Cruise low RPM on Continental

[CoachTom]

I have been reading some posts where folks are going low on their cruise RPM with Continental Engines.

 

Maybe somebody could clarify for me as I see my POH has minimum RPM for specific altitude blocks and Continental has a CSB published for low RPM to be avoided for counterweight issues?

 

Understand the lower the RPM, the more efficient the prop system is as well as efficiency of flame spread and power extraction from the combustion cycle, but which is it:  POH/CSB RPM or what your engine "will do" LOP??

 

And if you could share if you fly a 231 with inter-cooler and Merlyn; do you use the 252 POH charts or do you use your 231 (LB1 Engine) POH for cruise engine management and adjust power per the differential temp of the inter-cooler?  Also as a caveat to both of those POH's: if you fly cruise LOP, what are your your power, RPM, and fuel flow settings?

 

Thanks in advance to anyone whom might be able to assist!

 

Tom McDermott

1980 M20K

Merlyn/Inter-cooler/ LB1

[jlunseth]

Hi Tom.  My aircraft is same as yours, just a year younger.  The "low RPM limits" in the POH are there because the factory engine can't make the necessary power at lower RPM's and the turbo is already working very hard to stuff the necessary molecules of air into the cylinders.  For example, there are no 2200 RPM settings in the chart for 75% power above 14k.  14 or 15k is the critical altitude for the factory engine ( I can never remember which it is because it does not apply to the Merlyn equipped engines like ours).  The engine becomes, in effect, a normally aspirated engine above that altitude, and to produce the desired HP the RPM's need to go up.  Eventually, if you look at the charts, only the 2700 RPM setting (max for the engine) is shown in the charts for the higher altitudes.

 

The Merlyn wastegate continues to close and therefore to provide the air flow to produce 100% HP, up to about 22k.  You can use lower prop. settings if you like.  I have tried everything from 2200 to 2700.  I generally set the prop at a speed that produces a smooth engine, and after many flights that is usually 245 for me.  Don't ask me why the "50," 2400 works just as well and so does 2500, what can I say I like 2450.  I have used this at all altitudes, up to FL220 at least.  I have noticed that 2200 seems to be a little low for my engine unless I am using lower power settings.  It produces more vibration and airspeed slows a little.  Use whatever you like though.

 

I know that the GAMI/APS guys say that flame front spread is supposed to be better at lower RPM's when operating LOP.  They necessarily generalize when they talk about settings (because they are talking to a group with every type engine represented).  They generally fly the big bore Beechcraft engines, and not everything seems to apply to our small bore 360's in the same way it does in their 540-550's.

 

I have not had luck running LOP by setting a cruise MP, say 29", and then leaning to the LOP side.  The power and MP fall off pretty sharply.  I have had some luck with a pull-push technique, where I make the cruise MP setting, pull over to the LOP side, then put MP back in up to a number between 32 and 36", with a fuel flow of 11.5.  The GAMI/APS people teach that on the lean side, fuel flow dictates HP, unlike on the rich side where it is a function of MP, RPM, temp., etc.  The formula for our engines is GPH x 13.7 = Total HP, and 11.5 GPH produces 157.5 HP which equals 75% power in our 210 HP engines.  So according to the GAMI/APS teachings, you should be able to run 11.5 GPH and pretty much any MP and get 75%.  In practice, what works best is something between 32 and 36 ', and I generally use around 32-34.  So I pull to the lean side, and then push in MP to get what I want.  You probably understand though, that the fuel system in our engines will add or subtract more fuel flow with changes up or down in MP.  So you have to fiddle with the mixture at the same time to get it go the 11.5 setting (or whatever you want for %HP). 

 

I don't totally trust that everything is working in the engine as the GAMI/APS people predict, so I generally run ROP at 75%, 13.3 GPH, 2450 RPM.  I have tried the "push-pull" to the lean side, but only under 12k.  I might try it higher up this summer.  Someone in another thread recently wrote about putting the fire out trying to go to lean at higher altitudes, and that concerns me with our engines because of the high altitude restart issue (the turbine stops spinning).

 

I honestly can't say I notice any significant change in prop efficiency in the altitudes our aircraft is certified for.  The prop system will of course change the prop pitch to maintain an RPM setting, so the prop may be seeing something I don't see on the instruments, in other words it may be running at a coarser pitch higher up, than it would at say 10k, but I don't notice any big speed drop if I reduce cruise RPM's at any altitude.  The Merlyn pretty much ensures that the engine will get all the air it needs, unlike the factory fixed wastegate that becomes NA at 14 or 15k.

[CoachTom]

I wonder as well as I know you are also wondering why you cannot run LOP.  You have been trying for a while now haven't you?  

 

 

Any reason you run at 13.3?  Is it the function of horsepower you are looking for?

 

So, for all intents and purposes, if you dial in the mixture at 11.5 gph, you are producing 75% power.  By increasing the MP, all you are doing is leaning out the actual burned mixture because you are using more air, technically getting leaner?  By getting leaner, the CHT should descend the more MP you add until it gets rough (too lean).

