Max power, 19”

[M20E78merlin]

I just flew a 4.5 and I am wondering if I have an issue.

Disclaimer - New plane to me and I am trying various RPM and MP settings to see what speeds and fuel consumption I get before I make a flight from Tucson to central FL.

On the way out, I set 2500 RPM and 18” MP, 120-125 KIAS.

On the way back, I set 2600 RPM and wanted 24” MP but, even at max throttle, MP would not go above 19” (actually topped out just below).

Fuel efficiency was out of this world, but I was hoping to trade a bit of fuel efficiency for more speed on the return.  The POH and checklists suggest that 2600 RPM/24” MP is achievable…any ideas as to what may cause the MP to stay so low?

[MikeOH]

What was your altitude?  (Unclear what 'flew a 4.5' means)

What MP did you see at take-off (and what was field elevation)

Do you have fuel flow data?

[AndreiC]

If you were at 4500 you should be able to get around a 25" MP maximum. (At sea level the pressure is more or less 30" -- standard is 29.92" -- and you lose 1" of MP for every 1000' of altitude. This is not precise, as the engine is not 100% efficient, but should be close enough.) Certainly 19" is very low. Is your throttle control rigged properly? If that is correct, then the culprit could be the MP gauge itself.

[bigmo]

Assuming the a/c is in Tucson now (3kish feet). Once you're at 7K AGL, 19 inches is all you'll make. Pressure altitude can vary quite a bit, so unless you were down low buzzing around 3-4K AGL, you probably are just fine.

 

[redbaron1982]

Looking at ADSB Exchange there was flight yesterday which was at 11k. So 19 inches sounds about right.

[Bolter]

10 hours ago, M20E78merlin said:

I just flew a 4.5 and I am wondering if I have an issue.

Disclaimer - New plane to me and I am trying various RPM and MP settings to see what speeds and fuel consumption I get before I make a flight from Tucson to central FL.

On the way out, I set 2500 RPM and 18” MP, 120-125 KIAS.

On the way back, I set 2600 RPM and wanted 24” MP but, even at max throttle, MP would not go above 19” (actually topped out just below).

Fuel efficiency was out of this world, but I was hoping to trade a bit of fuel efficiency for more speed on the return.  The POH and checklists suggest that 2600 RPM/24” MP is achievable…any ideas as to what may cause the MP to stay so low?

Some opinions are coming... as someone new to Mooney's and Mooneyspace, you may believe that the POH is a great reference.  It is outdated, and perhaps was never very honest.  It is a common OPINION that the best operation is to keep the throttle full open from takeoff, through cruise (except low altitudes where you may make too much power), and until descent.  Do not intentionally run a lower MP because of a book setting.  Use RPM and Mixture to control total power.  Avoid the peak performance point of the POH of 50F rich of peak.  Either be 100F rich or run lean

A little unclear on what your conditions were, but 125 KIAS could be around 150 KTAS, which is a decent true airspeed for an E without speed mods running ROP.  If you are reporting KIAS, it helps to know altitude, altimeter setting, and outside temp.  Same when concerned about performance in general.  

[midlifeflyer]

The overall thrust of the comments is that you might need some ground refresher on the effect of altitude on engine performance, manifold pressure, and indicated vs true airspeed. 

If @redbaron1982's pickup of 11K is correct, depending on ambient temperature, ~18" MP sounds about right and your 125 KIAS equates to ~150 KTAS. Is that OK?

[M20E78merlin]

2 hours ago, Bolter said:

Some opinions are coming... as someone new to Mooney's and Mooneyspace, you may believe that the POH is a great reference.  It is outdated, and perhaps was never very honest.  It is a common OPINION that the best operation is to keep the throttle full open from takeoff, through cruise (except low altitudes where you may make too much power), and until descent.  Do not intentionally run a lower MP because of a book setting.  Use RPM and Mixture to control total power.  Avoid the peak performance point of the POH of 50F rich of peak.  Either be 100F rich or run lean

A little unclear on what your conditions were, but 125 KIAS could be around 150 KTAS, which is a decent true airspeed for an E without speed mods running ROP.  If you are reporting KIAS, it helps to know altitude, altimeter setting, and outside temp.  Same when concerned about performance in general.  

I was at 10.5-11.5k and my TAS was somewhere in the 148-152 range…so, you are spot on.

