Fluctuating Manifold Pressure

During a flight today I had widely fluctuating manifold pressure, at cruise it would constantly bounce between 20 and 30 or higher. The conditions of flight were as follows:

YIO360 M1B Lycoming with CS Prop.
RPM 2300, Fuel Flow 8.2 GPH.
Outside air temp 24 degrees.
Cylinder temps were around 290 with not more than a 10 degree spread. EGT was in the 1400 range with about a 50 degree spread.
LOP and ROP operation had no effect on the MP variations.

Interesting note is that I have a smoke system and turning it on caused MP to spike above 30...

I use a Dynon FlightDEK 180 for all my engine monitoring needs.

The engine was running smooth with no power fluctuations or other issues that I could identify.

Thoughts?

Do you, or have you ever had a restrictor installed in the MP line?

That is an indicator problem. Sensor ,wiring or the 180. The jump due to the smoke system would have me looking at wiring first.

Now I have a project to work on

I do not have a restrictor in the MP line...its as it came from Dynon...will look at wiring as well...

Under normal conditions, is the MAP reading consistant?
I bet it is-

Tom

TS Flightlines said:

Under normal conditions, is the MAP reading consistant?
I bet it is-

Tom

Click to expand...

He said that he is indicating >30" when he turns the smoke system on. A pretty good clue that it is electrical in nature, in addition to the 10" swings he is seeing. It's pretty rare to read greater than 30" of MP, unless he is WOT and 50' off the deck on one of the coasts.

I missed that Larry----other than how it previously behaved with or without the smoke system.

Tom

Orifice installed

I fabricated and installed an orifice in the MP line last night, will fly this evening to see if that has any impact on the fluctuation. The MP has always moved around a bit but the most recent seemed a bit extreme. Here in Anchorage my flight level is pretty close to sea level and, I was only at about 700 feet most of the time. Barometric pressure was 29.54 so air was pretty dense. I still have to ponder the electrical implications of the smoke system as to how it may impact the readings. I will post what I find tonight.

Berchmans said:

I fabricated and installed an orifice in the MP line last night, will fly this evening to see if that has any impact on the fluctuation. The MP has always moved around a bit but the most recent seemed a bit extreme. Here in Anchorage my flight level is pretty close to sea level and, I was only at about 700 feet most of the time. Barometric pressure was 29.54 so air was pretty dense. I still have to ponder the electrical implications of the smoke system as to how it may impact the readings. I will post what I find tonight.

Click to expand...

HOpe it works, but have doubts. On my first plane, I saw 1-2" swings in some cases. I fab'ed a restrictor and it went away. 10" swings seem to be well beyond the typical oscillations we see when using an individual intake port vs an intake manifold port. I suppose that an intake leak in the cyl that is tapped for MP could cause much larger swings that those normally seen. The leak would allow that port to drop it's vacuum, when the valve closes, due to the leak. Normally the vacuum in the intake system keeps the port from losing much vacuum when the valve closes. That said, none of this explains the interaction with your smoke system.

Larry

Some things baffle you

Flew last evening with the new restrictor in the MP line. Seems to have done the trick. Where the pressure was fluctuating by 10 its now in the decimal range. I operated over the full range of power from 65% cruise to WOT. All other variables except Barometer which was 30.32 were the same. As to changes when I turned on the smoke system...there were none. All I can surmise is that when I used it the first time other factors might have been in play, I only turned it on once during the first flight but did numerous tests this time. So for now it seems to be a non issue. The engine has always ran great and continues to do so.

Good to hear you have it resolved!

When we were developing the EICommander we realized that some engine monitors smoothed the MAP numbers via software and others did not. I didn't track, nor do I recall which did what.

Those that did not smooth the MAP number with software, such as the early Dynon units required some type of restrictor to smooth the power pulses.

While we think of the Manifold Pressure as being a constant vacuum, there are shore duration, high intensity positive pulses created when the intake valve slams shut.

It is these positive pulses that wrecked havoc with the timing of early P-mags. To the ignitions it appears as those someone was blowing in the tube and because the engine was spinning, the timing would get set to some random value. This issue was fixed a number of years ago with firmware version 40.

For those with P-mags, you want some type of restrictor upstream of the P-mags, otherwise they could be picking up some unknown value to set the timing, this is regardless of what you engine monitor does with the value.