 

I am usually flying between 10.5 and 11.5 depending on direction unless i am heading to NY which is a 4 hour flight at 17.5.  I use the 252 charts for power (2400 rpm\ 29.6 MP\ 12.7 GPH) but for my low altitude flights, i am going to try the 2400\ 11.5\ and 32 to 34 to see what my temps are.  Number 1 CHT is always 30° higher than all of the other CHT's.  Only thgink it has to be the Merlyn sitting underneath and Turbo restricting airflow...  Any ideas on that would help too...

 

Thanks again.  Maybe i can get to a point where i won't have to fly with cowl flaps in trail.

 

TMcD

[Buster1]

There is a CSB that restricts going below 2300 RPM on some big bore Contis.  Check out the details and see if yours falls into this restriction.

 

http://www.tcmlink.com/pdf2/CSB09-11.pdf

 

Lots of engines are affected by this:

 

MODELSAFFECTED: O-470-G; IO-470-N; IO-520-BB, CB, MB, P; IO-550-A, B, C, D, E, F, G, L, M, N, P, R; IOF-550-B, C, D, E, F, L, N, P, R;

TSIO-520-AE, BB, BE, CE, DB, EB, JB, KB, LB, NB, UB, VB, WB; LTSIO-520AE; TSIO-550-A, B, C, E, K; TSIOF-550- J;

TSIOL-550-A, B

[jlunseth]

I can run LOP.  I can certainly run LOP if I want to settle for 65%HP or less.  But when I am cruising, I usually want to actually get somewhere.  I can run LOP in higher power settings, but am not totally comfortable with what might be going on inside the engine, so not rushing into it.  And in any event I have done quite alot of flying in the last half of the year, but most of it manuevering to get ready for my Commercial (which I passed, yay!) and not much cruising.

 

I run at 13.3 because time and time again when I set cruise power, that gives me 100dF or better, Rich of Peak.  CHT's are cool and so are EGT's.  I know lots of people run in the mid 12's at 75%, from what I see on my JPI that is right in the red box, and probably right at peak or close to it.  If you are running at 29.6/2400/12.7 I would have to say you are right about at peak or maybe in the 50-80 degrees ROP range, which is not a good place to be.  The factory POH's have settings like that, but they recommended running at peak for Best Power, and that is just exactly where GAMI/APS says from their engine tests, not to run.

 

"So, for all intents and purposes, if you dial in the mixture at 11.5 gph, you are producing 75% power. By increasing the MP, all you are doing is leaning out the actual burned mixture because you are using more air, technically getting leaner? By getting leaner, the CHT should descend the more MP you add until it gets rough (too lean)."

 

You are exactly correct, and what should be happening in theory except for two things.  You can't increase MP without also increasing fuel flow, because the two are interlinked.  So what concerns me about this procedure, is that in theory if fuel flow alone dictates HP on the lean side, why is it that when I get over on the lean side, establish 11.5, and then add MP, the plane speeds up.  One reason is that more fuel flow has been added, so I am closer to peak than I wanted.  So I dial out the mixture a little bit to get back to 11.5 and find that, if I have gone from say 29" and 11.5, to 34" and 11.5, the plane is faster.  Why would the plane be faster?  That tells me that despite what the GAMI/APS theory is, the engine is increasing HP over on the lean side, by simply increasing MP, keeping GPH constant.  Bear in mind, I am talking about increasing MP while lean of peak, attempting to maintain the same fuel flow, I am not comparing the LOP setting to a ROP setting.  Makes me a little concerned that what is going on is not exactly what the theory says should be going on. 

 

I should add that the CHT's and EGT's look as they should if I am operating LOP.  The CHT's go down from around 380 at the 75% power ROP side, to about 320 at the 75% power LOP side, and the EGT's go up and are in the low 1500 range, say 1525.  TIT is a little higher but well within limits, usually at the high end of the 1500's like 1580.  My A&P says they have seen engines come back after 300-400 hours running LOP, using a technique where the engine is leaned out and power is added back in. I think their experience is with people doing that in the real race cars, like the Acclaims, and maybe not allowing for the fact that if you go over on the lean side, make a setting, and then add MP, you are also changing fuel flow to be richer.   I have also discussed it with Bruce Jaeger, and the question he raises is what are the higher EGT's doing to the valves?

 

I know that GAMI's information is to the contrary, that LOP is healthy for the engine.  But having been to their seminar, they don't spend any significant time on LOP in small bore turbo engines like the 360, they run big bore engines mostly NA but some turbo.  If the plane's speed did not pick up when going from 11.5 at say 29" on the lean side, to 11.5 at 34" on the lean side, I would go with GAMI.  But it does pick up, so what they say does not appear to strictly apply to yours and my TSIO-360-LB's.

 

It is colder than crap here right now, and I am going to try to sell the plane in the spring to move to a Bravo, so not doing any experimenting.  I have in mind when the weather improves, to try simply climbing at takeoff MP (about 36") and then do the "big pull" over to the lean side.  I bet I will wind up with 11.5 and about 34".  Would be a simple way to do it. 