Basically, it sounds like you are saying that I shouldn’t worry that my MP won’t get up to 19” under normal operating circumstances.  Is that fair?

[Ragsf15e]

5 minutes ago, M20E78merlin said:

I was at 10.5-11.5k and my TAS was somewhere in the 148-152 range…so, you are spot on.

Basically, it sounds like you are saying that I shouldn’t worry that my MP won’t get up to 19” under normal operating circumstances.  Is that fair?

It should definitely get above that, but not at 11,000’.  It should be approximately 29” at sea level and decrease with altitude.  If you cruise lower, you can get a higher mp.  However, your kias and true airspeed and MP seem about right for 11,000’.  So no worries there.

[EricJ]

5 minutes ago, M20E78merlin said:

I was at 10.5-11.5k and my TAS was somewhere in the 148-152 range…so, you are spot on.

Basically, it sounds like you are saying that I shouldn’t worry that my MP won’t get up to 19” under normal operating circumstances.  Is that fair?

Yes, MAP is Manifold Absolute Pressure, which means it is the actual, uncorrected for altitude or anything else, pressure inside the intake manifold.   It can't be higher than the ambient pressure in a naturally aspirated engine, and when the throttle is wide open it won't be far from the actual ambient pressure.    As altitude increases, the lapse rate is roughly 1" per thousand feet, so you'd expect to be 10"-11" less than the roughly 30" you'd expect at sea level.   So, spot on about 19".  

 

[M20E78merlin]

10 minutes ago, Ragsf15e said:

It should definitely get above that, but not at 11,000’.  It should be approximately 29” at sea level and decrease with altitude.  If you cruise lower, you can get a higher mp.  However, your kias and true airspeed and MP seem about right for 11,000’.  So no worries there.

Awesome!  Thank you!  The learning is coming at an insane pace right now.

[M20E78merlin]

2 hours ago, midlifeflyer said:

The overall thrust of the comments is that you might need some ground refresher on the effect of altitude on engine performance, manifold pressure, and indicated vs true airspeed. 

If @redbaron1982's pickup of 11K is correct, depending on ambient temperature, ~18" MP sounds about right and your 125 KIAS equates to ~150 KTAS. Is that OK?

Ground refresher or a better CFI for my complex.  I am learning about unknown unknowns right now.

[redbaron1982]

26 minutes ago, M20E78merlin said:

Ground refresher or a better CFI for my complex.  I am learning about unknown unknowns right now.

Is this the first time with a constant-speed prop airplane? If so, the manifold pressure in a NA (normally aspirated) engine can never be higher than the atmospheric pressure at the altitude the airplane is flying. So, on a standard day (ISA 15C 29.92inHG), if WOT (wide open throttle), you would get ~29.9 at sea level, and drop 1" every 1000ft of altitude. So, lets say you are flying at a density altitude of 11kft, then, WOT, you won't have more than ~30-11=19inHg.

 

[Will.iam]

Does yours have the ram air scoop on the front of the cowl? That’s good for an inch or 2 because the air doesn’t have to go through the filter and pipe bends which lowers the MP by an inch or so from ambient pressure but it’s unfiltered air going into the engine so only use it at cruise altitude. I. E. Not on the ground or when there is dirt in the air.  

[midlifeflyer]

6 hours ago, M20E78merlin said:

Ground refresher or a better CFI for my complex.  I am learning about unknown unknowns right now.

Yeah, I always think it’s weird when someone brags that their complex transition only took a couple of hours. As you are learning, there’s more to it than managing landing gear. 

But if you’re in Arizona, I’m surprised at this not being covered. Between  relatively high elevations and very high temperatures, the effect on engine performance and its indicators would normally be a major topic.

[M20E78merlin]

On 12/9/2024 at 6:14 PM, Will.iam said:

Does yours have the ram air scoop on the front of the cowl? That’s good for an inch or 2 because the air doesn’t have to go through the filter and pipe bends which lowers the MP by an inch or so from ambient pressure but it’s unfiltered air going into the engine so only use it at cruise altitude. I. E. Not on the ground or when there is dirt in the air.  

It does and I had it open above 6k MSL.  Thanks!

[AJ88V]

Great discussion above.

