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leaning at high power settings

T

Tom Martin

Well Known Member
HTML: 
Originally Posted by Kahuna  
Bob, Today I got the following
DA 8500', WOT, 2700RPM, 50ROP, 206ITAS, 16.0GPH

Without checking my power chart I would say that you are above 75% here and I have always been a bit nervous about aggressive leaning in that power range. I regularily run LOP below 65%. What are the leaning guidlines above 75% power. Last year I ran full power and full rich in the Rocket 100 race and I am curious to know how much I can safely lean at these high power settings.
I was at 30.5", 2600 rpm and around 26gph. This is with a MT three blade that does not seem to show a speed advantage at higher rpms. During this race my engine was running very cool in terms of CHT and oil temp so I certainly could have leaned more in that regard but these are not the only concerns at high power settings. Leaning even a little, would increase my power but how much is safe?
 
Over 75% power is not available at 8500' DA on a normally aspirated engine. Under 75% power Lycoming says you can lean all you want to.
 
You can't lean safely

Tom Martin said:
HTML: 
Originally Posted by Kahuna  
Bob, Today I got the following
DA 8500', WOT, 2700RPM, 50ROP, 206ITAS, 16.0GPH

Without checking my power chart I would say that you are above 75% here and I have always been a bit nervous about aggressive leaning in that power range.
Click to expand...
You should be worried about aggressive or any leaning above 75% power. But as Mel said its impossible to be at more than 75% power at 8,500 feet DA.

I regularly run LOP below 65%. What are the leaning guidelines above 75% power. Last year I ran full power and full rich in the Rocket 100 race and I am curious to know how much I can safely lean at these high power settings. I was at 30.5", 2600 rpm and around 26gph.
Click to expand...
There is no procedure; You can't or at least with out extreme risk of damage to the engine. 30.5" of MAP is outstanding. Not sure what altitude / ambient pressures you where running in, but 30.5" is very high. Typically you see 29" at full throttle on take off at sea level. I suspect you must have been going fast and getting some induction ram rise. Any way to the point you where near, at or above 100% power. Leaning at this MAP/RPM would be ill advised in my opinion and if you call Lycoming they will tell you the same.

This is with a MT three blade that does not seem to show a speed advantage at higher rpms. During this race my engine was running very cool in terms of CHT and oil temp so I certainly could have leaned more in that regard but these are not the only concerns at high power settings. Leaning even a little, would increase my power but how much is safe?
Click to expand...
You mention CHT, which does have some thing to do with with leaning, but its all about Detonation, an uncontrolled spontaneous explosion verses a controlled combustion burn. You can't tell if you are detonating with cockpit instruments or seat of the pants.

Lean mixtures burn at faster rates than rich. When you have high power you have high combustion pressures. Combine volatile lean mixtures and high pressure you will detonate, which is like a hammer pounding verses a push on the piston with controlled combustion. The rich mixture slows the combustion and suppresses detonation and of course does cool the engine. Detonation is literally a hammer on the piston. Don't believe it if some one tells you detonation is no big deal. It can burn pistons, damage bearings and a whole bunch of other things that are not good. The good news is its fairly easy to avoid with proper operation, which means leave the red knob alone above 75% power.

Now it would be nice to run near detonation and not in it, which is efficient, but in an air cooled aircraft engine that is impossible to control or determine, thus we run slightly rich above 75% power. Detonation is said to have the symptom of rising CHT and dropping EGT. Well that is real great is you can detect it, but by than it may be to late, if you even can tell. If you are racing at high power, race full rich. You may save fuel and even make a few HP more but its not worth it. A lot of smart people have tried to OUT SMART Lycoming and have ended up with a pile of worthless metal, that was formally known as their engine.
 
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Tom Martin said:
HTML: 
Originally Posted by Kahuna  
Bob, Today I got the following
DA 8500', WOT, 2700RPM, 50ROP, 206ITAS, 16.0GPH

Without checking my power chart I would say that you are above 75% here and I have always been a bit nervous about aggressive leaning in that power range. I regularily run LOP below 65%. What are the leaning guidlines above 75% power. Last year I ran full power and full rich in the Rocket 100 race and I am curious to know how much I can safely lean at these high power settings.
I was at 30.5", 2600 rpm and around 26gph. This is with a MT three blade that does not seem to show a speed advantage at higher rpms. During this race my engine was running very cool in terms of CHT and oil temp so I certainly could have leaned more in that regard but these are not the only concerns at high power settings. Leaning even a little, would increase my power but how much is safe?
Click to expand...


Tom,
As others have stated, your at less than 75% power at DA8500'. Also I was 50ROP which is also plenty safe at any power any alt. on a normally aspirated engine.

I was given guidence by Don at AFP and he said I could safely run LOP at any MP below 24".

If you want the best power, ~75ROP is the best power. If you were running full rich on the race, you left a bunch of power on the table and left a bunch of unburnt fuel in your wake. The only reason to run more ROP than 75, is for cooling. If your chts are in the green, any richer is just a waste of fuel and power.

Best,
 
Ditto what others have said. One of the reasons I like my GRT display is that it displays percent power. It uses a table that I loaded from the power chart graph for my engine. Above 75% power, I usually run full rich, but I'm only in this range during takeoff and climb. I immediately pull back upon reaching altitude to less than 75% power. I lean to peak because I understood that's where the most power is, not 50 or 75 ROP. If I'm cruising for a long time, I run LOP as long as it runs smooth.

I know you were down low for the ROcket Run, and I believe you can lean to 50 ROP at full power as long as your temps are okay.
 
I just read the real scoop from Jim Ayres in another thread. Best economy = peak, best power = 100 ROP.
 