 

I have the mechanics close the cowl flaps for winter, then put them in trail for the summer, the engine seems a little easier to keep cool with a slight trail.  My engine cools fairly well, my standard cruise setting usually gives me 380 CHT's. 

 

I would guess your number one is high because it and number two are the last cylinders in line for cooling air.  In my engine it is number two that is usually higher.  You could have your mechanic fiddle with cooling air flow, they can sometimes rig something to block or increase flow to one cylinder.

[jlunseth]

There is a CSB that restricts going below 2300 RPM on some big bore Contis.  Check out the details and see if yours falls into this restriction.

 

http://www.tcmlink.com/pdf2/CSB09-11.pdf

 

Lots of engines are affected by this:

 

MODELSAFFECTED: O-470-G; IO-470-N; IO-520-BB, CB, MB, P; IO-550-A, B, C, D, E, F, G, L, M, N, P, R; IOF-550-B, C, D, E, F, L, N, P, R;

TSIO-520-AE, BB, BE, CE, DB, EB, JB, KB, LB, NB, UB, VB, WB; LTSIO-520AE; TSIO-550-A, B, C, E, K; TSIOF-550- J;

TSIOL-550-A, B

 

Tom and I have TSIO-360's, not affected by the CSB.  Bolduc did some work on my engine a couple of years ago, and it is smooth as silk at any RPM, but I generally only go down as far as 2200 if I am using some kind of reduced power setting.  What I don't like about 2200 and cruise settings like 75%, is listening to the engine pressure up when I add the MP. 

 

Interesting, I just looked at the POH for my aircraft and the Cruise Power Schedule shows 13.3 GPH for Best Power, 75%HP, at 2500, 2600 and 2700, 13.1 at 2400, it also shows 11.1 at 2400, 11.2 at 2500, and 11.5 at 2600 and 2700 for 75% and Best Economy.  MP's vary depending on altitude.  So the Mooney test pilots saw pretty much what I am seeing on my JPI, and even better, the Mooney POH, without calling it that, is actually advocating LOP operations for Best Economy.  That makes me feel better about my LOP technique, I am going to give it another try.

[Buster1]

Why are you guys moving the MP?  John Deakin, Atkinson and all the APS guys recommend Wide Open Throttle (WOT) from application of power at T/o to all the way to Top of Decent (TOD).  I agree!  Why close off air (MP) when at altitude?  Get all the air you can and then control combustion / speed with mixture and prop.

[carusoam]

Don't do it,

[jetdriven]

2-3 inches of MP over the ROP book power settings ought to get you close to the power setting you are looking for on the LOP side. Instead of leaning it out and then adding MP, why not just set it 2" higher then do the BMP from that?

[jlunseth]

Buster you are thinking normally aspirated.  For one thing, us guys with TSIO-360's don't want all the air we can get, at least most of the time.  That would overboost the engine.  I could probably get 45" or better on takeoff if I wanted to, but the engine would not like me anymore, probably would unfriend me permanently, parts of it might even depart the aircraft in a huff.  I am thinking I could leave it at 36" (takeoff power) and do the BMP to 11.5, and that will probably give me 32-34".  I just cringe at the idea of going through the red box, even really quickly, while at 100% HP. 

 

One issue for us TSIO-360's is a little bit of negative bootstrapping.  As you pull mixture out and lose horsepower, the turbo slows down and you lose some MP, so you lose more HP.  The idea is to put some MP back in to overcome this.  I believe there are automatic controllers that manage MP to a specific setting, I believe the Bravo does that in fact ( I don't know, I don't fly one).  That is not what happens in the TSIO-360's whether with the Merlyn wastegate or the factory fixed wastegate.  The Merlyn is more of an equilibrium manager, i.e. if all other things remain equal, it will keep MP where you put it.  In this instance, other things are all changing.  Another issue is not wanting to run the turbo harder than necessary.  Sure, I can ask it to put out 36" at cruise, 11.5 GPH lean of peak.  But if all that is doing is keeping the engine further lean without producing more HP, and maybe also putting more air through the cylinders to cool them better, I am trading a hotter turbine for a colder cylinder, so why?  Best to put in only as much air as is needed.

[Buster1]

Okay, thanks...that makes sense. Didn't know you were talking turboed.

Appreciate it!

[David Brown]

Just a couple of points here regarding the above posts.

 

Small bore or big bore......the fuel knows no different. The theorhy is the same whether it is a 360TCM or and IO550, a Lyc 540 or a P&W radial. so no need to worry about that.

 

At 75% power ROP you should be aiming for something like 180dF ROP to stay out of the red box, at the fuel flow you talk about the 100dF ROP is not enough.

 

Alternatively about 40dF LOP would do,

 

As for adding back MP, when you do this the F/A ratio is already set, and the extra MP increases the mass airflow and the fuel flow at the same time to maintain the same approximate F/A ratio. The mixture is not getting richer except if there is a bit of non linear behaviour which is only small any way. Do not stress about that.

 

How you are operating, fearing that you are avoiding something you are not sure of, by doing exactly the opposite of what the science and data shows is less stressful, is somewhat confusing to me. Makes me cringe actually. Perhaps another visit to the course. Email John Deakin and ask can you do it again and how much. He will cut you a pretty good discount for sure.