I'd just add that it would still be worth pulling the side panels off your engine cowl and inspect the corrugated rubber intake boot.  Just read a "Never Again" story where a the spring wire holding the duct open had broken free of the rubber and was letting the duct collapse partially under vacuum.  Recommend both looking for holes and actually squeezing the duct in a few dimensions just to verify integrity. 

And in the realm of truly bizarre, I discovered a 5/8" combo open/box end wrench inside the intake boot of my plane when I first bought it.  No idea how it got there, but  glad it was heavy enough to not get sucked up to the carb!

[Ragsf15e]

10 hours ago, AJ88V said:

Great discussion above.

I'd just add that it would still be worth pulling the side panels off your engine cowl and inspect the corrugated rubber intake boot.  Just read a "Never Again" story where a the spring wire holding the duct open had broken free of the rubber and was letting the duct collapse partially under vacuum.  Recommend both looking for holes and actually squeezing the duct in a few dimensions just to verify integrity. 

And in the realm of truly bizarre, I discovered a 5/8" combo open/box end wrench inside the intake boot of my plane when I first bought it.  No idea how it got there, but  glad it was heavy enough to not get sucked up to the carb!

Weird. I would have expected a 3/8” in there.

 

[AJ88V]

10 hours ago, Ragsf15e said:

Weird. I would have expected a 3/8” in there.

 

You may be right!  I was guessing after 25+ years of memory.  BUT!!!!  I actually still have the wrench as an orphan in my toolbox, so I can check! LOL!!!!

[bigmo]

When I shop for a mechanic, I look at their tool box and tool organization. Mine has a designated spot for every tool. Must be a former military mechanic SOP. No drawers or bins of tools allowed. 

My former job was flying MQ-1 & MQ-9 and spent a lot of time outside the GCS in the hangar. When a tool goes missing, the aircraft are grounded until it’s located.

[AJ88V]

22 hours ago, bigmo said:

When I shop for a mechanic, I look at their tool box and tool organization. Mine has a designated spot for every tool. Must be a former military mechanic SOP. No drawers or bins of tools allowed. 

My former job was flying MQ-1 & MQ-9 and spent a lot of time outside the GCS in the hangar. When a tool goes missing, the aircraft are grounded until it’s located.

Can't say I'm that super organized, but am still pretty good about remembering what tools I have used when I'm putting them away.  I'm still looking for a 10mm socket that's probably in my front lawn somewhere.  I hate losing tools!  But your exactitude especially makes sense for working on an airplane vice an old Miata.

[PT20J]

I still have Cleco that fell out of a lightening hole in the rear of C-152 Aerobat wing during my hammerhead turn and jammed up the right aileron.

I have a friend that worked for years as an A&P at United's San Francisco maintenance depot. She told me that they found so many tools inside airframes that they instituted a rule that you could never set a tool down on, or in, an airplane. 

When I volunteered doing maintenance at the museum, the mechanic had us stop work half an hour before quitting time and clean up and put all the tools away. If we were missing a tool, we had to find it before going home. We should have inventoried rags. Someone stuffed rags in the intake of the left engine of the B-25 to prevent FOD from falling down there (the intake is on the top of the cowling) when we were working on it and no one saw it and so it got left in there. When we ran it up, it ran OK, but later the museum owner complained that the left engine wouldn't make takeoff power. 

[Pinecone]

On 12/13/2024 at 1:47 PM, AJ88V said:

I'm still looking for a 10mm socket that's probably in my front lawn somewhere.

Why is it that ONLY 10mm short sockets go missing??????????

[M20E for me]

On 12/11/2024 at 9:07 AM, AJ88V said:

Great discussion above.

I'd just add that it would still be worth pulling the side panels off your engine cowl and inspect the corrugated rubber intake boot.  Just read a "Never Again" story where a the spring wire holding the duct open had broken free of the rubber and was letting the duct collapse partially under vacuum.  Recommend both looking for holes and actually squeezing the duct in a few dimensions just to verify integrity. 

And in the realm of truly bizarre, I discovered a 5/8" combo open/box end wrench inside the intake boot of my plane when I first bought it.  No idea how it got there, but  glad it was heavy enough to not get sucked up to the carb!

I found a 3/8" box end in the bottom of my cowling, I was hoping for a Snap On but it was a cheep crappy made in China wrench.