Power Boost & engine power calculations

[Matt Ward]

I'm getting more versed on running LOP and other engine calculations and I have a question:  a recent flight profile for me was 9500 feet, 2400 RPM, 21 MP, 45 OAT.  With that config, I am generating about 65% power which puts me in the Red Box if I'm Peak to about 100 degrees ROP.  However, my cruise was set up at 20 inches MP, I opened my Power Boost, and it raised to 21 inches.  That is important because if I were running 20 inches, I'd be at 60% power and able to run almost any mixture I like.

So my question is this: when you're calculating engine power, are you considering the MP you dialed in before opening the Power Boost or after?  To me, it seems like it's the former because the Power Boost is only changing the air input and doesn't seem like it would be adding additional strain to the engine.

[tmo]

Why would you consider anything than what is currently indicated?  If it says 21", the engine has 21" to work with, no?

[Matt Ward]

Maybe you're right, I don't know.  To me the thinking was somewhat analogous to Altitude vs. Density Altitude.

At the config I mentioned, I'm running WOT to get 20" and that has an associated fuel flow at whatever mixture I'm running.  If I pull the Power Boost, fuel flow doesn't change and now the engine has 21" to work with.  So, more air and the same amount of fuel.  It just seems that condition could have some consideration on how I lean.

 

[gsxrpilot]

Yeah, you can read MAP right off the gauge. Your Power Boost is just adding a little (1") of pressure (suction) to the air flowing through the intake. A turbo does the same, just more than 1".

[tmo]

And to not bring a turbo into the mix - think a dirty, clogged air filter - it will give you less MAP than a clean one would.  Your engine will be making the power indicated by the gauges, but with a dirty one, perhaps not all it could.  Same with closed / open ram air / power boost (which I imagine is the same thing).

What really gets confusing is the basic M20K-style turbo, where touching any of the knobs messes up the relationship - but you still look at what the gauges are showing.  Thankfully this forum has people that have tamed the beast, I hope to be able to follow in their footsteps without breaking too much...

[kortopates]

1 hour ago, Matt Ward said:

I'm getting more versed on running LOP and other engine calculations and I have a question:  a recent flight profile for me was 9500 feet, 2400 RPM, 21 MP, 45 OAT.  With that config, I am generating about 65% power which puts me in the Red Box if I'm Peak to about 100 degrees ROP.  However, my cruise was set up at 20 inches MP, I opened my Power Boost, and it raised to 21 inches.  That is important because if I were running 20 inches, I'd be at 60% power and able to run almost any mixture I like.

So my question is this: when you're calculating engine power, are you considering the MP you dialed in before opening the Power Boost or after?  To me, it seems like it's the former because the Power Boost is only changing the air input and doesn't seem like it would be adding additional strain to the engine.

When you opened the Ram air to get an additional 1" of MAP, and if your FF doesn't change you should be able to see a small rise in EGT if you were on the ROP side of the curve or a small decrease in EGT if you were on the LOP side. Depending on what kind of engine monitor you have, you may well have to look at the downloaded data to discern any difference.

True in that in your E model, a 5% power change is only a difference of 1.5 off the key numbers, Key numbers being 45 for 65% meaning any combination of MAP + RPM/100 = 45 will have you in the ballpark of 65% power. The numbers I use for the E, both ROP and LOP are below. But anytime we're rich of peak, I'd go by the ROP numbers and only when all cylinders are Lean of Peak then switch to LOP numbers. 

ROP HP%	KEY#	GPH		LOP %PWR	GPH
75%	48	14		75	10.1
65%	45	10		65	8.7
55%	42	8		55	7.4

Edited May 14, 2020 by kortopates

[Matt Ward]

Thanks for that insight, @kortopates.  I have been using the Mooney PPP guidelines before I started reading the Advanced Pilot stuff.  The PPP has me at 9.0 GPH with a key number of 45 for 65%, which is typically about 2400 + 21" for me.  At 9.0 GPH, I suspect I'm near peak EGT but I'm not certain.  According to Advanced Pilot, that would be a no-no.  My first test with the JPI 700 + Hoskins puts me around 10.5 GPH to run 100 degrees ROP but I want to test that again - it seemed high.  On the other end, it had me near 7.5 GPH when I ran 25 LOP - I need to test it again, too.  My first tests on KTAS showed 145 vs 135, respectively, so if that holds, running LOP is a no brainer: 7% trade in speed for 23% in fuel.  Again, I am suspect of those results and need to test again.

