engine was originally designed to produce) or 75% of 170 HP = 127 HP??
Fin
9A Australia
 
Good question

Finley Atherton said:
This may be a dumb question, but I don?t think it has been specifically raised before. Is 75% still the correct reference if an engine has been modified to produce more HP? Fin 9A Australia
Not dumb at all. I can't give you a definitive answer but may I suggest you write Lycoming and tell them what you have. Lycoming has long been against engine modification, not because it is bad but because it is untested. Detonation margins are less with higher compression pistons. That is a fact. Also every engine configuration Lyc makes is tested for 100's if not 1000's of hours on test cells. Further each airframe maker (certified) do an inflight detonation survey, because the installation also affects the picture.

With that said I have a 8:50 to 1 compression ratio O360, which is stock. In general 8.70 to 1 and above starts to get sporty. Of course Lyc has engines with compression ratio's way higher so it is not new. However looking at a long list of C.R. for all Lycs most are 8.50 and below. I found one 10.00:1 helicopter engine and a handful 8.70's (200 hp angle head 360's). I think Lycoming is telling us something when most engines are well under 8:50:1.

I don't know what our resident LOP expert Walter will say, but I think most of his experience is with factory engines, mostly Contenentals and mostly lower compression.

Some experimental's bump their C.R. to 10 to 1. My gut feel is you may not want to not lean at 75%, may be 70% or less, way less? High compression does increase engine efficiency, but fuel economy is not usually the goal of higher compression for those who choose this option to make power.

Personally for other people reading this, thinking about high compression pistons, my advice to them is don't do it, at least over 8.5 or 8.7 to 1. I was working with a new STC for high compression pistons on a Lyc O235. During the flight test I burned a piston. Lycoming made this higher HP version but removed it from the market. Lyc found on the test cell things where fine, but in service the higher compression ratio was problematic, at least in this engine.

Bottom line with out testing or a detonation survey of your engine in your airframe, you don't know what you have. Of course you can go by others who have the same C.R. and config and follow their procedures and hope for the same. Add electronic ignition timing advance, free flow intake and exhaust, who knows. They don't call it experimental for nothing. Just be aware detonation can and does damage. Detonation is hard if impossible to detect in an aircraft engine.
 
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Where does altitude fit into this?

Walter Atkinson said:
Hp calculations can be estimated quite closely when ROP by using mass air flow. (FF, in the commonly used mixtures, is not a variable.) FF differences are essentially ineffectual to Hp when ROP, unless one is VERY rich. You can attain mass airflow without a MP gauge, but it is much easier with one. The ROP Hp equation is:

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

I'm puzzled. I don't see an altitude term in this equation. But, if I look at the Lycoming power charts, they tell me that if I keep the rpm and MP the same, the power increases as the altitude increases. Comments?

What is the Max MP value? The max mp at full throttle sea level? The ambient pressure? The MP you get if you set full throttle at the current altitude? Something else?
 
gmcjetpilot said:
With that said I have a 8:50 to 1 compression ratio O360, which is stock. In general 8.70 to 1 and above starts to get sporty. Of course Lyc has engines with compression ratio's way higher so it is not new. However looking at a long list of C.R. for all Lycs most are 8.50 and below. I found one 10.00:1 helicopter engine and a handful 8.70's (200 hp angle head 360's). I think Lycoming is telling us something when most engines are well under 8:50:1.

Personally for other people reading this, thinking about high compression pistons, my advice to them is don't do it, at least over 8.5 or 8.7 to 1. I was working with a new STC for high compression pistons on a Lyc O235. During the flight test I burned a piston. Lycoming made this higher HP version but removed it from the market. Lyc found on the test cell things where fine, but in service the higher compression ratio was problematic, at least in this engine.

Just a couple of quick notes. The O360 that Mooney used in the M20C (64 model in this case) uses the helicopter engine and 9.5:1 compression. The Heli and 10:1 is used in other *airplane* engines.

George is correct that Higher CR degrades the detonation margin, but I suspect (actually I know) that Walter has experience with High CR engines and LOP. I'll let him provide the basis for his knowledge.

I think we've discussed this, but pre-ignition is *way* worse for your engine than detonation (hopefully you'll have neither, but if given lemons... well you know the rest of the story). The Cont IO-550 guys use the 10:1 CR pistions all the time in experimental applications. I do know of one that has had a problem but it was "supercharged" on top of that (which was the problem causer).

While George offers good perspective, please treat it just as that. 10:1 and higher have been run successfully and can be run with little problem, it's just like anything else high performance, you have to know how to use it, and you have to be knowledgeable about it's performance limitations.

Experimental aircraft are just that, experiments. They require hypothisis and testing to verify the results, and changes when problems are found. Engines just happen to be a critical component so do be careful.

For me, I'm having a 10:1, ported, polished, blueprinted and balanced IO-550 built that should make between 340-370HP. Lots of other mods included, too many to mention. But then it isn't going in an RV either :)...

Be careful out there
 
Kevin Horton said:
Walter,

I'm puzzled. I don't see an altitude term in this equation. But, if I look at the Lycoming power charts, they tell me that if I keep the rpm and MP the same, the power increases as the altitude increases. Comments?

