What manifold pressure should be seen at 7000', 9000', and 11000'? At 7000' MSL and better I don't see anything higher than about 20" (mostly 19"). I know that once you reach about 7000' altitude a normally aspirated engine can only produce 75% power with the throttle wide open, so that would be your power setting for that altitude and up. I was surprised to see so little manifold pressure I suspected a restriction (ice or plugged air filter) might be indicated. Something that throws me off is setting the altimeter to 29.92 at FL 18. Didn't think about making notes of static pressure on the ground at the airport, and haven't been able to read anywhere about what manifold pressure would be seen at these altitudes, so I thought I'd field the question here.

In theory your maximum manifold pressure at sea level should be 30 inches (29.92, but you can't measure .08 inches on most gauges). In reality most engines will give a max of about 26-28 inches at sea level because of restrictions to airflow in the air filter, manifold, carb., etc. For every 1000 foot altitude you loose approximately 1 inch because of decreased air pressure (it's actually logarithmic, you loose 50% every 18,000 feet, but at lower altitudes 1 inch per 1000 feet is a good approximate). Having said that 19-20 inches at 7,000 feet is about right. It's been a long time since school, if my numbers are off someone else go ahead and correct me.

For me, nearly every flight is above 7000 MSL. Looking back through some pictures, here are some MP readings I have off the CRG-30P. I used the GPS altitude as it's closer to actual DA altitude.

7049 FT = 22.1
8566 FT = 21.2
13494 FT = 17.7

I live at 6600ft.. runway at 6450ft... sitting static mani pressure is 23" (11.4-11.6 psi of pressure vs. 14.7 at sea level)... 7000ft is about 22.5ish and of course as mentioned you are going to see a little less on the gauge due to carb/intake etc. - Mine is usually around 1" less iirc

I see ballpark what Matt and GK seeing. Today's flight at 7,500 was around 22".


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Grassstrippilot wrote:I see ballpark what Matt and GK seeing. Today's flight at 7,500 was around 22".


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Same here

Since mine was showing an inch or two below what others have listed here, I installed a new air filter. It'll be a while until I can compare with the last flight. Thanks for the help and info.

If installing the new filter doesn't do it, look for other restrictions. I don't know how your filter is connected to your engine's intake, but if it's with scat or sceet, look for a collapse, especially where it bends.

Cary

MP gauges go bad.

Most MP gauges use aneroids, and if corrosion is not an issue, they may be fairly easily adjusted. There is typically an adjustment screw associated with each of the two aneroids (one is sealed and references the case pressure, and the other is connected to the input and references it to the case pressure). It is a simple procedure to adjust and calibrate these.

Paying someone with a gold plated miniature flat head screwdriver to do this for you, with a yellow tag, will set you back around 200 or less including shipping.

We are fuel injected and might get a little ram air effect with our induction design. Of course MAP also varies with RPM too. Here's what we see:

WOT at about 7,000ft gives us 25 inHg.

Here's a photo at 9,400ft with WOT, manifold pressure just under 22 inHg


WOT at 10,000 is about 21.5 inHg

Up at 13,000" we are seeing around 19 inHg - again that is with wide open throttle.

Here's a photo at 7,000 and 21 inHg - but you can see the throttle is half-way closed.

WOT at about 7,000ft gives us 25 inHg.

You are definitely seeing a ram air effect. There is not 25" available at 7000ft in the atmosphere. That's pretty impressive because you are seeing around 1.5PSI of increased pressure assuming gauge/sensor error is not contributing to those numbers!!

Battson, thats a NICE panel.

The Maule is currently at 600' MSL and the pressure outside is 29.64"; the MP gauge is indicating a hair over 29" static. I'm going to take better notes next time but I recall seeing 19" at 8000 to 9000' MSL with WOT. I'm also burning 2 gph more since the carb overhaul (was 8 gph; is now using 10 gph). If the weather holds out this weekend, we're headed to Sulphur Springs TX for a Maule gathering hosted by Russell Armstrong; I'll do some more checking then as well.

GravityKnight wrote:

WOT at about 7,000ft gives us 25 inHg.

You are definitely seeing a ram air effect. There is not 25" available at 7000ft in the atmosphere. That's pretty impressive because you are seeing around 1.5PSI of increased pressure assuming gauge/sensor error is not contributing to those numbers!!

Yeah I hope so! I went to a bit of trouble designing the intake to maximise ram effect. I know it's something which a lot of RV builders (and similiar) pursue with great interest.

Instrument error is less than 0.2 inHg at rest, i.e. >0.6% which I am pretty happy with. I have no way of testing for error when the engine is running.

Battson wrote:

GravityKnight wrote:

WOT at about 7,000ft gives us 25 inHg.

You are definitely seeing a ram air effect. There is not 25" available at 7000ft in the atmosphere. That's pretty impressive because you are seeing around 1.5PSI of increased pressure assuming gauge/sensor error is not contributing to those numbers!!

Yeah I hope so! I went to a bit of trouble designing the intake to maximise ram effect. I know it's something which a lot of RV builders (and similiar) pursue with great interest.

Instrument error is less than 0.2 inHg at rest, i.e. >0.6% which I am pretty happy with. I have no way of testing for error when the engine is running.

That's very cool. I had an intake setup I built on mine for a while that was getting a little ram air effect... but it was causing mine to lean out, even with larger jets I still had trouble controlling the mixture properly. I may re-visit this idea again. 'free' power is a great thing

Gump, I had read that article (very informative reading, and he did columns on the prop and mixture control, too), but I had to reread it to digest more of it. After getting what others report here I had a better base from which to understand what to expect. Like I said, I wish I had a greater understanding prior to this trip that would have allowed me to know to take better notes. They don't talk about this stuff in primary training, but then again trainer aircraft don't have CS props. MP gauges are reading in absolute pressure, but in the automotive world they're called vacuum gauges.

DeltaRomeo wrote:They don't talk about this stuff in primary training, but then again trainer aircraft don't have CS props. MP gauges are reading in absolute pressure, but in the automotive world they're called vacuum gauges.

It's a real shame that flight schools don't teach engine management. Its not uncommon to meet flight instructors who don't know know their way through an engine, so it's clearly not being taught well at higher levels either. I guess they call it flight school and not engine school.

In the 70ish hours it takes the average student to get through primary, this would add at least 10 more hours to the program, unless the CFI managed to squeeze it in concurrently with other cross country activities. As for me, I was ready to get my wings and start learning the stuff I needed to know to fill our mission. There are a fair amount of "students" such as myself that rely on the experience that is found here that would not be available otherwise; this is hangar talk you cannot find just anywhere. I'm glad to be learning at a time it is a resource I can use!

Can anyone report the MP they see on their Continental with cross flow cylinders (intake on top of the engine)? I'm wondered if there is any ram air effect from intake being on the pressure side of the cowl.

I do most of my blasting around at 1500' MSL or so here in the Puget Sound area.
Went up to 4500' yesterday crossing the Salish Sea coming home from the San Juan islands.
I normally cruise at 22 squared and when it was time to descend I didn't reduce power, I just trimmed the nose down and let the airspeed come up. Got down close to 1000' pattern altitude and noticed I was close to 24" MP. No big deal at the low power settings I run, but that increase in MP as you descend might be an issue if you're already running at high power.