[David Brown]

2-3 inches of MP over the ROP book power settings ought to get you close to the power setting you are looking for on the LOP side. Instead of leaning it out and then adding MP, why not just set it 2" higher then do the BMP from that?

 

Great tip.......but Byron, why make it so complicated ? 

[Nemesis]

I should add that the CHT's and EGT's look as they should if I am operating LOP.  The CHT's go down from around 380 at the 75% power ROP side, to about 320 at the 75% power LOP side, and the EGT's go up and are in the low 1500 range, say 1525.  TIT is a little higher but well within limits, usually at the high end of the 1500's like 1580.  My A&P says they have seen engines come back after 300-400 hours running LOP, using a technique where the engine is leaned out and power is added back in. I think their experience is with people doing that in the real race cars, like the Acclaims, and maybe not allowing for the fact that if you go over on the lean side, make a setting, and then add MP, you are also changing fuel flow to be richer.   I have also discussed it with Bruce Jaeger, and the question he raises is what are the higher EGT's doing to the valves?

 

I know that GAMI's information is to the contrary, that LOP is healthy for the engine.  But having been to their seminar, they don't spend any significant time on LOP in small bore turbo engines like the 360, they run big bore engines mostly NA but some turbo.  If the plane's speed did not pick up when going from 11.5 at say 29" on the lean side, to 11.5 at 34" on the lean side, I would go with GAMI.  But it does pick up, so what they say does not appear to strictly apply to yours and my TSIO-360-LB's.

 

jlunseth,

I too have been to the APS. Was that about the best money ever spent on aviation or what?

 

Even given that statement, I too found the info lacking with regards to turbo-supercharging.

 

For those that don't know, turbo-supercharging (TSIO) is where the air induction system generates more than normal atmospheric pressure for 100% HP ~ 36" to 40" MP or even higher for some older engines. Turbo-normalizing (TN) is where the system generates only normal atmospheric pressure for 100% HP ~ 30" MP or so - in other words, normal sea level pressure. Both systems are turbo charged but supercharged engines use lower compression ratios - for TSIO-360 purposes 7.5 : 1. Normalized (TN), normally aspirated (NA), and carbureted engines use 8.5 : 1 for the most part.

 

As Buster1 pointed out, much is made at the APS about always using WOT, but WOT can not apply to us TSIO guys because the fuel system would likely go to cut off before we ever made it to LOP or the stoichiometry between peak EGT and cut off would be very narrow and unstable. In addition to that, as you pointed out, there is no way that I would want to try a BMP beginning anywhere close to 100% HP. Moreover, we could never operate at 40" MP (37" MP or so for intercoolers) because of excessive TIT and/or CDT.

 

So, the question remained in my mind, in a TSIO engine, what MP could be considered as WOT? The answer is simple and of course is about 30" MP because that is what is approximately normal pressure at sea level. There still remains a difference however and that is the lower compression engines that we use. Braly, Atkinson, and Deakin all thought that going to perhaps 32" MP in a TSIO engine would be enough compensation for the compression issue. I still think that 32" MP is a bit low, but higher MP will once again get that TIT and CDT creeping up. I know that CDT is probably not an issue for you but it is for me. I think there is an inherent advantage that a TN engine has over a TSIO engine.

 

I am sure you remember the part about what happens at peak EGT, on the rich side of peak EGT, and on the lean side of peak EGT. For those that have not been to APS or have not really thought about it, at peak EGT the fuel / air charge is equal and there is nothing left over except some trace gases. If on the rich side, there is excess fuel that is not being burned, that is wasted, that more readily turns to carbon, more readily creates deposits inside the cylinder, and most importantly creates the highest cylinder pressures and temperatures. On the lean side of peak, there is excess air but all of the fuel is burned, cylinders stay cleaner and the internal pressures and temperatures are lower. The key to that last sentence is that there is excess air in the fuel / air charge. That is why when LOP, adding even more air (MP) or even reducing some air (MP) makes no difference to power output. All of the fuel is burned and there is always leftover air. Adding more MP does nothing because all of the fuel is already burned.

 

The one thing that I have not seen in any of your writings is how far LOP that you are trying to operate. Remember also that as the percentage of HP increases, so does the size of the red box, even on the lean side of peak EGT. At 60% or less, the red box pretty much disappears and an engine can be run with the mixture set anywhere. At 65% the red box extends up to peak EGT. At 70% it goes to maybe 10dF LOP. AT 75% the red box goes to about 30 dF LOP. So, to safely run at 11.5 GPH LOP and pretty much any MP that will support that, for myself I would be shooting to be at least 50 dF LOP and maybe a little more.