What is the Max MP value? The max mp at full throttle sea level? The ambient pressure? The MP you get if you set full throttle at the current altitude? Something else?

Kevin, I asked the same and I guess Walter didn't see the query. However, isn't Altitude as a byproduct included in Walters calc? If MaxMP is MP at sealevel (say 29" for this discussion), then MP is indicated, and will be decreasing with Altitude, hence its inclusion.

Course I'm curious as well, to the answers to your questions
 
aadamson said:
Kevin, I asked the same and I guess Walter didn't see the query. However, isn't Altitude as a byproduct included in Walters calc? If MaxMP is MP at sealevel (say 29" for this discussion), then MP is indicated, and will be decreasing with Altitude, hence its inclusion.

Course I'm curious as well, to the answers to your questions

The numbers are: RPM for maximum horsepower at sea level and MP for maximum HP at sea level. These can be taken from the Lycoming charts.

Different Lycoming charts have different data, and the Superior charts are different again. At some point we may have to just take our best guess.

I'm using the spreadsheet for preliminary HP settings, and running peak rpm and peak egt fuel flow tests to correlate with BSFC curves.

This will take some time to gather a lot of data at different MAP/Rpm/PA/FF and OAT settings. Fortunately, I can just record the numbers any time I'm stabilized in cruise, then curve fit later on.

Those of you with fancy engine/air data monitors may be able to data log all of this easily.

Vern Little
 
aadamson said:
Kevin, I asked the same and I guess Walter didn't see the query. However, isn't Altitude as a byproduct included in Walters calc? If MaxMP is MP at sealevel (say 29" for this discussion), then MP is indicated, and will be decreasing with Altitude, hence its inclusion.
Alan, according to the Lycoming power charts, if you keep the rpm and MP the same, the power varies with altitude. For example, on an IO-360-A series engine, if I set 2400 rpm and 24" MP, the power at sea level will be about 139 hp (69.6%), and the power at 4000 ft would be about 147 hp (73.5%).

If I use Walter's formula I get 76.4%.
 
Simple is as simple does

vlittle said:
I've spent quite a bit of time going through the Lycoming power curves. If I had a constant speed prop, I could work out HP directly from MAP, RPM and altitude. Unfortunately, I have a FP prop, and I'm finding it difficult to come up with a simple horsepower table...
Vern,
We have something in common. We both fly with 160 HP fixed pitch prop combinations. Call me a simpleton, but my longtime relationship with simplicity is part of my genetic makeup. I'll leave the hair splitting to those with more hair.
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Kevin:

Sorry, I've been out of town speaking at the Cirrus Convention and flying the new Turbo Cirrus. It's fast.

Yes, HP changes as altitude changes with the same MP and RPM. Why? The alteration in volumetric efficiency due to the changes in exhaust back pressure. These changes are relatively small and my formula is a quick estimate calculation when ROP while very accurate for LOP. If you need a really accurate ROP Hp calculation, mine is not what you want.

For George:

We have more experience with Lycoming engines testing for things like detonation indexes than we do with TCM engines. We have a modest amount of experience TESTING the engines that are being built with the high CR and electronic timing. We have been impressed with two findings:

1) these setups can have seriously high ICPs.

2) So far, none of the ones TESTED have produced the power that they claim to produce. In some cases they produce LESS power than the mag-equipped, stock engines.

I know this is heresy here to say those things, but you asked and that's what the real tests showed. I have no dog in the fight--just reporting the measured findings. I'm not in a position to give details as some of these issues are propriatory. Based on my experience I think your advice was dead-on correct when you noted that you really have no idea what's really happening unless you test it properly.

Walter
 
You are teasing me

Walter Atkinson said:
1) these setups can have seriously high ICPs.

2) So far, none of the ones TESTED have produced the power that they claim to produce. In some cases they produce LESS power than the mag-equipped, stock engines.

I know this is heresy here to say those things, but you asked and that's what the real tests showed. I have no dog in the fight--just reporting the measured findings. I'm not in a position to give details as some of these issues are proprietary. Based on my experience I think your advice was dead-on correct when you noted that you really have no idea what's really happening unless you test it properly. Walter
Thanks Walter for the info. The only real test I have seen has been done in the 3 part article by Cafe Foundation Org. It was just on the original electroair design. They did a fine job flight testing on their Mooney and explored stock dual mag, single mag/EI combo and dual EI configurations. It appears it works. At Reno most of the fast boys run the lightspeed. The E/P-mag seems to be a nice package and good for daily flying. They don't make spacific claim to performance as much as a sold replacement for magneto with the inherent advantages EI gives. LASAR I don't know much about, but from what I have read on their site and what pilots report it seems to cause higher temperature. I can guess as to why but it appears it advances sooner or faster than the other non cert "experimental" units.

