WOT/7500 ftMSL/2200RPM/22" MP

[sleepingsquirrel]

I climbed to 7500 feet today just to show my wife the tops of the clouds.  I admit this is the highest I've been in years in a "small" plane. OAT 64 degrees; IAS, 136 MPH. All in all, pleased with the performance, the climb rate was only diminished slightly from take off from 600 ft MSL like 900 FPM. I was rather surprised to only achieve 22" of manifold pressure at WOT is this why people need a turbo or is my engine sickly?

I guess I could have pulled the throttle back and oversquared the RPM but by how much? Why do I want to do that?

[jetdriven]

You could run it at 1900 RPM at that altitude no issue. "Oversquare" is another wives tale. You can run 27" and 2400 RPM down low if you wish. At your density altitude, probably 10,000 feet,  every 100 RPM lower you go, lowers the % of power and TAS.  You save  a little fuel, but you are losing lots of airspeed as well. You might consider raising RPM to recover some % of power.

[rob]

I almost always run at 2500 in cruise. I'm not sure why I'd want to be anywhere else. The MAPA eval articles, and my engine and plane's vibrations seem to really like 2500. That aside, 2200 is in the "do not operate" range for me.

[jetdriven]

We did some testing and 2300 RPM is quite a bit quieter, but a little slower as well.  Stock McCauley prop.  Thing is, if you run LOP, you are a little slower.  At 10,000' density altitude, a couple knots lost there too. Pull it down to 2300 RPM? Lost a couple more knots.  Eventually like we were, at 6500', 68 degrees, full throttle, running at 2100 RPM, burning 7.5 GPH and like 132 KTS true.  Fine and efficient, but just pretty slow.

 

here is our test data.

6500', 68 degrees, full throttle. 40-60 LOP.

rpm    TAS      GPH     % power   NM/gal

2500     139     8.4          62.5          16.57

2400     138      8.15       60.7          16.94

2300    135.8    7.8        58.1           17.41

2200    133.6     7.4        55.1         18.05

2100     132.4    7.35       54.7         18.01

 

 

 

[sleepingsquirrel]

I have one exhaust temp probe and one CHT probe and lean to rough then enrich slightly. I have read about leaning then  enriching and watching airspeed peak. I did this a couple of times as I fiddled around with the knobs. I'll try increasing the RPM for speed. DA was 9700 ft. I checked the manual and see I probably need to go WOT then back it off slightly to get it off the auto rich feature. I suppose that leaning like I was doing does the same thing as backing off the throttle? CHT was 380F like always .

 Thanks for the information !

 

[Shadrach]

Hiya Squirrel,

What are you flying? 7500ft is prime traveling altitude in any Mooney. I would have to try very hard to hit 380df CHT at that altitude in level flight. I've no time in non-injected Mooenys (it appears as if that is what you're flying), but I've never heard any one suggest that any "enrichment ciruit" cannot compensated for with the mixture knob. In fact I think How long have you been flying this particular bird?

Precision Airmotive's MSA Float Carburetor Handbook (linked below, but read-only :-( unfortunately)

Section 2 c. says it all...

home.comcast.net/~r123rs/Documents/Carb%20Manual.pdf

 

[sleepingsquirrel]

Thanks for the link to the carb stuff. I've been flying this for about 6 months . I made one trip in the Spring and got book numbes for fuel consumption at 5000 ft. Ths engine is probably still tight from overhaul ,about 50 hours SMOH, I've put 30 hrs time in the last 6 months before that it was rarely flown.

M20B 1961 NA O360A1D

[Shadrach]

I doubt that the CHTs have anything to do with the recent TSOH... 380 should not hurt anything, just not much in the way of margins. 

[Hank]

Hey, Squirrel.

At 7500 msl, I typically run my C-model at 22"/2400 for 74½% power. As I go higher, I leave the throttle wide open and stay at 2500 RPM. These give me the most speed while still letting me lean nicely; at low altitudes I keep it ~50 ROP, higher up is less, and around 8-9000 I lean to rough then enrichen to smooth, which is generally pretty close to peak. It helps to back off the throttle just enough to make the MP needle move. The thinking is that cocking the throttle body in the carb creates additional turbulence, which increases atomization of the fuel and makes for a more uniform air-fuel mixture.

My POH [O-360-A1D] shows a maximum of 22.5" MP at 7500 msl. Don't forget the effect of OAT on engine power--your OAT of 64ºF is 32º above standard, so you should subtract an additional 3% from the power. That would enable running per my POH at WOT/2500, especially if the throttle was backed off enough to move the needle. [22.5/2500 = 78.8%; 22/2500 = 76.6%]

[Hank]

And yes, your A-model most likely has a different model engine, so it may run a little bit different. Does your Owner's Manual have Performance Tables?

[sleepingsquirrel]

Hi Hank,

 I have a B model with the same engine you have O360 A1D. I've been flying off the previous owners cruise check list and decided to check my owner's manual and with the rest of you guys with more experience on this engine. I don't have the modern engine instruments so I'm flying the old fashioned way. Just fiddle with the knobs until it takes off. I do keep an eye on the exhaust after every flight , I know I'm running hot enough to move the lead out but not hot enough to run the aluminum out.

Thanks for the info!

 

 

[Hank]

Yep, I also have the factory single-probe oil temp and CHT, plus an "optional" EGT gage with actual numbers on it. I typically cruise at 1450-1500º on the EGT, but the absolute numbers are meaningless. It does make it easier to tell if anything is changing, though, and makes it simple to ease the  mixture forward and maintain cruise EGT during descent.

