JPI EDM 930 %HP Constant

[Bob_Belville]

I suspect I need to adjust the %HP constant for my JPI EDM 930. The manual has a procedure for doing that by setting up 70% based on POH MP/RPM at an altitude between 5000 & 8000. My POH indicates 21/2600 is 70% @ 7500'. "Standard Atmosphere".

 

Question 1: I suppose I need to calculate DA for non standard OAT?

 

On a recent flight the data download:

 

On FT climb out:

OAT 16

Alt. 1811 (=DA 2350)

MP 27.5

RPM 2650

FF 17.8

% HP displayed: 87%

 

Stabilized cruise:

OAT 14

Alt. 5007 (=DA 6044)

MP 23

RPM 2550

FF 10.8

% HP displayed: 69%

 

So... it looks to me that the indicated HP is low. at 5000' the POH lists 23/2500 as 73% and 2600 as 77% so 23/2550 would be 75%.

 

Question 2: can I just change the HP constant by 6/69 = 8.7% from the factory setting in the range 45-180 to get the indicated %HP closer to POH?

 

[Bob_Belville]

'66E IO360A1A if that make a difference.

[jlunseth]

I have a 930.  I have not altered the %HP constant so can't be of specific help on the mechanics of doing that.  However, I can tell you this, which you may know.  The %HP readout on the 930, while useful, is an algorithm and the algorithm is not published so there is no way of knowing how it arrives at the displayed number.  It is a little bit of "black magic." I have noticed that in my aircraft, the %HP that is displayed is not directly related to MP and RPM's only, there are other factors in the algorithm, either OAT or Induction Air Temp (IAT) or both.  My aircraft is turbocharged and there is an intercooler that cools the induction air downstream of the turbo and before the induction system.  That is where IAT is measured.  On takeoff, to give you an example of the variability, I may set an MP that results in 100% HP being displayed (RPM's are always 2700 on takeoff).  At rotation or shortly after I will check the %HP again, and it will say 105%.  I changed nothing on the engine controls and the altitude has changed only insignificantly, the only thing that has changed is that the aircraft is moving through the air much faster.  This could affect %HP in one of several ways, (1) with more air moving over the intercooler it may be cooling the turbo output better so IAT drops and %HP goes up, or (2) there may be some ram air effect making it easier for the turbo to push air through so although MP does not change, the turbo is not working as hard and therefore the air coming out of it is cooler, or (3) the movement of the aircraft through the air reduces the effort the engine needs to exert to produce a given HP.  I don't know which it is.  I can tell you this though, I have seen MP settings varying from as little as 33.3 to as much as 35.5 MP all of which produce 100%HP on takeoff, depending mainly on OAT. 

 

I have used takeoff readings as an example, but things are subject to change in virtually every regime of flight depending on any number of factors considered by that "black magic" formula.  There is enough variability that I would hesitate to use readings from just one or two flights to decide that the HP constant needs to be changed, or to decide how much it needs to be changed.  I would instead record data from a number of flights including the MP, RPM, OAT, Alt., and even the humidity (which affects air density) and see what the trends look like before tweaking the constant.  Then I would make relatively small changes and test again over a number of flights.

[Bob_Belville]

jlunseth> thanks for the comments. As you may know, the "Pilot's Guide" for the 930 provides instructions for "adjusting the HP Constant for Rich of Peak."  To summarize the steps: @ an altitude between 5000' and 8000' set the plane up at 70% power based on MP & RPM from the POH and then change the HP constant to make the power read 70%. 

This procedure does not seem to deal with the factors you mention. I suppose one should set up the 70% based on DA not PA which is implied with reference to Standard Atmosphere in the old Owners Manual power setting charts.

[Bob_Belville]

I was hoping someone would comment on whether you pay attention to the %HP info from the EDM930 and use to to set up MP/RPM.. 

[jimosborn]

Beyond assuring that your very expensive 930 is correct in all parameters, who really cares about % HP?   Seriously...... MP, RPM, FF, CHT sure, but knowing the percent of horsepower that a given power setting produces has no practical value! 

 

My approach is to find the sweet spot for current conditions and fly that.

 

Cheers

[jetdriven]

I was hoping someone would comment on whether you pay attention to the %HP info from the EDM930 and use to to set up MP/RPM.. 

It's easy. MP, max available. RPM, 2300-2500. Mixture, 15-25 LOP, maximum of 10.0 GPH. If you want to go slower, less of one or all three.

