Manifold Pressure Calibration

[hydroguy2]

How do I check to ensure my manifold pressure reading is correct.  I have RamAir on my RV-7 and it almost always reads higher than other planes. One "expert"     says no way my MP readings can be as high as it is.

I see 21.5" at 10500 (11500DA) and around 23.7 at 8500'. Am I offbase here or close.

 

[dynonsupport]

You check it on the ground.

Set your altimeter "baro" setting until altitude reads zero. In this case, your baro setting and your MAP should match.

Checking it at lower pressures would require a way to produce vacuum and a calibrated gauge.

Some fast math:

10,500' =20.2 inHg atmospheric pressure (Even with a 30.03 assumed baro)
154 knots = 1.1 inHg

So the max MAP you can read at that altitude, assuming your ram intake was perfect, would be 21.3". So it does sound like something is off, because no intake is that good.

 

[hydroguy2]

ok thanks. is there an adjustment procedure to fix it?

 

[dynonsupport]

There is not. If it's out by too much, it's broken and needs to be replaced.

 

[hydroguy2]

Thanks

I went to the hangar today and checked it. I couldn't lower the Altitude to 0. Lowest I could achieve was 1400' and 27.40 baro reading. I assume that if I was able to reach 0 altitude, I would be 26.00inhg...Is that a correct assumption?

I was showing MP of 26.4", so will guess I have a 0.4" error.
Does this mean I have a bad sensor or ??? Just trying to figure this out, as it might explain my high %power readings also.

Thanks again

 

[Dynon]

What altitude are you actually at when on the ground? Also, what's your altitude read when your BARO is set to 29.92 (along with all of the other numbers again when you get a chance to check again).

 

[hydroguy2]

3893' is field elevation. Probably closer to 3894 or 95 at the hangar.  I'll be out there again tomorrow.

Earlier I snapped this pic after I ran it back up to make sure it was still moving and not bottomed out(if that's possible)

 

[dynonsupport]

On a standard day (29.92"), 3,900' altitude is 25.94 inHg, so that's what your MAP should read on the ground. Offset that 1:1 with how far off your baro is that day.

So if you are getting 26.4" on the ground on a 30.0X" day, you are about 0.3"-0.4" high.

 

[hydroguy2]

Here's my numbers at 29.92 this morning in the hangar.


And this was roughly field elevation when I first powered it up.


27.50 is as low as I can go with the Baro...is this correct?


Looks like I do have a error.  Can it be fixed with a new sensor or ???

 

[Dynon]

Thanks for the additional data. That confirms what we were seeing earlier, and it looks like a new sensor is in order here. Sensors are warranted for a year from purchase.

 

[hydroguy2]

OK thanks. I'll get one ordered and swap it out during my condition inspection next month. I've had this one about 4.5yrs or so...might be out of warranty.

 

[hydroguy2]

well I haven't flown yet, but it appears that didn't work.

Installed a new sensor and the readings on the ground are unchanged.

Is there anything else I should be checking or doing?

 

[Dynon]

You're getting close to stumping us... try measuring the 5V (red) wire that goes to the MAP sensor. Is it 5V? Is it going anywhere else? Also, does the black ground wire go back to the EMS? (it should)

 

[hydroguy2]

I'll do some checking tomorrow. I don't think it is connected or share with anything else.

My panel and wiring are pretty simple.....but I did the work myself, so might not be perfect.