[Shadrach]

Squirrel,

If your "380" is from the original factory guage, I'd be skeptical until it was verified...

 

Hank,

If running ROP to peak, what is the point of cocking the throttle?  It seem unlikely that there will be any appreciable fuel savings at those settings. A touch of carburetor heat will increase atomization as well, but it will also rob power slightly. It probably doesn't slow you down but a tiny bit, but it probably does not benefit you much either.  Seems like a lot of extra fiddling for a "zero sum game"... Have you seen numbers that suggest a benefit?

[Hank]

"If running ROP to peak, what is the point of cocking the throttle?  It seem unlikely that there will be any appreciable fuel savings at those settings. A touch of carburetor heat will increase atomization as well, but it will also rob power slightly. It probably doesn't slow you down but a tiny bit, but it probably does not benefit you much either.  Seems like a lot of extra fiddling for a "zero sum game"... Have you seen numbers that suggest a benefit?"

Ross--

I figure improved atomization and more uniform fuel-air mixture will burn more uniformly in all four cylinders. There may not be a measurable fuel savings, but I would expect more complete combustion, and therefore more power output [maybe even as high as shown in The Book], instead of the carb-heat power loss. My Hartzell 3-blade seems to return pretty close to book speeds.

As far as Peak to ROP, even at 10,000 msl she starts stumbling right around peak, maybe just as the needle starts to move back. With the spacing of those 25º hash marks on the small gage down at the bottom of the panel, I'll never notice if I'm actually running 5º or even 10º LOP, so I take what I can get. No fuel flow yet, but I'm still hoping. Even then, 9gph isn't too bad. Stumbling at 10,000' is much more noticeable than at 3000' and not something I like to feel.

There's really no extra work, just pull the throttle until the MP needle moves [can be a long pull at 10,000 msl], set RPM to 2500, wait a minute and start leaning. Not exactly "a lot of extra fiddling" in my book. If I'm at 7500 or below, I typically pull the throttle back some anyway; for my plane, MP + RPM ≤ 47, so that's pretty much 23"/2400 or 22"/2500. IO-360s are different, at least in the nice notebook I have from the MAPA PPP I took as a new pilot.

As for the factory gage, there is no way to tell what my CHTs are with any precision. I can only assume that Squirrel has something additional if he has an actual number. I fly in the green part of the gage, then there's a bit with no stripe, then a red line marked with an improbably high number [something foolish like 500ºF], so I just try to stay a needle's width or more below the top of the green. Not difficult except in summertime climbs to altitude, and then the oil temp rises faster than CHT.

 

[sleepingsquirrel]

Cocking the throttle plate is the poor mans' intake Tornado. It does not require an STC nor does it cost $20 as seen on TV. The idea is that a slight disturbance will cause all the cylinders to get a more uniform fuel air mix .

 

 

[Parker_Woodruff]

22" MP is to be expected at 7500' MSL.  I'd expect somewhere around 21-22" MP at that altitude in a normally aspirated aircraft.

I'd run your RPM higher for some extra power.

[Shadrach]

"There's really no extra work, just pull the throttle until the MP needle moves [can be a long pull at 10,000 msl]"

Hank,

2 things:

1) If what you say is the case and you can move the throttle a bit without a change in MP,  then why pull until MP drops. Just moving the butterfly should create turblence.

3) If your engine stumbles at peak, then I'd look into moving the probe, because it seems one of the other cylinders is running leaner than your reference cylinder...

[Hank]

1) If what you say is the case and you can move the throttle a bit without a change in MP,  then why pull until MP drops. Just moving the butterfly should create turblence.

3) If your engine stumbles at peak, then I'd look into moving the probe, because it seems one of the other cylinders is running leaner than your reference cylinder...

Ross--

1) the throttle butterfly has to move farther at altitude to create turbulence in the air flowing into the carb venturi because as altitude the air is less dense, meaning there are fewer particles and they will compress down smoothly for a while. Once turbulence is achieved, the MP begins to drop; no decrease in MP means no turbulence, so no improved atomization or mixing.

2) with marks every 25º, that are the width of the needle and a needle's width apart, I can't tell exactly when the engine starts to stumble. On your analyzer, if you print out a slow leaning cycle to peak instead of the "one big pull" to reach LOP, how flat is the temperature peak versus red knob movement? My C has the throttle quadrant with levers, so there are no vernier to turn. With my limited instrumentation, this is all I can do. Can you please improve my knowledge with applicable data? "Happy wife, happy life" in general; "Happy engine, happy flight" is more appropriate but not as catchy.

[Shadrach]

Nice 2 things followed by a 1) and a 3) in my last post...this lack of editing will hopefully make me a better proofreader.

1) Why is it that you believe that you must necessarily see a drop in MP for there to be intake turbulence. Any movement of the butterfly should affect airflow, if not MP. 

2) I have an older JPI and do not have the proper port to retrieve data. I've therefore never downloaded and graphed the data.  However, I can tell you that peak is in a pretty precise place WRT the red knob. I have experimented at alt and found that if I carefully lean for peak egt (which varies cycle to cycle about 3º), any movement of the knob will cause the temp to fall off one side or the other... If the knob was a clock hand, finding true peak would be an exorcise in slowly fine tuning between the 11 and 1 o'clock positions overshooting a few times and backing up until you get the absolute max number...

 

In my early days flying LOP I was usually flying leaner than necessary because I would enrichen too quickly after the pull - going rich again before seeing peak...this would give me a "false peak" and I would set accordingly. The speed sucked; so I experimented further, eventually learning the error of my ways.