But I still think there is something fundamental that I'm not understanding.  Reading a bit about estimating engine output with a Lycoming here (https://www.kitplanes.com/determining-engine-power/), the inputs to calculating power output are MP, RPM, Pressure, and Alittude.  Fuel flow is an indirect byproduct of those, I suppose.  But if I can generate the 45 key number on either 10.5 GPH or 7.5 GPH, my power output wouldn't be any different.  It would be 65%.  But my KTAS would be different.  Why is that?

My going-in assumption on this LOP stuff was that I wouldn't be able to generate the 45 key number LOP but I'm not sure that's the case.

Thanks for the help, lots to learn!

[kortopates]

4 minutes ago, Matt Ward said:

Thanks for that insight, @kortopates.  I have been using the Mooney PPP guidelines before I started reading the Advanced Pilot stuff.  The PPP has me at 9.0 GPH with a key number of 45 for 65%, which is typically about 2400 + 21" for me.  At 9.0 GPH, I suspect I'm near peak EGT but I'm not certain.  According to Advanced Pilot, that would be a no-no.  My first test with the JPI 700 + Hoskins puts me around 10.5 GPH to run 100 degrees ROP but I want to test that again - it seemed high.  On the other end, it had me near 7.5 GPH when I ran 25 LOP - I need to test it again, too.  My first tests on KTAS showed 145 vs 135, respectively, so if that holds, running LOP is a no brainer: 7% trade in speed for 23% in fuel.  Again, I am suspect of those results and need to test again.

But I still think there is something fundamental that I'm not understanding.  Reading a bit about estimating engine output with a Lycoming here (https://www.kitplanes.com/determining-engine-power/), the inputs to calculating power output are MP, RPM, Pressure, and Alittude.  Fuel flow is an indirect byproduct of those, I suppose.  But if I can generate the 45 key number on either 10.5 GPH or 7.5 GPH, my power output wouldn't be any different.  It would be 65%.  But my KTAS would be different.  Why is that?

My going-in assumption on this LOP stuff was that I wouldn't be able to generate the 45 key number LOP but I'm not sure that's the case.

Thanks for the help, lots to learn!

The POH number on FF are usually low because they do have you running at or near peak ROP which is bad till below 65%. To understand your engines power curves its best to go by the manufacturers Operators manual for your engine. But you can get excellent approximations that are really close enough using a formula such as you reference - but its even easier than that since it really can be simplified down to MAP and RPM as follows %Hp= ( %Hp = (100-(((Max RPM-RPM)/100)*2.5+(Max MAP-MAP)*3.5))/100

Key numbers are good for ROP power settings but have no relevance for LOP power which is entirely determined by FF as you well know after doing the APS class. So to generate the equivalent power for the key number of 45, what we're really saying is to set at 65% power LOP which you can certainly do by just having enough air or altitude to have sufficient MAP in order to have all cylinders LOP at 10 GPH.  But really to do this in practical terms, you'd keep MAP at WOT and after climbing to altitude, do the big mixture pull to entirely on the LOP side. Then you can enrich to 10 GPH, that will ensure 65% power as long as still LOP. Then to verify how LOP you are, you can safely enrich till your see the first cylinder go through peak. This will be be your richest cyl used to set LOP settings, then lean again till 10 GPH to see exactly how LOP you are. If not at least 15F LOP lean some more till you are or until ~60% power where it no longer matters (no limited red box). 

