M20J Cruise Power Settings

[Slick Nick]

It's not "a few minutes" though. It's 8-10 minutes a flight, times that by tens of flights per year, and you've got hours and hours of overly rich engine operation. Who fed you the BS regarding detonation if you don't run full rich below 75%?

You're doing more damage to your engine now than you'll ever do by properly leaning it. It's sloppy airmanship not to look after your engine, especially in a single it's basically your lifeline. 

Edited 17 hours ago by Slick Nick

[Bolter]

5 hours ago, Shiroyuki said:

Maybe I should start leaning it then. 

But here's my question, when should I be worried about detonation? I have this thought which come from primary training that leaning above 75% may cause detonation and damage the engine, that's why I keep it rich. Above 7000ft the engine won't make 75% power anymore so I start leaning. That was my rational and I never put too much thought in it. I also see little benefit in leaning early, maybe 200fpm faster climb, saving a gallong of fuel... And I doubt three minutes of running rich will kill an engine...

Detonation risk is from running within the "red box".  If go overly lean at high power.  You can run 150F ROP at any power setting.  You can only run 0-50F ROP when power has dropped.  The red box is smaller as your power drops.  At 10k, I would often run right at peak EGT since power was down so low.  

[0TreeLemur]

7 hours ago, Shiroyuki said:

Maybe I should start leaning it then. 

But here's my question, when should I be worried about detonation? I have this thought which come from primary training that leaning above 75% may cause detonation and damage the engine, that's why I keep it rich. Above 7000ft the engine won't make 75% power anymore so I start leaning. That was my rational and I never put too much thought in it. I also see little benefit in leaning early, maybe 200fpm faster climb, saving a gallong of fuel... And I doubt three minutes of running rich will kill an engine...

As stated before, there are many reasons to lean in the climb.  Provided that you have an engine monitor that displays EGT and CHT for each cylinder, there is very little risk.  On the other hand, not leaning and running way rich invites creation of carbon deposits that can glow and act like little spark plugs and cause pre-ignition. 

Notice your EGTs the next time you take off.  That's the full power EGT target.   Mine is 1270F.  As you climb,  slowly lean once every couple of minutes to re-establish that EGT target.  Watch your CHT's.   If your engine has good cooling air distribution they will stay below 400F.   In my J they only climb to about 390F in the summer and 350F in the winter while leaning in the climb this way.  If your CHTs start to reach or exceed 400F, then richen it up a bit.  By maintaining the EGT target, you are still running quite far ROP.  In my J, 1270F is about 200F ROP because my cruise max. power EGT's up high tend to be about 1470F.

[0TreeLemur]

On 12/18/2024 at 7:50 PM, PT20J said:

Mooney had different versions of the power charts in different POH versions. Who knows why? Here’s one that shows 2500 rpm. I use 2500 most of the time. Bob Kromer (former factory test pilot) has written that the prop on the J was optimized for 2500 rpm.

I find it surprising here that this data sheet says that best economy is 25F ROP.   I'd wager that at MP>23" for a NA IO-360 that's in the red box.  I recall reading somewhere that to stay out of the red box for ROP ops, you need to be at least 80F ROP at 75% power?

[Bolter]

28 minutes ago, 0TreeLemur said:

I find it surprising here that this data sheet says that best economy is 25F ROP.   I'd wager that at MP>23" for a NA IO-360 that's in the red box.  I recall reading somewhere that to stay out of the red box for ROP ops, you need to be at least 80F ROP at 75% power?

If not in the detonation regime, it is at least an operating point of high internal pressures, making excess heat and stress.  

[ArtVandelay]

If not in the detonation regime, it is at least an operating point of high internal pressures, making excess heat and stress.  

Higher pressures = higher horsepower.

As long as the engine is operating correctly, it’s no problem. That’s reason they went to 20° timing BTW, was to improve the detonation margin.

[Bolter]

8 hours ago, ArtVandelay said:

Higher pressures = higher horsepower.

As long as the engine is operating correctly, it’s no problem. That’s reason they went to 20° timing BTW, was to improve the detonation margin.

Per the Lycoming chart, peak power is not at the peak CHT (which i think is peak internal pressure).  

[PT20J]

2 hours ago, Bolter said:

Per the Lycoming chart, peak power is not at the peak CHT (which i think is peak internal pressure).  

I believe that George Braly has shown that peak pressure occurs at about peak CHT. 

The shape of the power curve as a function of mixture strength is not obvious. LOP it is driven by thermodynamics. ROP it is driven by combustion chemistry. What George has done is explained it in terms of peak pressure as a function of crankshaft angle which is more easily visualized. 

Peak EGT occurs at the stoichiometric mixture where all fuel and all oxygen molecules should combine. This never happens in a practical engine because the mixture isn't perfectly homogeneous and there isn't enough time for the reaction between the hydrocarbons and the oxygen (which actually involves a number of intermediate reactions) to complete.  What does occur at this mixture is high temperature. The high temperature causes chemical dissociation of the combustion products that frees up additional oxygen. This is why a slightly richer mixture (which provides additional fuel to combine with the freed up oxygen) produces higher power and higher CHT at the max CHT mixture. Adding yet more fuel actually cools the mixture but adds more substance to the cylinder which expands during the power stroke providing more power at the max power mixture.