[Bob_Belville]

It's easy. MP, max available. RPM, 2300-2500. Mixture, 15-25 LOP, maximum of 10.0 GPH. If you want to go slower, less of one or all three.

 

 

It's easy. MP, max available. RPM, 2300-2500. Mixture, 15-25 LOP, maximum of 10.0 GPH. If you want to go slower, less of one or all three.

Byron, doesn't Lycoming still advise 65% HP or less, 400d CHT or less, oil T 200d or less, for max engine life?

 

So, if I want to operate @ 65% or less I either have to consult Mooney owners manual charts or adjust MP/RPM "by the numbers" either "45" or "46" for 65%, I've seen both for this model? (BTW, That rule of thumb does not match the Owners Manual all that well at all altitudes.)

 

OR... is the modern wisdom that 75% for continuous operations is OK as long as temps are controlled? I have not run LOP but @ 75% ROP I have to either run at least 125 ROP and or open cowl flaps to keep cyl #3 below 400 and oil T around 200. 

 

"want to go slower"? I bought a Mooney - in general I want to go faster. I want to go as fast as I can without sacrificing engine health and life. 

[Bob_Belville]

Beyond assuring that your very expensive 930 is correct in all parameters, who really cares about % HP?   Seriously...... MP, RPM, FF, CHT sure, but knowing the percent of horsepower that a given power setting produces has no practical value! 

 

My approach is to find the sweet spot for current conditions and fly that.

 

Cheers

 

 

Beyond assuring that your very expensive 930 is correct in all parameters, who really cares about % HP?   Seriously...... MP, RPM, FF, CHT sure, but knowing the percent of horsepower that a given power setting produces has no practical value! 

 

My approach is to find the sweet spot for current conditions and fly that.

 

Cheers

Jim, thanks!

 

My fancy JPI 930 probably can be right if I adjust the "factor" so that it matches the book for MP/RPM/DA(/FF?). I am trying to find that sweet spot. I like the way it runs at FT & 2550 but FF and hottest cyl CHT (#3) are a little high. And I wonder how much weight to give to Lycoming's 65% HP or less for max engine life.    

 

FWIW, I flew an earlier '66E for about 2500 hours in the 70s/80s without the multi cylinder instrumentation now available. I swallowed a valve with a factory new, Norm Bender, engine @ mid time. That's more excitement than I wish to experience again if I can avoid it. I suspect I was routinely operating with CHT well above 400. In those days we watched the 475 red line but I do not recall shooting for a lower #. I do recall typical FF as 9.1, probably operating 25 ROP per contemporary wisdom for best economy.

[Bob_Belville]

Bob,

Fly WOT at the RPM of your choosing. I typically choose 2500 RPM. Lower RPM will reduce noise and slightly increase efficiency due to the reduction in internal engine friction, but lower RPM also increases CHTs and internal cylinder pressures. 2500 RPM seems to be a good compromise most of the time for me. Then pull the fuel flow back to 8.6 GPH and, assumimg your fuel flow instrument is accurate, you will be at 65 percent power. Below about 8500 feet all of your cylinders will be LOP. If you fly higher monitor your richest cylinder to see if it goes ROP at that fuel flow. At the point that that occurs you will have to switch over to ROP engine management practices (i.e. the POH power setting tables) if you want to maintain 65 percent power.

Down low at WOT you might not be able to lean all of the way back to 8.6 GPH without encountering engine roughness. If that is the case reduce manifold pressure as necessary for smoothness. 24" or so will probably do and will keep you well LOP and your temperatures down.

See how your engine monitor's reported percentage of power compares to what I am telling you above and you will have a baseline for computing it's error when LOP. Personally I would just disregard it, but if I paid as much for the functionality as you have I would probably feel differently.

One last thing, there is a simple formula for computing LOP percentage of power based on fuel flow. For our engines, multiply your fuel flow times 15 and then divide the resulting number by 200, which is our rated maximum horsepower.

Jim

 

 

Bob,

Fly WOT at the RPM of your choosing. I typically choose 2500 RPM. Lower RPM will reduce noise and slightly increase efficiency due to the reduction in internal engine friction, but lower RPM also increases CHTs and internal cylinder pressures. 2500 RPM seems to be a good compromise most of the time for me. Then pull the fuel flow back to 8.6 GPH and, assumimg your fuel flow instrument is accurate, you will be at 65 percent power. Below about 8500 feet all of your cylinders will be LOP. If you fly higher monitor your richest cylinder to see if it goes ROP at that fuel flow. At the point that that occurs you will have to switch over to ROP engine management practices (i.e. the POH power setting tables) if you want to maintain 65 percent power.

