jM20

M20C Manifold Pressure

Recommended Posts

Hello,

I was hoping somebody with a C could give me guidance.
I think I'm at least 1" short of manifold pressure. I took it up yesterday when outside pressure was at 30" of mercury.

On the ground at full power, I was at 2700 R.P.M and 27" of mercury. When I got up to 2500 MSL, I was at 2700 R.P.M and 25" of mercury.
I'm concerned that the throttle isn't going in all the way. 

What do you guys think?

Thanks!

JM20


 

Share this post


Link to post
Share on other sites

*Members that donate $10 or more do not see advertisements*

I am anxious to read what the experienced Cer's write about this.  As a new ie to my C my first I pression is that you are overthinking this.  When I push everything forward and climb out at 105 MPH I am seeing about 26" or maybe a little more.  That is plenty of power for the job.

I fully expect to learn something from this thread.

My $0.02,

Share this post


Link to post
Share on other sites

Check your MP gauge on the ground with the engine off.  At full power, you should be near to that number.  1" of loss is not unheard of.

27" at sea level to 25" at 2500' is a normal decrease based upon pressure loss at altitude.

  • Like 1

Share this post


Link to post
Share on other sites

You will never show outside pressure when the engine is running. When your altimeter is set to field elevation  efore you start the engine, the MP gage should read pretty close, but because there are induction loses, it will read less after engine start.

My Owners Manual shows 28" at Sea Level, and 27" at 2500 msl.

When I get home, I can do a check with the engine running,  but I'm based at a whopping 326 msl. Where are you based, and what is your field elevation?

Share this post


Link to post
Share on other sites

Hank field elevation is  830'.


Andy,

At field elevation, without the plane running, I believe the MP gauge was sitting at 28.(Something,I forgot :() even though the pressure nearby was 30". I was coming up with 2" loss with the gauge and 1" just missing. Although now I'm confusing myself, i'm not sure what I was at during takeoff, but I thought it said 27". I can't imagine why it would lose another 1.something inches after startup.

Thanks!

 

Regards

Share this post


Link to post
Share on other sites

I think you're probably right on, I wouldn't worry about it.  The next time you have the lower side cowling off on the right side, you will be able to ensure the linkage is going full forward to the stop on the carburetor.

The MP gauge will show the barometric pressure at EXACTLY sea level and EXACTLY 15°C/59°F.  You will lose about 1" for every 1000' above sea level and you'll also lose for higher temperatures.  The 30" barometer is corrected for temperature and elevation.  

Share this post


Link to post
Share on other sites
On ‎8‎/‎15‎/‎2017 at 4:06 PM, Andy95W said:

I think you're probably right on, I wouldn't worry about it.  The next time you have the lower side cowling off on the right side, you will be able to ensure the linkage is going full forward to the stop on the carburetor.

The MP gauge will show the barometric pressure at EXACTLY sea level and EXACTLY 15°C/59°F.  You will lose about 1" for every 1000' above sea level and you'll also lose for higher temperatures.  The 30" barometer is corrected for temperature and elevation.  

So it goes to say - if you are at field elevation of 1000 ft, and temperatures are 100 degrees, it would be a decent amount lower than altimeter setting? correct? maybe by a couple of inches?

Share this post


Link to post
Share on other sites
4 minutes ago, Supercop0184 said:

So it goes to say - if you are at field elevation of 1000 ft, and temperatures are 100 degrees, it would be a decent amount lower than altimeter setting? correct? maybe by a couple of inches?

Yes, easily. But that's with the engine off. 

With the engine running, the O360 will only make so much MP based on Density Altitude. It's been awhile since a flew a C, but for example if I can only make 21" at 8000 ft. at full power, and now I'm taking off from an airport on a hot day with a DA of 5000 ft. I wouldn't expect to be making more than about 24" or so on take off.

Share this post


Link to post
Share on other sites
Just now, gsxrpilot said:

Yes, easily. But that's with the engine off. 

With the engine running, the O360 will only make so much MP based on Density Altitude. It's been awhile since a flew a C, but for example if I can only make 21" at 8000 ft. at full power, and now I'm taking off from an airport on a hot day with a DA of 5000 ft. I wouldn't expect to be making more than about 24" or so on take off.