As far as testing would you care to divulge what model EI was tested you big tease? :D Just my guessing I would think an EI with hotter spark would produce a little more power than a magneto? However even the Cafe research states a magneto provides a spark the is sufficiently hot. The real advantage is extending lean misfire limits. As far as timing advance....... uhaa I don't know what to say, but my feeling is it's more efficiency than power, i.e., you can lean further. Since most (if not all) EI's don't advance over 25 degrees until you're at lower power anyway. Per the cafe foundation reseach most benifit comes above 10,000' (70% power or less).

http://cafefoundation.org/v1/aprs/ignition1.pdf
(Part one of three makes some conclusions on page 6 & 7)

http://cafefoundation.org/v1/aprs/ignition2.pdf
(not page 12, starting at the par above the bolt type "wide experience has shown..." , bold par's and conclusion are all very interesting. It touches on LOP issues and timing advance. )

http://cafefoundation.org/v1/aprs/ignition3.pdf
(More data and same conclusion but the graphs are interesting. It is clear EI is making more HP by virtue of greater airspeed for given fuel flow. However to back up what you said Walter there are a few conditions there the magneto actually was the same or slightly better. However overall it is clear EI does provide a gain in overall efficiency, but as I said my conclusion of the data is you need to fly above 10,000, below 70% power to make max benefit of the EI's hotter spark, longer duration spark and timing advance. The mag my be old technology but it still does a remarkable job. After performance EI should give superior reliability and less maintenance.)

I highly recommend anyone wanting to know how all that leaning, electronic ignition and how an engine works take the time to go over these reseach papers. To be honest there is a lot there and going over them for the second time I still have not deciphered all the details, but I think I have the big picture.
 
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George:

I think you have a pretty good handle on the issues. Here's the BIG question that never seems to get answered:

"Since the ICPs are so radically different ROP and LOP, how does the EI know which is appropriate?"

That's a huge issue. If it's optimized for LOP in cruise it could be very risky at ROP settings. If it's optimized for ROP, it will be sub-optimal at LOP cruise settings. How do the EI maps take that into account? I don't think they do.

Also, we did a lot of things at Reno back when we had a Gold Race T-6 that we would never do on an engine we planned to fly cross country or expect to have decent longevity. It only had to last 6 laps and it was circling the airport where the tools were!

Walter
 
Walter,

This EI map issue has bothered me ever since I started testing a Pmag on my fuel injected O-360-A1A.

Let me ask this: If I could run my EI at 26 BTDC for normal operation, then, when I am cruising at LOP, switch it to 34 BTDC or 38 BTDC, would I be operating in "safe" timing zones?

Thanks for your insight,
Ben Beaird
-6A
 
Ei Rop/lop

These and the posts under emag/pmag really do get one thinking. Cockpit timing controls as suggested may be the workaround.

Sounds like the subject for some Sunday expirimenting. It should be relatively easy to switch in and out out the manifold pressure advance at a few mixtures at 60 percent manifold pressure and say 40 and 20 degrees LOP peak, 20 and 40 degrees ROP and see what happens to speed, temps, vibration.

I've always had an uncomfortable vibration slightly ROP and low loads that drove me to a smoother point LOP. Hartzell not all that happy with what they see in vibration analysis either. Could be all that advance and an early thetaPP.

Duane Zavadil
IO-320, 6A
 
Ben:

**If I could run my EI at 26 BTDC for normal operation, then, when I am cruising at LOP, switch it to 34 BTDC or 38 BTDC, would I be operating in "safe" timing zones?**

The absolute truth is that there is NO way to know without testing it. Based on my gut and experiences with other engines on the test stand, I'd guess that the 34 degree timing might be OK when LOP in cruise at modest power settings. Again, without testing, there is no way to know for sure.

Walter
 
Duane:

**I've always had an uncomfortable vibration slightly ROP and low loads that drove me to a smoother point LOP. Hartzell not all that happy with what they see in vibration analysis either. Could be all that advance and an early thetaPP. **


THAT is a very estute observation. The effects that advanced timing can have on crankshaft torsional stresses and, therefore, the effect that they can have on propellers can be profound. Again, without TESTING these issues, all bets are off as to the effects. We've seen some very profound effects from rather subtle changes.

Walter
 
Not to put words into Walter's mouth . . ..

What I think I hear him saying (and having shared several adult beverages with him while discussing this stuff as he tried to educate this moron), is that they have seen some stuff on the test stand, relative to crankshaft stresses and propeller vibration that would simply scare the snot out of you.

"Profound Effects" my empanage. <grin>

Some things should not be done without checking it on a million dollar test stand.
 
**Some things should not be done without checking it on a million dollar test stand.**


As a pal of mine once said, "An adventure is what happens with poor planning."

That can be true of a hunting trip or an engine build-up, right, Tom? :)

I recall the day that the spark plugs were blown out of the Lycoming TIO-540J2BD (and over the 12' wall and onto the ramp) during an engine test run while working on spark advance and detonation testing. That was the day that we discovered that the spark plug bosses are blown out of the cylinder at around 1200 psi ICP. THAT was an adventure.

The normal peak internal cylinder pressure during takeoff power on that engine can be in the 1050-1100 psi range. That's not a lot of wiggle room. One might consider this when thinking of messing with the timing.

OTOH, one could be looking for an "Adventure." :eek:

Walter
 
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