Down low at WOT you might not be able to lean all of the way back to 8.6 GPH without encountering engine roughness. If that is the case reduce manifold pressure as necessary for smoothness. 24" or so will probably do and will keep you well LOP and your temperatures down.

See how your engine monitor's reported percentage of power compares to what I am telling you above and you will have a baseline for computing it's error when LOP. Personally I would just disregard it, but if I paid as much for the functionality as you have I would probably feel differently.

One last thing, there is a simple formula for computing LOP percentage of power based on fuel flow. For our engines, multiply your fuel flow times 15 and then divide the resulting number by 200, which is our rated maximum horsepower.

Jim

Thanks Jim, that's helpful. Your 201 probably runs a good bit cooler than my old Super 21. I have replaced and tweaked the baffle seals but the airflow around the cylinders probably is still not nearly as efficient and yours.

[Bob_Belville]

Thanks Jim, that's helpful. Your 201 probably runs a good bit cooler than my old Super 21. I have replaced and tweaked the baffle seals but the airflow around the cylinders probably is still not nearly as efficient and yours.

 

 

Thanks Jim, that's helpful. Your 201 probably runs a good bit cooler than my old Super 21. I have replaced and tweaked the baffle seals but the airflow around the cylinders probably is still not nearly as efficient and yours.

My scimitar "top prop" has a different restricted zone - 2350-2550 when >24". Not an issue about 5 or 6000 but I've gotten in the habit of running @ 2350 down low and 2550 up high. 

[Bob_Belville]

Hi, Bob.  That is interesting about the Top Prop's RPM restrictions.  I didn't know that.  The important take away is the formula for computing percentage of power when LOP.  That'll give you a simple baseline for determining the accuracy of JPI's percentage of power algorithm, at least when all cylinders are LOP.  The only other variable being the accuracy of your fuel flow equipment.

 

All the best.  Beautiful bird you have there from what I can see.

 

Jim

 

 

Hi, Bob.  That is interesting about the Top Prop's RPM restrictions.  I didn't know that.  The important take away is the formula for computing percentage of power when LOP.  That'll give you a simple baseline for determining the accuracy of JPI's percentage of power algorithm, at least when all cylinders are LOP.  The only other variable being the accuracy of your fuel flow equipment.

 

All the best.  Beautiful bird you have there from what I can see.

 

Jim

Thanks Jim. I will have to try LOP. (The %HP feature of the JPI may only apply to ROP operations. The manual is ambiguous in that the instructions for setting the HP Constant is labeled "for ROP Operation.") You and Bryan have been very helpful, it would be nice to hear from someone who has set up this parameter in the 930. 

[KSMooniac]

Jim's and Byron's advice for LOP ops is spot-on, and can be useful for you to tweak whatever you need to in the JPI to get the ROP algorithm correct.  But, like Jim O, that is more of a "neat" thing and not necessarily useful or helpful IMO, but I can understand your desire to get everything dialed-in as best as you can.  

 

Simple test... figure out a POH ROP setting for 65% or close to use as a test point.  For the IO-360, 65% power is 8.67 GPH ( = 200 hp * 65% /15).  IF (that's a big IF) your fuel flow K factor is correct AND your engine is happy flying LOP, then simply setup cruise with WOT and whatever RPM you plan to use from the previously mentioned 65% ROP setting.  Say 2600 for example.  Lean directly to 8.67 GPH and wait a few minutes in still air and record your airspeed.  Next, set 65% according to the POH ROP, which might require a MP reduction and then wait a few minutes and check the airspeed again.  The speeds should match.  Check the JPI readout and see if it says 65% HP.  If not, then you know it will need an adjustment...

 

I'm not sure about your LOP confidence or knowledge, so I'll recommend doing the GAMI Lean Test first, which could also be done on the same test flight at 65% power or less.  A data recording helper and test card is nice to have, but you could do it solo and dump your JPI data for analysis post-flight.  Almost every Lyc IO-360 will run LOP if the ignition system is up to par, the injectors are clean, and there are no induction leaks. It would also be worth the hassle for you to attend the Advanced Pilot Seminar in Ada, OK next month to learn all about engine management and engine monitor use, in addition to LOP ops.  I took it a few months after purchasing my J, and it was far-and-away the best money I've spent as an owner, bar none.  I learned more there in 2.5 days than in some of my semester-long engineering courses.    You've spent a lot of money perfecting your E, so I think you should spend a little on yourself now.  