Yup that makes sense. Thanks! HEY by the way how are ya? Was in the area not too long ago and tried callin! Hope you're doing well!

  • Like 1

Share this post


Link to post
Share on other sites
2 minutes ago, Supercop0184 said:

Yup that makes sense. Thanks! HEY by the way how are ya? Was in the area not too long ago and tried callin! Hope you're doing well!

Thanks... yeah, been pretty busy and traveling a ton for work. Also the Mooney was down for MONTHS. All better now. We should get together for lunch somewhere when it cools down... say October.

Share this post


Link to post
Share on other sites
Just now, gsxrpilot said:

Thanks... yeah, been pretty busy and traveling a ton for work. Also the Mooney was down for MONTHS. All better now. We should get together for lunch somewhere when it cools down... say October.

Sounds perfect to me! Lets do it!

 

HEY HOW DO I GET THAT COOL MAP?

Share this post


Link to post
Share on other sites

Well I ran it again. I'm at a field elevation of 870. The airport was reading 30.15.

The gauge was 29.10 with power off. On the runway at full power, it read 27.3.

 

Regards,

Jim

Share this post


Link to post
Share on other sites

Sounds like you're right about where you should be.  I'm at 920' and get about 27-27.5" on takeoff on a typical day.

Share this post


Link to post
Share on other sites

According to the C POH...

28.0" produces 99.5%bhp using 18.2 gph @2700 rpm...

Cruise and range @SL....  expect to fly for 2 hours doing this.  Use caution, boats may get in the way....

27", 97.5%bhp using 17.8 gph @2700rpm altitude 2,500'... (JM20 reported 2" shy of this number)

Another thing to check is the MP reading at full idle while on the ground.

The MP line is susceptible to collecting blue goo from 100LL evaporating in the intake of the nearest cylinder.  The MP line has a very tiny vent/weep hole near the gauge. If a crack forms at the hole, the gauge won't read a good power number at idle...

For Alex... I'm not sure how oil can get to the MP line, but the valve rocker and intake are close together. Check with you mechanic to see if cleaning under this valve cover would be helpful...

PP thoughts only,

-a-

Share this post


Link to post
Share on other sites

I don't think anyone commented on your throttle question. I believe the throttle (and mixture for that matter) cable should be adjusted such that the arm on the carburetor (or fuel injection servo for my plane) hits the physical stop before the knob hits the instrument panel. IOW, pushing balls to the wall should not literally get to the wall. This assures that the business end of the cable goes to the stop. 

  • Like 1

Share this post


Link to post
Share on other sites

Jim,

I re read your statement... Because Bob brought up an interesting question...

Did you push the throttle all the way in for the whole flight?  Full throttle or FT

I assume you did. But the magic number 25" at 2500' raises a few questions...

Simplified laws of physics dictate that we should expect to lose 1" of MP for each 1k' of altitude... but, my C POH doesn't seem to follow Simplified laws of physics....

How old is the air filter? Check the MP with the air filter removed...(?) they sell low back pressure air filters...

Best regards,

-a-

Share this post


Link to post
Share on other sites

What type of air filter do you have? A dirty old Brackett foam one will also restrict airflow to the engine. Is the carburetor heat control rigged correctly? Partial carb heat decreases air density and lowers power output.

Clarence

Share this post


Link to post
Share on other sites

Does anybody know pretty accurately, what the full stroke of your throttle handle is?
Thanks!

Share this post


Link to post
Share on other sites
2 hours ago, jM20 said:

Does anybody know pretty accurately, what the full stroke of your throttle handle is?
Thanks!

I have the quadrant, and my throttle stops ~1\4" short of the end of the slot. The Prop lever goes further forward.

Share this post


Link to post
Share on other sites

All the way out to all the way in, minus a small distance...

The important part is at the other end... Make sure it is covering the whole distance at the carb.

There is probably a procedure for setting the carb operations up properly...

Don't forget to check the adjustable friction device for proper operation, and the cut-out for the throttle position sensor when fully out.

Sorry i don't have actual distance data for the throttle shaft. Hoping this is helpful.

PP thoughts only,

-a-

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now