 

[gsxrpilot]

5 minutes ago, kortopates said:

as you well know after doing the APS class

I just loaned Matt the manual from the APS class I attended... not quite the same as attending the class 

[Ragsf15e]

1 hour ago, Matt Ward said:

Thanks for that insight, @kortopates.  I have been using the Mooney PPP guidelines before I started reading the Advanced Pilot stuff.  The PPP has me at 9.0 GPH with a key number of 45 for 65%, which is typically about 2400 + 21" for me.  At 9.0 GPH, I suspect I'm near peak EGT but I'm not certain.  According to Advanced Pilot, that would be a no-no.  My first test with the JPI 700 + Hoskins puts me around 10.5 GPH to run 100 degrees ROP but I want to test that again - it seemed high.  On the other end, it had me near 7.5 GPH when I ran 25 LOP - I need to test it again, too.  My first tests on KTAS showed 145 vs 135, respectively, so if that holds, running LOP is a no brainer: 7% trade in speed for 23% in fuel.  Again, I am suspect of those results and need to test again.

But I still think there is something fundamental that I'm not understanding.  Reading a bit about estimating engine output with a Lycoming here (https://www.kitplanes.com/determining-engine-power/), the inputs to calculating power output are MP, RPM, Pressure, and Alittude.  Fuel flow is an indirect byproduct of those, I suppose.  But if I can generate the 45 key number on either 10.5 GPH or 7.5 GPH, my power output wouldn't be any different.  It would be 65%.  But my KTAS would be different.  Why is that?

My going-in assumption on this LOP stuff was that I wouldn't be able to generate the 45 key number LOP but I'm not sure that's the case.

Thanks for the help, lots to learn!

Your speed and fuel flow numbers are very similar to what I see ROP/LOP.  If you’re off, it’s not by much.  One thing to keep in mind, you lose power much quicker on the LOP side as you decrease fuel flow.  My engine runs smooth deep LOP, and I can get the ff way down, but it eventually gets slow enough to be less efficient then being about ~15-25LOP.  
 

So, with that in mind, my technique to set LOP after level off is leave full power (WOT), Ram on (probably already on from ~5,000’agl), then set cruise rpm (I use 2500), Then pull mixture slowly until all cylinders go LOP and the last one is ~10-15LOP.  Then I make sure I’m at 65% or less power (based on fuel flow).  I generally cruise 7,500-10,500’, so it’s usually good.
 

You will likely be 5-10 ktas slower than 100ROP at similar MP, RPM.  You should see all cylinders much cooler than ROP.

[Tcraft938]

Probably an ignorant question, is this notorious red box thing I keep reading in here only apply to fuel injected engine?  Thanks. 

[Hank]

51 minutes ago, Tcraft938 said:

Probably an ignorant question, is this notorious red box thing I keep reading in here only apply to fuel injected engine?  Thanks. 

No, it applies to all engines. But it's more difficult to get a carbureted engine to run smoothly LOP.

Edited May 14, 2020 by Hank

[afward]

@Tcraft938 not just fuel injected.

Basically, it's a way to ensure safe mixture settings for any given power setting.  There's a lot of math & science behind it, but in general you can trust that it'll help keep you out of trouble.

https://www.jpinstruments.com/wp-content/uploads/2013/05/Mike-Bush-Red-BoxRed-Fin.pdf

[kortopates]

41 minutes ago, Tcraft938 said:

Probably an ignorant question, is this notorious red box thing I keep reading in here only apply to fuel injected engine?  Thanks. 

Not at all, the red box come about from detonation margin and maximum Internal Cylinder Pressure (ICP) and applies to our entire piston fleet. To put in in simpler terms it comes down to what is safe leaning which is relative to % power being operated. You may well see some debate on this between our engine manufactures in that manufacturer may say its entirely safe to lean the engine any way you want as well as at or below max cruise power. But the manufacturer is more concerned about selling engines with best performance numbers possible and does not warranty them long enough to care about engine longevity. So the redbox principle is to say at roughly power levels from about 65% and on up, we don't want to operate too close to peak causing ICPs and higher CHTs which can be harmful to the engines longevity.    

[Matt Ward]

5 hours ago, Ragsf15e said:

Then I make sure I’m at 65% or less power (based on fuel flow)

How do you do that in flight? I have a little tool to calculate power but it’s based on just MP, RPM, pressure, and temp. I *think* it’s only useful ROP. 