I think best power is 150F

f1rocket said:
I just read the real scoop from Jim Ayres in another thread. Best economy = peak, best power = 100 ROP.
Click to expand...
Best (greatest) power is = 138 ROP not 100F. The Best power range is from 100-175F ROP. See Lycs charts.

100 F is a compromise good number for power and econ, but right at 75% power or say above 70% power, I'm more comfortable with at least 125F to 150F ROP.

The reason 50-70F ROP is less than desirable is because its peak CHT (as shown in Lycs power chart). However when get down to 60% power or less, flying at high altitude its OK to be at 50F ROP because you are running cool (low power) any way.

People use Peak EGT because its good econ but not power, and the CHT is less than 50-70F ROP. The Econ range as you see in the chart is 20F ROP to 75F LOP. The LOP side is usually limited by rough running and increased loss in power. I would not run peak at 75%, but if you less than 60% power range, peak is OK. Lyc says you can run at Peak. This is where the LOP debate comes in. If you can run LOP than do it. You see cooler temps but also note the drop in power. A little LOP is good but not if you are running rough or the power has dropped appreciably.

 
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(AFR) guage seems to work for me....

Tom Martin said:
HTML: 
Originally Posted by Kahuna  
Bob, Today I got the following
DA 8500', WOT, 2700RPM, 50ROP, 206ITAS, 16.0GPH

Without checking my power chart I would say that you are above 75% here and I have always been a bit nervous about aggressive leaning in that power range. I regularily run LOP below 65%. What are the leaning guidlines above 75% power. Last year I ran full power and full rich in the Rocket 100 race and I am curious to know how much I can safely lean at these high power settings.
Click to expand...

Tom, I use a rich lean indicator that uses an O2 sensor in the exhaust to measure the oxygen content. The guage shows the air fuel ratio (AFR), it's very simple and works great. I have over 350 hours on the O2 sensor will no problems. Some people say the 100LL fuel will create problems because of the high lead, but this is Voodoo, each 100 hours take the O2 sensor out and clean it with a butane torch to get rid of the lead on the sensor and screw it back in.

Now back to leaning at high power/full throttle at low altitudes. Take this at your own risk but it works for me. I lean back to an air/fuel ratio of 12:5:1 at high power. The 12:5:1 AFR is about right for max power and keeping it out of detonation. At less than 75% I lean back to around 14:3:1-----14:7:1 range. Hope this helps. BTW you can buy the AFR guage through anyone like Summit Racing/Jegs, etc. along with a 2 wire O2 sensor. Good luck, hope this helps.
Click to expand...
 
you all have not spent any time reading John Deakin's engine management articles on AvWeb. 50 to 100 deg ROP is EXACTLY where you don't want to run on a NA engine producing > 65 % power. The Lycoming recommendations were written by marketers not their engineers. We monitor our engines and EGT's because they are a decent proxy for where maximum internal combustion pressures (ICP) are generated. empirical info show that in fact this occurs between peak and 100 ROP.

There are whole courses given on this topic so one post really is not even the beginning of what is needed for this important a topic so i think i would do better by posting links:

www.advancedpilot.com
http://www.avweb.com/news/pelican/list.html (Where should i run my engine?)
 
The power chart that I have stops at 28" and 2500 rpm and gives a fuel flow of 19.2 gph and 232 hp, or 93%. So I must have been running at close to 100% power. I checked my notes and my MP was 29.9 to 30.1" during the race, not 30.5 as mentioned earlier. I did see 30.5" last sunday at 1300 asl with a an altimetre setting of 30.1.
I am not interested in the fuel savings for a short race like this, just more power without damaging the engine. If I am burning 26gph, what about 22 or 24gph if everything is in the green. My engine was running very cold last year during the race and if nothing else it would be nice to get the temperatures higher in the green and not have to worry about detonation.
 
Mel said:
Over 75% power is not available at 8500' DA on a normally aspirated engine. Under 75% power Lycoming says you can lean all you want to.
Click to expand...
It depends on how efficient the air box is, and how fast you are going. The following is for the IO-360A series. On a standard temperature day, at 8500 ft, at 2700 rpm, you would need about 21" MP to get 75% power at 2700 rpm. The ambient pressure at 8500 ft is 21.8" MP. At 206 KTAS, there is up to 1.6" of ram pressure rise to take advantage of, so the pressure at the entrance to the air inlet on the outside of the cowling could be as high as 23.4".

So, it may be possible to get more than 75% power at 8500 ft DA, if you go fast, and if you have an air box that doesn't sap too much of the pressure.
 
rv969wf said:
Tom, I use a rich lean indicator that uses an O2 sensor in the exhaust to measure the oxygen content. The guage shows the air fuel ratio (AFR), it's very simple and works great. I have over 350 hours on the O2 sensor will no problems. Some people say the 100LL fuel will create problems because of the high lead, but this is Voodoo, each 100 hours take the O2 sensor out and clean it with a butane torch to get rid of the lead on the sensor and screw it back in.

Now back to leaning at high power/full throttle at low altitudes. Take this at your own risk but it works for me. I lean back to an air/fuel ratio of 12:5:1 at high power. The 12:5:1 AFR is about right for max power and keeping it out of detonation. At less than 75% I lean back to around 14:3:1-----14:7:1 range. Hope this helps. BTW you can buy the AFR guage through anyone like Summit Racing/Jegs, etc. along with a 2 wire O2 sensor. Good luck, hope this helps.
Click to expand...

Alan,
It has been about 18 months since you posted this message, is the 02 sensor still working OK with 100LL? I really like the idea of leaning with the A/F ratio information, it is how we arrived at a compromise fuel pressure to control mixture with the Subby injection system.
The only down side of my install is I have 4 pipes, I'd have to rely on information from one cylinder.
Click to expand...
 
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