This winter I spent some time learning how to use my engine monitor (EDM900) better. I did the free class at Advanced Pilot and was going to buy the full course but John Deakin and George Braly seemed to be having some internal strife and the class wasn't available until my window of opportunity had closed. Since I run a P-ponk carbureted engine I lost a little interest in LOP as well.

Anyway, one of the things I wanted to research was the reason I see very slight fluctuations in my manifold pressure. Often, with the throttle unchanged I notice the MP going up and down a couple of few 10ths (24.2, 24.3, 24.4, 24.3, 24.2). This happens at all throttle settings.

Is this normal, or indication of a problem? It would not be noticeable with an analog gauge. My instinct is that it is normal, but I'll feel sort of silly if my pricey instrument is clearly telling me to fix a problem that I'm ignoring.

I'm using an Electronics International M-1 manifold Pressure gauge on my O-470R and it holds rock steady with no fluctuations. Yours may have a higher sampling rate, and is catching variations that mine is not.

I have an EI CGR30. The manifold pressure graph shows fluctuations of +/- 0.1" from the baseline value, sampling at 1 Hz. It is also apparent that the recorded measurement is digitized, not analog, and the quantization is 0.1" for the least significant display increment for manifold pressure.

I use 4 different EDM930s for work and +/- 0.1" happens regularly, I would consider it normal. On an analog gauge that is about 1/4 of a needle width.

I would guess that the JPI sensitivity settings can be adjusted just like the EI. I adjusted my MVP-50 for the same reason. It would not stay at a particular setting. It would wander. More of an annoyance than anything, but I still adjusted it and now it’s rock solid. FWIW

Thanks all. I'll see about a sensitivity setting but won't worry in the meantime.

Allan

The fact is that you're dealing with a MUCH more sensitive device than a standard analog gauge. You'll likely note RPM variations as well.....drives some folks batty at first, but no big deal. Fact is, the engine DOES actually vary a tiny bit. A needle wouldn't show that much fluctuation, unless you got down there and really stared at it.

These are great machines, in any case.

MTV

When I had my Horizon digital tach installed, that was the first thing I noticed--rpm going up and down and never rock solid, although I couldn't hear any change in apparent engine noise. It's just something to get accustomed to. MP is similar. Besides the obvious changes in engine load that will occur if the airplane goes from cruise to climb to descent, we fly in a constantly changing environment, and that will change the manifold pressure continually, albeit not by much, just noticeably. When you start seeing MP changes of a couple PSI rather than a couple tenths, then become concerned.

Cary