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Manifold Pressure Make Sense?

J

jpharrell

Well Known Member
I'd like some help making sense of manifold pressure readings from my last flight. I'm using the Dynon-supplied sensor with my Skyview system. The engine is an Aero Sport Power IO-360-B2B with Air Flow Performance FM-200 fuel injection and dual P-Mags. Before start-up the MAP reading was 30.0 inches which is 0.6 inches higher than the local barometric pressure. Is that much error typical of the Dynon sensor? I expected less static error.

After start-up I saw 12.5 inches indicated at idle (750 rpm). The weird thing is Aero Sport Power recorded 10.1 inches at idle in the performance section of my engine log book. Why is my pressure more than 2 inches higher? I've checked for an induction leak but haven't found anything to explain it. Could there be that much error in my sensor?

On takeoff I saw manifold pressure peak at 30.1 inches during roll-out which is 0.7 inches above ambient barometric pressure, and decrease slowly on climb out to 29.2 inches before reducing power on downwind. There is some ram air effect from the cowl snout (vertical induction) but I didn't expected to see that much manifold pressure due to the flow losses of the airbox and filter, induction tubes, etc. This pattern repeated for each takeoff. I loaded the log data into SavvyAnalysis so it's easy to review the history.

What should I make of these pressure readings? :confused:
 
Calibrate the sensor, it's easy

An easy way to calibrate your MP sensor is to take a known good altimeter and set it to 0 (zero) elevation. On our electronic EFIS's it's real easy. Just trun the BARO setting until you read Zero feet in the Altimeter window. It's a little hard to do if you live at high elevations. Then read the baro setting in the Kolsman window. That is what your manifold pressure should read with the engine off. If it doesn't, usually you can go into the calibration menu and adjust it.

Vic
 
Ground

signal ground and aircraft ground is not same thing check if you connected correctly

this is from general electronics rules not related to Dynon
 
jpharrell said:
I'd like some help making sense of manifold pressure readings from my last flight. I'm using the Dynon-supplied sensor with my Skyview system. The engine is an Aero Sport Power IO-360-B2B with Air Flow Performance FM-200 fuel injection and dual P-Mags. Before start-up the MAP reading was 30.0 inches which is 0.6 inches higher than the local barometric pressure. Is that much error typical of the Dynon sensor? I expected less static error.

After start-up I saw 12.5 inches indicated at idle (750 rpm). The weird thing is Aero Sport Power recorded 10.1 inches at idle in the performance section of my engine log book. Why is my pressure more than 2 inches higher? I've checked for an induction leak but haven't found anything to explain it. Could there be that much error in my sensor?

On takeoff I saw manifold pressure peak at 30.1 inches during roll-out which is 0.7 inches above ambient barometric pressure, and decrease slowly on climb out to 29.2 inches before reducing power on downwind. There is some ram air effect from the cowl snout (vertical induction) but I didn't expected to see that much manifold pressure due to the flow losses of the airbox and filter, induction tubes, etc. This pattern repeated for each takeoff. I loaded the log data into SavvyAnalysis so it's easy to review the history.

What should I make of these pressure readings? :confused:
Click to expand...
There are two separate issues here:
  1. An apparent 0.6 in HG error in your indication. As a point of reference, my Grand Rapids EIS MP reads about 0.25 in HG high. This is determined by setting my altimeter to 29.92, reading the pressure altitude, and then converting from pressure altitude to ambient pressure.
  2. Is the high MP reading during take-off credible? Your MP indication during take-off was 0.1 in HG higher than it was with engine off, so it was 0.1 in HG higher than ambient, not 0.7 above ambient. On my RV-8, with IO-360-A1B6 engine, and the horizontal induction intake, the MP slowly increases during the take-off ground roll, and is very slightly above ambient (i.e. the MP reading with engine off) by the time I reach 60 kt, and is about 0.7 in HG above ambient during a climb at Vy.
I've also been puzzled by the high MP readings during take-off and climb. It is plausible that the induction system is recovering a significant percentage of the available ram air pressure from the airspeed - the available ram pressure is about 0.4 in HG at 90 kt.

The pressure behind the prop is also a bit higher than ambient - a rough back of the envelope calculation for a 200 hp engine with 80% prop efficiency at 90 kt suggests a 0.3 in HG pressure differential between the air ahead of the prop and the air behind the prop, assuming the pressure is the same over the whole prop disc.

There has to be some pressure loss in the induction system though, so I cannot make the numbers add up. But, I see this in the data on every flight for which I have recorded the data.
 
I get like 14-15 at idle... Seems high. Nothing abnormal during cruise. Maybe I'll check that calibration Vic suggests.
 
Ambient Barometric Pressure

Kevin,

The reason I said 0.7 inches above ambient barometric pressure is the altimeter setting was 29.90 and the field altitude was 500 feet so the ambient barometric pressure would be approx. 29.40 and that is what I would expect the MAP sensor to read with the engine off. But it seems to be reading 0.6 inches high at static conditions. So during takeoff roll it is reading 0.7 inches higher than true ambient pressure on the runway, not relative to what it reads with the engine off which seems to be in error.

Looking closely at my data, the indicated manifold pressure rises to 30.1 inches as the IAS rises to 70 kts and then slowly decreases to 29.2 inches as the altitude climbs to 1000 ft AGL. That decrease makes sense because the pressure drops about 1 inch per thousand feet of elevation increase.

I suspect my MAP sensor calibration is off, causing a error of 0.6-.07 inches at sea level pressure and perhaps as much as 2 inches at the low end. I may invest in a vacuum tester to check the accuracy and look for leaks in the lines.
 
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