 

As I have written about before, I am severely limited by TIT when trying to operate LOP. Anything over about 62% for me and 9.5 GPH LOP, my TIT shoots up to over 1650 dF so my turbo is just less efficient than I would like or need. However during those brief excursions that I have tried at 70% or 75%, I have found that the balancing act of MP vs. FF has been very tedious and unstable. The engine is smooth all the way to 80 or 90 dF LOP, the MP and FF are stable at a given setting, but trying to adjust FF while trying to keep MP the same so that I can check on the number of dF LOP is almost impossible because I run out of time for the rising TIT and because I want to be in the red box only briefly while I check. When I am at 62% or less, I have all of the time in the world to check how many dF LOP I actually am operating. At higher power settings I do not.

 

I would suspect from my own experiences that somewhere before, during, or after your transition from 29" MP to 34" MP, that your FF as related to the number of dF on the LOP side has changed and some or all of your cylinders may have intruded into the red box and back to the rich side where changing air flow does affect HP output. Let me know what you think.

 

With regards to your discussion with Bruce Jaeger about rising EGTs, my TIT is high because of a less efficient turbo and that it has to spin faster, not because of rising EGTs. Burned valves are not because of LOP, ROP, or even peak. Valves burn because of poorly mated valve seats which TCM is famous for. Remember, we are shooting for LOP operations therefore by definition the EGTs are lower. EGTs are max at peak and are lower on both sides of peak, lean or rich. If your EGTs are rising for any reason, then your mixture settings are changing and getting closer to peak EGT, regardless if you are coming from the rich side or lean side, and at higher power settings, that peak EGT can be a rough place for an engine to be.

 

Dave

1980 M20K 231, LB, Merlin, no intercooler

[fluffysheap]

One reason is that more fuel flow has been added, so I am closer to peak than I wanted.  So I dial out the mixture a little bit to get back to 11.5 and find that, if I have gone from say 29" and 11.5, to 34" and 11.5, the plane is faster.  Why would the plane be faster?  ...  Makes me a little concerned that what is going on is not exactly what the theory says should be going on.

 

Why are you guys moving the MP?  John Deakin, Atkinson and all the APS guys recommend Wide Open Throttle (WOT) from application of power at T/o to all the way to Top of Decent (TOD)

 

I think I can answer both of these with one chart.  See Pelican #18: http://www.avweb.com/news/pelican/182084-1.html?redirected=1

Scroll down about 25% to the chart where he says "do not just skip over that chart."  The chart is for an IO-550, but at least it's a Continental.  The independent variable (x-axis) in this chart is the fuel flow, and the MP and RPM are constant.

 

The important curve to look at here is the BSFC.  BSFC = "brake-specific fuel consumption" and it is a measure of the amount of HP put out by the engine for a given amount of fuel, under particular conditions.  Put simply, it's a measure of fuel economy.  Assuming you hold other factors such as RPM constant, your mixture controls your fuel economy.  If you improve BSFC, you can either burn less fuel for the same power, or get more power for the same fuel!  But leaner isn't always better.

 

What we think of as "peak" is peak EGT, but it isn't actually quite the same as peak efficiency - even though POHs often pretend it is.  You can see on the chart that BSFC actually peaks well to the lean side of peak EGT.  So, as you go leaner, for a while the engine actually gets more efficient, and puts out more power for the same fuel flow.  And so, the plane goes faster.  But the effect only goes so far.  After a while, both efficiency and power start to drop off, and not long after that the mixture gets too lean to burn.  So that explains not using WOT at all times (plus, it works the turbo harder).

 

Now, these charts are written in terms of manifold pressure, not fuel flow.  So, increasing MP slightly means the chart isn't 100% identical between two different mixture settings.  But the peaks will move gradually and you should still be able to perceive the effect.

 

What causes the change in BSFC?  Well, mostly, a difference in the combustion process.  The engine needs to reach peak pressure at exactly the right time; if the combustion pressure peaks too soon detonation can result, and if it peaks too late, the piston has already moved and the expanding gas has nothing to push against.  Running leaner mixtures (assuming LOP) slows the combustion rate.  Up to a point, this moves it closer to the "right" time, and even a little past that point BSFC might still improve because even though less power might be produced, less fuel is also burned.  But go too far, and the combustion is just too slow, and too much of it happens after the piston has moved too far along the expansion stroke.  Pelican mentions this in one of his articles, where extremely lean mixtures cause not only loss of power, but actually a rise in EGT because the fuel is still burning during the exhaust stroke!

 

Anything over about 62% for me and 9.5 GPH LOP, my TIT shoots up to over 1650 dF so my turbo is just less efficient than I would like or need.

This is similar to me.  I don't have the foggiest idea how many degrees LOP I run.  I just run somewhere LOP, and adjust MP and fuel flow to keep the TIT under control.  At about FL200 or below, I run something like 10.5GPH and about 33-34" (intercooled) MP, which keeps TIT in the 1500s.  I bet I'm something like 100 or 150 LOP, but I really have no solid evidence.  Above FL200, I don't have enough surplus turbonium and I usually end up with something like 10GPH and 30" MP.  You might be limited by your lack of intercooler.

[jlunseth]

I understand the chart.  What concerns me is that if I am truly lean of peak (and my monitor says so) then 34" and 11.5 GPH should be leaner than 29" and 11.5 GPH, and there should be no difference in HP, or if there is, the leaner mixture (the 34" one) should produce less power.  But it produces more power, which suggests to me that I was not really lean of peak, or the other possibility, which is that be moving MP, peak has changed. 

