Questioning manifold pressure gauge accuracy

corbinace · #1 01-05-2010, 03:26 PM

Please help the ignorant.

O-320 E2D fixed pitch

The manifold pressure gauge reads 30 engine off.

In flight at 2350 rpm 3500' it reads 26.
Go higher to 10500 it is stilll reading 26 @2350.
In either case push the throttle in and it goes to near 30 and RPMs climb to, or near 2700
At 2500rpm any altitude it reads near 30" but I can still advance the throttle and increase RPM.

Can I really be getting 29-30" of MP at 10500 feet?

At first I thought that I was over pitched on the prop but now I question the validity of the MP readings. This, because it would not be able to get 2700 RPM if it was already at atmospheric at a lower RPM.

Am I understanding the issue correctly or do I need more education. I am ready for more education or a different MP gauge.

Thanks, Tim.

n5lp · #2 01-05-2010, 03:44 PM

Quote:

Originally Posted by corbinace

Please help the ignorant.

O-320 E2D fixed pitch

The manifold pressure gauge reads 30 engine off.

That is about right assuming you are near sea level.

Quote:

Can I really be getting 29-30" of MP at 10500 feet?

No, but I don't know where the problem is. The rough rule-of-thumb is that you lose about 1" of maximum MP per 1,000 feet of altitude gain.

AZtailwind · #3 01-05-2010, 03:54 PM

Quote:

Originally Posted by corbinace

Can I really be getting 29-30" of MP at 10500 feet?

....Turbo on that?

That is the only way to reach 30 inches at 10500.

I have seen 30 at sea level on a cool day but the MP should decrease as you climb in altitude and loose air density/engine power- Sounds like a gauge or MP pressure sensor going: My Vans MAP gauge died as it started to jump around on the low end. Your failure mode sounds different but may still be the gauge. Search the forum on MP and manifold. You will find your answer or at least a place to start.

dedgemon · #4 01-05-2010, 08:31 PM

Do you have a restrictor of some kind in the manifold pressure hose?
Its needed. Helps average out the readings and slow down the changes.
Might be the issue.

Also, try taking the hose off and putting some air pressure on it with a compressed air regulator set to 20 or 30 and see if it makes sense. It should read that nice steady pressure. If it does that, then I would suspect you need a restrictor in the line from the jug to the sensor.

gasman · #5 01-05-2010, 08:37 PM

I had a WESTACH MP gauge that did just what you said................. and I tissed it!

n5lp · #6 01-05-2010, 08:50 PM

Quote:

Originally Posted by dedgemon

...Also, try taking the hose off and putting some air pressure on it with a compressed air regulator set to 20 or 30 and see if it makes sense. It should read that nice steady pressure. If it does that, then I would suspect you need a restrictor in the line from the jug to the sensor.

I would advise caution on this one. The MP gauge measures absolute pressure so It will be around 29 or 30 inches of mercury just sitting there. If you add another 20 or 30 PSI to that it may be over the range of the gauge. 1 PSI is equal to 2.036 inches of mercury. Put another way, if you put 30 PSI on the gauge it would read not that pressure, but 60 inches plus the absolute pressure of 30 inches or so or around 90 inches MP, if it doesn't break and if it is working correctly.

corbinace · #7 01-06-2010, 01:32 AM

Thank you one and all for the education. I had never had a MP gauge to look at before, so I am learning.

The guage did not originally have enough restriction to take out the vibration from the single cylinder source. I fixed that with a .5" piece of #6 screw in the rubber hose section.

Gasman: I think I need to put my carbon copy gauge with yours. I didn't really want to mention any brand names.

Now I need to find an good one to replace it.

Thanks again.

PCHunt · #8 01-06-2010, 01:42 AM

Exact same issue. Same engine: O320-E2D. FP prop.

I get 29" on the MP gage at 8'000 PA WOT. (I now ignore the gage.)

Any of you engineer guys have a clue what's the cause??

On the ground, the gage appears to operate normally. 10-12" at idle, and "ambient" (29") at WOT. Is MP a differential pressure, or, as I suspect, an absolute pressure?

Yet another aviation mystery!!!!!!!!!!

Kevin Horton · #9 01-06-2010, 04:08 AM

Quote:

Originally Posted by PCHunt

Is MP a differential pressure, or, as I suspect, an absolute pressure?

MP gauges measure absolute pressure.

The ambient pressure at 8,000 ft pressure altitude is 22.2" HG (and it is 20.2 " HG at 10,500 ft). There is a bit of ram pressure rise at the cowl air inlet, due to the high speed of the RV. That would be worth 1.1" HG, or less, at 150 kt CAS. Thus the pressure at the cowl air inlet would be somewhere between 22.2 and 23.3" HG. There is some pressure lose to push the air through the carb, induction system, etc, so the pressure at the inlet port, where the MP is measured, is a bit less than this.

29" HG on the MP gauge at 8,000 ft is only possible with a turbo, or a broken gauge.

dedgemon · #10 01-06-2010, 08:05 AM

Just goes to show what happens when you post too late!

Certainly right about absolute gauges.

Quote:

Originally Posted by n5lp

I would advise caution on this one. The MP gauge measures absolute pressure so It will be around 29 or 30 inches of mercury just sitting there. If you add another 20 or 30 PSI to that it may be over the range of the gauge. 1 PSI is equal to 2.036 inches of mercury. Put another way, if you put 30 PSI on the gauge it would read not that pressure, but 60 inches plus the absolute pressure of 30 inches or so or around 90 inches MP, if it doesn't break and if it is working correctly.