[Hank]

Just now, Matt Ward said:

How do you do that in flight? I have a little tool to calculate power but it’s based on just MP, RPM, pressure, and temp. I *think* it’s only useful ROP. 

When LOP, there's a simple formula (which I forget, having a carb; it's something like FF * 14.5 = hp, and 65% of 200 = 130).

Or you can scroll up to @kortopates's post above, where he has a small table he uses when instructing in E models, with % power for both LOP & ROP. I'd make several copies, including one to fly with.

[gsxrpilot]

What's the formula to calculate HP based on the type of pistons in the IO360? 

For my TSIO360 Continental it's 13.7

 

[Matt Ward]

In the kitplanes article I linked to above they say
 

If we are running lean of peak EGT, the BSFC should be on the order of 0.4 pound/hp per hour. We can get a very approximate power by multiplying the fuel flow in gph by 15

I just never heard that before so I didn’t know how trustworthy that was as I hadn’t seen any derivation!

[Tcraft938]

2 hours ago, afward said:

@Tcraft938 not just fuel injected.

Basically, it's a way to ensure safe mixture settings for any given power setting.  There's a lot of math & science behind it, but in general you can trust that it'll help keep you out of trouble.

https://www.jpinstruments.com/wp-content/uploads/2013/05/Mike-Bush-Red-BoxRed-Fin.pdf

Very interesting. Thank you very much. I need to read it several more times to sink in. Wonder what my O360-A1D will think of it.  Lol. 

[gsxrpilot]

Try 14.85. This only works on the Lean side of Peak LOP.

(GPH * 14.85)/Engine HP=%HP

(10.1 * 14.85)/200 = 75%
(9 * 14.85)/200 = 67%
(8.5 * 14.85)/200 = 63%

[Ragsf15e]

1 hour ago, Matt Ward said:

How do you do that in flight? I have a little tool to calculate power but it’s based on just MP, RPM, pressure, and temp. I *think* it’s only useful ROP. 

Good question.  I use 9gph or less... it’s close enough and it works just fine.  Chts are cool.  At worst, I’m running 67%. 15 lop.

[MikeOH]

57 minutes ago, Ragsf15e said:

Good question.  I use 9gph or less... it’s close enough and it works just fine.  Chts are cool.  At worst, I’m running 67%. 15 lop.

BINGO! Just how I do it.  It's a dead simple procedure; I don't even use the engine analyzer anymore.  Once I've set MAP and RPM I just pull the mixture back to the desired fuel flow.  Then, after things settle for a few minutes I check CHTs and EGTs.  I'm not too hung up on the exact percentage; I figure if I'm much over 5,000 DA, I really can't hurt anything.  End up somewhere between 20 and 40 LOP depending on which cylinder.

[ArtVandelay]

Good question.  I use 9gph or less... it’s close enough and it works just fine.  Chts are cool.  At worst, I’m running 67%. 15 lop.

Kinda depends on your altitude. At 8000’ my FF will be about 9, but at 11000 it’s 8.2, 13000 7.7, 15000 7.2.

[Matt Ward]

10 hours ago, ArtVandelay said:

Kinda depends on your altitude. At 8000’ my FF will be about 9, but at 11000 it’s 8.2, 13000 7.7, 15000 7.2.

This is consistent with what I was seeing as well.  It seems like there are two schools of thought on this: 1) the Mike Busch pull it 'til it's rough, and 2) the target EGT method (ie, 20LOP or whatever).  Do you do it one of those two ways?  My early bias is the latter but then I read stuff about spending any time in the peak EGT zone is abusive so folks do the Big Mixture Pull and all that.  Still trying to separate rational from irrational risk!

[ArtVandelay]

This is consistent with what I was seeing as well.  It seems like there are two schools of thought on this: 1) the Mike Busch pull it 'til it's rough, and 2) the target EGT method (ie, 20LOP or whatever).  Do you do it one of those two ways?  My early bias is the latter but then I read stuff about spending any time in the peak EGT zone is abusive so folks do the Big Mixture Pull and all that.  Still trying to separate rational from irrational risk!

I do both, #1 if less than 7000, #2 if 7000 or above, at 2400 rpm and 7000’ I’m close to 65% maxed out.