 

But having found the 11.5 setting in my factory POH, that gives me a little more confidence that the engine is fine.  It will be a little bit, but I am going to try it again. 

 

PS I have not tried LOP above 12k.  Cooling would be part of the issue, but also the turbo would need to work quite hard to produce the MP that seems to be needed.  Maybe this summer I will try some more.

 

Nemesis I just read your post.  Very good.  You ask what degrees LOP I am running at, and the answer is that despite having very good engine instrumentation (JPI 930) I have no way of knowing.  In my experimenting, when I initially lean, the MP falls and so does the power, as I have said.  And the fuel flow changes.  So I lean to around -10 dF LOP.  Then I readjust the settings to put back in some MP and pull out some mixture because putting in the MP adds fuel, so that I wind up at say 34" and 11.5 GPH.  My JPI tells me that I am still about -10 dF LOP, but that is meaningless because peak has changed with the various setting changes.  In theory, if 29" and 11.5 GPH was -10, 34 and 11.5 GPH is further from peak and probably by quite a bit, but I don't know by how much an never will because it is not possible to capture a valid peak, as the settings change that peak is going to change. 

 

I will say that at the APS seminar, the rule of thumb for redbox in a turbo as I recall, was 125 ROP and 60 LOP.  Maybe it was even 150 ROP, I think that is correct.  At any rate, I have found in my engine, with good instrumentation, those numbers are just no valid.  125 ROP is about as rich as I ever go.  Anything higher is just flooding the engine, the cylinders do not run much cooler but the power starts to fall off and fuel flow goes into the 14's.  Same thing on the lean side, the engine runs fine at anything past -10 dF LOP, and -60 in my engine is at the point where the fire is about to quit.

 

I think you asked about higher EGT's.  The EGT's are higher just because they are.  If you run 125 ROP, that means if peak EGT was hypothetically 1600, the EGT is now 1475.  If you run 50 LOP, EGT will be 1550. 

 

I like Byrons idea so am going to try it.

 

David, I did email John Deakin.  He was not helpful, said they do not know the Merlyn product.  Big bore and small bore are different because of the volume of mixture that gets moved. 

[jlunseth]

PS David.  I read your post.  I did not find it very helpful.  Two things.  First, the advice to run my particular engine 180 dF ROP is bad advice.  That setting results in a fuel flow in the vicinity of 15 GPH for my little 360, and is rich enough that power begins to fall of in the same way that it does when going too far LOP.  125 dF is about max, and the engine is quite happy there.

 

Second, since you know this stuff so well, please answer me this.  If (having a JPI 930) I use the LOP lean function and lean from the rich side to a setting that is about 29" and 11.5 GPH, and I am about -15 dF LOP, and I then add MP to 34", adjusting fuel flow so that i remains at 11.5, which means in theory I have a mixture leaner than -15 dF (I am trying to get down to your -40), why does my aircraft speed up?  Why does it speed up significantly, say 5 - 10 knots.  I am on the lean side right?  Only fuel flow should dictate HP, right?  So why does changing MP, change my HP?

[CoachTom]

I am with you John... There is so much with the small bore that is anomalous than with the regularity apparently observed in the big bore.  Using your process of 11.5 gph at 29"MP then pushing up to 34" at the same fuel flow could only mean you weren't using all of the available fuel (right?).

 

I am usually flying 10-12K or up at 17.5K: is the only difference to our engines being the outside air density relative for cooling? LOP should be the same due to the Turbo?

 

This seems to be so easy to derive on paper, but when we try to actualize the concepts, there seems to be so many opinions both from the POH, APS graduates, and Continental. Then how low do you go with the RPM? 2200? 2300? 2400?  with the TSIO-360?

[Nemesis]

This is similar to me.  I don't have the foggiest idea how many degrees LOP I run.  I just run somewhere LOP, and adjust MP and fuel flow to keep the TIT under control.  At about FL200 or below, I run something like 10.5GPH and about 33-34" (intercooled) MP, which keeps TIT in the 1500s.  I bet I'm something like 100 or 150 LOP, but I really have no solid evidence.  Above FL200, I don't have enough surplus turbonium and I usually end up with something like 10GPH and 30" MP.  You might be limited by your lack of intercooler.

 

"turbonium" - That word rates a post all by itself. Thanks fluffy!!

 

It is true that I have seen the CDT getting pretty warm, but my TIT will always get to 1650 dF before I ever need to worry about the rising CDT. I think the relationship between lower CDT with an intercooler and lower output temperatures is very small, only a handful of degrees, so I do not think that an intercooler by itself would help with my high TIT by more than a few degrees at most. Where an intercooler really shines is keeping that induction air cooler so as to provide a much larger margin between normal combustion and detonation.

