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mlwynn

Well Known Member
I have been through a really frustating round of getting my engine to operate correctly. It is now running smooth and strong but I still can't make heads or tails of MAP readings. The strange reading have really complicated the process of finding why my engine didn't want to run smoothly.

IO360M1B Engine
Dual Plasma III Ignition
AFP FM 200 Injection system
GRT EIS 4000 and manifold pressure sensor.

On the ground, at idle, I cannot get the MAP reading below 12-13 inches. This is with the engine idling smoothly and taking throttle without problem. It goes up with adding throttle and indicates 29.9 with the engine off.

There are two read-outs: one from the EIS EFIS and a second separate indicator that comes from the Plasma Ignition. I would have thought there was a problem with the pressure sensor except both systems match.

Gets weirder. When flying, pull the throttle back and you see 9-10 inches. Once again, the engine is running fine. This is the reading I would expect to see at idle on the ground, and seems like the correct reading in the air.

So, why would it be different in the air and on the ground? I searched for a long time for an induction leak. Found one: a faulty sniffle valve. With that out of the system and the hole plugged, no change in the MAP readings.

Does anyone have an explanation for this?

Regards,

Michael Wynn
RV 8 Phase I
Livermore, CA
 
RPM. I'd venture to say that on the ground your RPM is 800-1000 while in flight it will be significantly higher and the prop is driving the engine and not the other way around.
 
Mike,

I'd say your MP readings are about right.
Remember, what's going on here is the engine is acting as an air pump, pumping air past the nearly closed throttle plate at idle. That closed throttle plate is a pretty significant restriction, so the intake is at a partial vacuum, e.g., pressure well less than ambient 29.5" (note LVK is 400 MSL, so you should be seeing about 0.4" less than the altimeter Kollsman setting, with the engine off).
At idle you're probably showing 900 rpm.
Now in a power off glide you're probably showing, what, 1500 rpm? Not quite twice the pumping speed, so the intake manifold pressure is lower as the engine is working harder to pump air thru. If you have a CS prop and you're within the governing range at idle in a glide, pull the prop control back, reduce the the rpms, and you'll see MP rise.
Even with "normal" power settings you'll see a small interaction between MP and prop rpm settings, for the same reasons.
 
Mike,
Simple answer: If you have the throttle pulled back all the way, you have a very small "crack" in the throttle letting air in. This crack is the same size no matter what RPM you are at.

At higher RPM, the engine is trying to pull more air per second, so the MAP is lower since the crack in the throttle is the same, hence physics says the pressure differential must be greater.

One way you can make logic of this: At zero RPM, your MAP is 29", no matter what the throttle setting, because their is no airflow. So you can see you are already expecting higher MAP with lower RPM, so it shouldn't be surprising if 900 RPM is less than 0 RPM that 1500 RPM should be lower than 900. Doesn't really matter if the engine is driving the prop or vice versa. RPM is the main thing that changes MAP for a fixed throttle setting.

--Ian Jordan
Dynon Avionics
 
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And to add to what others have already posted, when the engine is off your MAP will be your local barometric pressure so seeing 29.9 is actually pretty common.

:cool:
 
MP Readings

I think I understand all that, Bob. When I was working with Don Rivera at AFP, he gave me to understand that I would expect to see an idle MAP of 9-10 inches on the ground.

It makes sense at lower power and higher RPM, as in the air, the vacuum pump will be more active. If the throttle is closed, even at 700 RPM, shouldn't you see a lower MAP?

Regards,

Michael Wynn
Phase I
Livermore, CA
 
I checked my datalogs, and on my IO-360, at 700 RPM I am at 12" MAP while at sea level.

I never thought to look at this before, but here's a graph of MAP vs RPM over a flight. Kinda cool. Ignore all the speckles- it's the dense areas that count.

MAP.png
 
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mlwynn said:
I think I understand all that, Bob. When I was working with Don Rivera at AFP, he gave me to understand that I would expect to see an idle MAP of 9-10 inches on the ground.

It makes sense at lower power and higher RPM, as in the air, the vacuum pump will be more active. If the throttle is closed, even at 700 RPM, shouldn't you see a lower MAP?

Regards,

Michael Wynn
Phase I
Livermore, CA
Click to expand...

I think 12" at idle on the ground is pretty typical. That's a lot lower than 29". (???not sure I understood the question)
Your MP gauge may be reading slightly high. LVK is 400' MSL so engine off you should see altimeter setting minus 0.4", or about 29.5".
 
MAP

From looking at the other data here, I guess what I am seeing is normal. The engine is certainly performing well and the readings in the air are as expected. Time to put in a lot more flight time and go from there. Incidently, Sunday was my first flight in the aircraft after three test flights by a much more experienced pilot. I have a lot to learn about the airplane. I think that is what the flight testing is really about.

Thanks for everyone's comments and insights.

Regards,

Michael WYnn
RV 8 Phase 1
Livermore, CA
 
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