[Bob_Belville]

Jim's and Byron's advice for LOP ops is spot-on, and can be useful for you to tweak whatever you need to in the JPI to get the ROP algorithm correct.  But, like Jim O, that is more of a "neat" thing and not necessarily useful or helpful IMO, but I can understand your desire to get everything dialed-in as best as you can.  

 

Simple test... figure out a POH ROP setting for 65% or close to use as a test point.  For the IO-360, 65% power is 8.67 GPH ( = 200 hp * 65% /15).  IF (that's a big IF) your fuel flow K factor is correct AND your engine is happy flying LOP, then simply setup cruise with WOT and whatever RPM you plan to use from the previously mentioned 65% ROP setting.  Say 2600 for example.  Lean directly to 8.67 GPH and wait a few minutes in still air and record your airspeed.  Next, set 65% according to the POH ROP, which might require a MP reduction and then wait a few minutes and check the airspeed again.  The speeds should match.  Check the JPI readout and see if it says 65% HP.  If not, then you know it will need an adjustment...

 

I'm not sure about your LOP confidence or knowledge, so I'll recommend doing the GAMI Lean Test first, which could also be done on the same test flight at 65% power or less.  A data recording helper and test card is nice to have, but you could do it solo and dump your JPI data for analysis post-flight.  Almost every Lyc IO-360 will run LOP if the ignition system is up to par, the injectors are clean, and there are no induction leaks. It would also be worth the hassle for you to attend the Advanced Pilot Seminar in Ada, OK next month to learn all about engine management and engine monitor use, in addition to LOP ops.  I took it a few months after purchasing my J, and it was far-and-away the best money I've spent as an owner, bar none.  I learned more there in 2.5 days than in some of my semester-long engineering courses.    You've spent a lot of money perfecting your E, so I think you should spend a little on yourself now.  

 

 

Jim's and Byron's advice for LOP ops is spot-on, and can be useful for you to tweak whatever you need to in the JPI to get the ROP algorithm correct.  But, like Jim O, that is more of a "neat" thing and not necessarily useful or helpful IMO, but I can understand your desire to get everything dialed-in as best as you can.  

 

Simple test... figure out a POH ROP setting for 65% or close to use as a test point.  For the IO-360, 65% power is 8.67 GPH ( = 200 hp * 65% /15).  IF (that's a big IF) your fuel flow K factor is correct AND your engine is happy flying LOP, then simply setup cruise with WOT and whatever RPM you plan to use from the previously mentioned 65% ROP setting.  Say 2600 for example.  Lean directly to 8.67 GPH and wait a few minutes in still air and record your airspeed.  Next, set 65% according to the POH ROP, which might require a MP reduction and then wait a few minutes and check the airspeed again.  The speeds should match.  Check the JPI readout and see if it says 65% HP.  If not, then you know it will need an adjustment...

 

I'm not sure about your LOP confidence or knowledge, so I'll recommend doing the GAMI Lean Test first, which could also be done on the same test flight at 65% power or less.  A data recording helper and test card is nice to have, but you could do it solo and dump your JPI data for analysis post-flight.  Almost every Lyc IO-360 will run LOP if the ignition system is up to par, the injectors are clean, and there are no induction leaks. It would also be worth the hassle for you to attend the Advanced Pilot Seminar in Ada, OK next month to learn all about engine management and engine monitor use, in addition to LOP ops.  I took it a few months after purchasing my J, and it was far-and-away the best money I've spent as an owner, bar none.  I learned more there in 2.5 days than in some of my semester-long engineering courses.    You've spent a lot of money perfecting your E, so I think you should spend a little on yourself now.  

Scott, thanks. The JPI 930 has "Lean Find" functions for both ROP and LOP operation. In the course of identifying the leanest and richest cylinders (1st and last to go past peak EGT) the JPI reports the GAMI spread. Ain't that slick? My EGTs graphs are very tight, I hope that means the GAMI spread will turn out to be small. I have not had a chance to check it out yet.