 

For both fluffy and jlunseth, what I came away from APS with was that the key to LOP operations, #1, numero uno, is knowing precisely how many degrees LOP the engine is running. Lacking that knowledge is not healthy for the engine and is potentially dangerous for me and my passengers. I have very good instrumentation with my JPI-830 as well, but with my less efficient turbo and my apparent lack of turbonium, meaning that my turbo is spinning faster to give me the MP I want and therefore producing more heat, I have never been able to operate for any extended period of time LOP and at higher power settings. Because of the mechanical link between the MP and FF, I just have never had enough time to get a good precise reading of the number of degrees LOP before the TIT starts cooking. LOP, high power setting, it seems that I can have one of those at any given time but not both.

 

Dave

[jlunseth]

Thanks Dave.  That is precisely why I have gone through the exercise that I have, using the LOP lean function and going over on the lean side, then adjusting MP and fuel flow to get the setting that I want.  That way, I get a reading of degrees LOP on the 930.  I wanted to know how many degrees LOP the engine was, before I use the simpler and more elegant technique of just pulling it over and going with a setting, which is what Byron suggested, and I don't mean to say that is not a good suggesting, it is what I would like to do, I just want to know where the engine is operating if I am going to do that.  Since I own the engine, like the rest of you, I get to pay the maintenance if it happens to put the engine in the wrong spot.  The issue is, as I said, that the peak that was detected is no longer valid because the MP and mixture are being changed, so I have not accomplished what I wanted (knowing how many degrees LOP I am).

 

One technical correction Dave.  An intercooler will not affect CDT(compressor discharge temp. or the temp. coming out of the turbo), it will affect IAT (Induction Air Temp., or the temperature of the air going into the induction system).  I have both readings on my JPI.  The CDT will go over redline often somewhere up in the FL190-200 range, but it is no longer an issue as far as engine health is concerned.  The purpose of the redline is to protect against detonation, not to protect the turbo.  The turbo is protected by the TIT redlines.  The intercooler cools the CDT sufficiently that the IAT is never an issue.  That information comes from asking the question at the APS seminar, and directly from Mooney.

 

I like turbonium. 

[fluffysheap]

I do not think that an intercooler by itself would help with my high TIT by more than a few degrees at most. Where an intercooler really shines is keeping that induction air cooler so as to provide a much larger margin between normal combustion and detonation.

 

Well, here's my thought process.  If you increase MP but lean to keep fuel flow the same, then you're leaning out, and TIT and CHT should drop.  So, you're either limited by maximum throttle (no more MP possible, solutions, try lower altitude or reduce fuel flow), by CDT (solution, intercooler) or engine roughness (solution, GAMIjectors, or if you already have them, then rub down the engine with cheetah blood).  It's got to be one of those, right?  Unless I missed something.

 

What I came away from APS with was that the key to LOP operations, #1, numero uno, is knowing precisely how many degrees LOP the engine is running. Lacking that knowledge is not healthy for the engine and is potentially dangerous for me and my passengers.

 

Hmm, I am not sure I'd agree.  The trouble is there is no useful way to measure this.  If you're at low power, you can find the peak value but it doesn't really matter.  But if you're already at higher power, you can't make a real determination because you have to run the engine in the danger zone to find peak EGT.  So I prefer to pull mixture to "somewhere LOP" and then adjust it to keep CHT, TIT and fuel flow all more or less where I need them to be.  Excepting the GAMI lean test, I'm not sure I've ever known exactly where peak is.  I just know "lean enough" or "not lean enough" or occasionally "need to reduce power."

 

Since I usually run somewhere between 10 and 10.5GPH anyway, I'm not going much over 70% power.  So the danger area is pretty small and I'm confident that, even if I don't know exactly where I am on the curve, I'm not in the red box.

[Nemesis]

One technical correction Dave.  An intercooler will not affect CDT(compressor discharge temp. or the temp. coming out of the turbo), it will affect IAT (Induction Air Temp., or the temperature of the air going into the induction system).  I have both readings on my JPI.  The CDT will go over redline often somewhere up in the FL190-200 range, but it is no longer an issue as far as engine health is concerned.  The purpose of the redline is to protect against detonation, not to protect the turbo.  The turbo is protected by the TIT redlines.  The intercooler cools the CDT sufficiently that the IAT is never an issue.  That information comes from asking the question at the APS seminar, and directly from Mooney.

I get what the real purpose of an intercooler is. fluffy suggested that an intercooler might help my situation. Even though there is a very small but measureable effect on EGT while using an intercooler, my main problem is turbo speed and turbo heat. Thus, an intercooler won't do very much for me. If I could get the turbo running more efficiently through overhaul, and could spend more time LOP at higher power settings, my CDT might be up too high and then I would have a direct intercooler issue (or lack thereof). Then I would be looking to cool my induction temps.

 

Well, here's my thought process.  If you increase MP but lean to keep fuel flow the same, then you're leaning out, and TIT and CHT should drop.  So, you're either limited by maximum throttle (no more MP possible, solutions, try lower altitude or reduce fuel flow), by CDT (solution, intercooler) or engine roughness (solution, GAMIjectors, or if you already have them, then rub down the engine with cheetah blood).  It's got to be one of those, right?  Unless I missed something.