[jlunseth]

Hi Bob.  I do use my 930 to set RPM and MP, at least sort of.  Again, I have a turbo with an aftermarket intercooler. So to set MP and RPM by the book, I would need to look up a setting in the POH.  Lets say I am at 12000 and the book says if I set 32" and 2450 I will get 75% HP.  I am not looking at the book so am just making up these numbers to give you an example of what I do.  Then, because I have an intercooler, I have to do some math and make and adjustment.  The reason this adjustment is needed in my engine is that the POH charts are set up assuming that Compressor Discharge Temp. (CDT), which is the air coming out of the turbo, is the temp. of the air entering the induction system (Induction Air Temp.)  The intercooler comes with a chart that allows adjustments based on the differential temp. between CDTand IAT because that is what an intercooler does, it cools the airflow.  That differential temp. needs to be converted to degrees C.  Then I look up on the intercooler chart what actual MP I need to set given the differential temp., in order to get my desired MP of 32", and that will probably be in the 28-29" range depending on how well the intercooler is doing its job that day.

 

 

The 930 appears to me to already take all of this into consideration, so rather than do the mental gymnastics, I just set the RPM to where the engine runs smoothly and the prop is going to be efficient, in this case 2450, and then adjust the MP to give me 75% on the 930.  It is close enough.  If the temps are good and the airspeed is about right at that altitude for that airspeed, then it has all worked out.

 

Because I have a turbo and operate in the Flight Levels, I do not always use the same RPM.  2450 is where my engine is happiest until around 18k, then the prop starts to lose efficiency and I will run at 2600 or 2650.  I still use the 930 to set the %HP by dialing the MP to whatever is needed to get my desired %HP at that RPM and at that altitude.

 

 

 

The 930 algorithm is different on the LOP side than on the ROP side.  On the LOP side there is a straight formula, which in the case of my turbo is 13.7 x GPH, that provides HP.  The 930, from what I can tell, uses that formula on the LOP side.  So control switches much more from MP and RPM to use of the mixture knob.  The formula is different in a normally aspirated engine, I think it is 14.2 x GPH or something like that.  The difference is because my turbo has a lower compression ratio than your NA engine.  Someone who runs an NA can tell you the correct number off the top of their head, but you really don't need to know it.  All you need to know is that on the LOP side, the 930 uses a formula that correlates HP directly to fuel flow, where on the ROP side it is using much more "black magic" stuff.

 

To distill all this down to a simple formula, set the RPM you want.  A good place to operate at or below about 15,000 seems to be 2400-2450.  Then adjust your MP to get the desired %HP on the 930, or if you are operating LOP, adjust the fuel flow to the %HP on the lean side.

[Bob_Belville]

Final(?) comment on setting % HP display. I was able to verify that the change I had made to the constant for the %HP display based on JPI collected data was dead on. I had changed the constant by a proportional amount.

 

Yesterday while at 8000' coming back from KLYH I set up 70% by the Mooney cruise charts and what do you know, the JPI agreed. 

[KSMooniac]

That is great to know!  I hope to be going thru the same gyrations later this year myself, and might have to come back to this thread or talk to you directly.

[Bob_Belville]

That is great to know!  I hope to be going thru the same gyrations later this year myself, and might have to come back to this thread or talk to you directly.

 

 

Fine.

[Bob_Belville]

Scott, thanks. The JPI 930 has "Lean Find" functions for both ROP and LOP operation. In the course of identifying the leanest and richest cylinders (1st and last to go past peak EGT) the JPI reports the GAMI spread. Ain't that slick? My EGTs graphs are very tight, I hope that means the GAMI spread will turn out to be small. I have not had a chance to check it out yet.

 

 

Scott, thanks. The JPI 930 has "Lean Find" functions for both ROP and LOP operation. In the course of identifying the leanest and richest cylinders (1st and last to go past peak EGT) the JPI reports the GAMI spread. Ain't that slick? My EGTs graphs are very tight, I hope that means the GAMI spread will turn out to be small. I have not had a chance to check it out yet.

My GAMI spread on a recent flight was 0.3 gph. Sounds good. I'll have to try the LOP LEAN FIND feature the next time I have a decent length flight.

[KSMooniac]

Yeah, that is pretty good and not surprising for an IO-360 in good shape.

[carusoam]

Write the LF instructions out first. It's easy when you put it in your own shortened words.

Practice will help identify how quickly or slowly you may need to lean.

Best regards,

-a-