 

 

Hmm, I am not sure I'd agree.  The trouble is there is no useful way to measure this.  If you're at low power, you can find the peak value but it doesn't really matter.  But if you're already at higher power, you can't make a real determination because you have to run the engine in the danger zone to find peak EGT.  So I prefer to pull mixture to "somewhere LOP" and then adjust it to keep CHT, TIT and fuel flow all more or less where I need them to be.  Excepting the GAMI lean test, I'm not sure I've ever known exactly where peak is.  I just know "lean enough" or "not lean enough" or occasionally "need to reduce power."

 

Since I usually run somewhere between 10 and 10.5GPH anyway, I'm not going much over 70% power.  So the danger area is pretty small and I'm confident that, even if I don't know exactly where I am on the curve, I'm not in the red box.

The part that you are missing in your first paragraph is that by changing MP, peak EGT has also changed. True, I also would think that holding FF the same and increasing MP only should result in a leaner mixture overall. However, for me I would still be worried about - how much - LOP. I completely agree as to how difficult it is to measure those numbers with a TSIO-360. I think it is ok to run in the red box for short periods of time while trying to test for peak EGT. However, as soon as you touch the red knob, nothing is held as a constant. As soon as you touch the black knob, nothing is held as a constant. Trying to balance the two for me takes much more time than I am comfortable running in the danger areas. That is the biggest problem with the APS, that MP during the APS topics is always held constant unless fine tuning.

 

Lastly, sometimes it is easy to just get mired in the numbers. If I were trying to operate at 75% power, I would be very worried about the numbers because the red box has grown to a pretty large size on the LOP side. At 70% power, and with the red box only at about 10 dF LOP, perhaps even I would think that simply doing a BMP and trying to hold FF as a constant might be a reasonable approach given all of the other problems we have written about.

 

Dave

[jlunseth]

I posted a question on Mooneyspace awhile ago, about why TIT would be high with LOP ops.  My thought process was the same as yours, that if the mixture was lean and mostly burned, and then travels down the manifold to the turbo during which it has the opportunity to be cooled, why would it be high?  Someone, and I regret I don't recall who, with a greater understanding of the subject, posted that there is a secondary reaction that occurs in the manifold. 

 

I agree with Dave that knowing where you are running the engine is important (and was taught at APS).  I also agree that with the BMP technique that they teach, you will not know that, and I have proved to myself that it is difficult or not possible at all to know where the engine is operating in terms of degrees LOP.  The gist of it for me, is that when you have put the type engine on a stand many times (the Beech big bores), and seen in great detail how it operates, you can use a gross technique like BMP with confidence.  When you have not done that and when the engine has controller technology that you do not have experience with as John Deakin says, you are operating as a test pilot and cannot be certain what is happening with the engine.  They also warn at the seminar that CHT's should not be used to determine if you are sufficiently LOP (the Mike Busch technique), because if you happen to have superior cooling you can have good CHT's but high ICP's and even marginal detonation.  My engine is well cooled, and not only that I live where during a significant part of the year it is super cooled and therefore CHT's are not a big help.

 

I can tell you that if I use the technique I have described and pull to 11.5 GPH with an MP between 32 and 36, that my EGT's and CHT's look just exactly as they should if I am operating properly LOP.  The CHT's go from about 380 at a comparable ROP setting, to 320, and the EGT's are higher by about 80 degrees.  What I can't say with confidence is how far LOP I am, nor even if I am LOP, or as I suspect with if the MP is up near 36, running pretty close to peak.  I also do not know what the long term effect of the higher EGT's is on the valves, or the higher effort is on the turbo.  Since I pay for them, these are important issues to me. 

[Will.iam]

This is a fascinating topic as I am struggling with this exact issue myself.  Alot has been learned since 2014 and I would like to know if these guys have any lessons learned since then or things they do differently today than what they were doing back then.  I pull back the MP to 29" then I change the prop to 2200 RPM and do the mixture pull until the engine runs rough.  Then I slowly enrichen it till it runs smooth and then increase MP to 31 or 32" and the fuel flow settles in at 10.5 to 10.6.  This gives me 150kias eventually at 3500ft on a Mooney 252 I know the KIAS would be much higher above 10k.  I noticed that for any MP pressure and fuel flow setting LOP if the TIT starts to get above 1550, I can reduce the RPM and the TIT comes down.  I.E.  I can't run MP 32" and 10.5 FF at 2500 RPM and keep the TIT below 1550 (with cowling full closed) it's more like 1580 or 1590. My limiting factor always seems to be TIT temp as I try to keep it below 1550.  Has the last 6 years taught to keep the TIT that low or are y'all still running up to 1600 continuously? 

[jlunseth]

I am running around 1600 continuously and not having any issues with the turbo after several years, in fact it is over 1,000 hours since the last turbo rebuild. I use 1600 as my limit, if it creeps a little higher, 1603,1605 I am fine with that but I will usually do something about it, like reduce the fuel flow further, which reduces the power output. I haven’t tried reducing the RPMs, I will try that next time.

Depends on altitude too though, props are less efficient at high altitudes and need to run faster.