MAP Gauge fluctuates

I have my MP ported to my #3 cylinder and going into my MGL EFIS system. I've noticed that my MP fluctuates pretty wildly during flight. It's always "ballparked" around a number depending on RPMs, etc, but it will swing a whole number each way. For example, if it should be reading 24", it'll fluctuate 3-times per second (That's how often the EFIS updates the MP display) between 22.8 and 25.2.

I'm wondering if I need a smaller restrictor fitting, or if there's a problem with the sensor. Someone on another forum suggested connecting the MP line just north of the carb butterfly valve. I'm not sure about that...any ideas?

Thanks in advance!

Previous threads

... have indicated the need for a restrictor. I used a 10-32 tap to part way go thru the male fitting on the AN side (not the NPT side). I then screwed in a 10-32 aluminum bolt, then cut it off, drilled a 1/16 hole thru the center, then ground it smooth. Can't go anywhere that way. Some have used a rivet and loctite. Some have used a cigarette filter in the line.

Don

The MAP on a single cylinder does swing wildly, even more so than if MAP is tapped near the throttle, even though it still shows the average for all cylinders. You need to restrict the line from the cylinder to the sensor to smooth this out. Since there is no flow in the system, pressure only, you really want to put a very, very small restriction in the line, especially since you are not trying to follow rapid changes in throttle setting as with a car. The digital display is sampling the sensor output at a fixed rep rate and will show almost the highs and lows in the MAP, depending on the processing circuitry's filter time-constant. With a sampled data system you can get into really large errors whenever the sample rate is almost synchronous with the pulse-rate as demonstrated by the Shannon-Nyquist Theorem.

MAP Restrictor

I used Vans fitting but the Dynon sensor. I used a piece of 1/8th inch copper tube about an inch long and filled it with solder then drilled an itty bitty hole through the solder. Inserted the pipe into the plastic tubing. It was a tight fit. Seems to have done the job.

Thanks, guys. Sooo, what I'm understanding is that the Van's-supplied restrictor hole is probably too large, therefore I should find a way to make it smaller. What its' current size is, I don't know...I haven't looked at that hole in almost 2 years!

My other option is to maybe mount the MP fitting at the back of the throttle. I'm using an MA-4-5...I don't know if it has the brass plug or not, but I think it does. If this is a "better" option, why have I not heard of anyone doing this before? It seems that everyone mounts the MP line on the #3 cylinder.

Last thing I'll throw out there...because I'm using P-mags, and they require MP to advance properly, do you foresee any problems with either option? I would imagine as long as they get the right MP, they'll work just fine.

(BTW: The cigarette filter idea sounds easy, but I don't smoke!). Makes me wonder if there's an alternative, but similar solution that wouldn't ruin the engine if it were ingested.

bump

Can anyone offer suggestions on orifice size? Or a way to restrict the flow further? Is popping a rivet in an AN fitting and drilling a small orifice a bad idea?

lostpilot28 said:

(BTW: The cigarette filter idea sounds easy, but I don't smoke!). ....

Click to expand...

This is how we fixed ours. We don't smoke, so we borrowed one from someone that does.
We just slipped itsind the 1/4" line from the MAP sensor.
Woeks great...

I'm using...

... Vans restrictor in my Superior IO-360with dual P-mags and AFS 3500 EFIS.

No problems in 2 years/ 150 hrs. MAP-readings show steady and stable when they're supposed to.

Both P-mag curves used (the most restrictive one for the first 50 hrs) and no problems with either one.

Vans' restrictor fitting (at least the one I found) is a 45-degree angle fitting, and on my O-320 that doesn't work... the cylinder fins get in the way of rotating the fitting. So I had to make my own restrictor fitting as others have done. I bought a straight steel fitting (alum would be fine for this application) from spruce and inserted a rivet into it, securing it with loctite, and drilling a small hole in the middle.
On my first version, the hole ended-up getting clogged with loctite (discovered after first flight, when I realised I had no MP readings (not really critical being a FP prop)), so I drilled it up to a 3/32, which is too large - it now fluctuates by about 0.3". I'll be making a new one with a smaller restriction soon.
Note: I need to use a deep socket to be able to install the restrictor fitting... not enough room to turn a standard wrench on it (cylinder fins in the way).

MauiLvrs said:

This is how we fixed ours. We don't smoke, so we borrowed one from someone that does.
We just slipped itsind the 1/4" line from the MAP sensor.
Woeks great...

Click to expand...

Thanks, Dave...what was it doing before the filter that it required fixing? Was it the same problem I'm having?

Alf, I wonder if your EFIS has software that averages out the fluctuation. My P-mags also work great, but my EFIS sensor seems to be sensitive.

Try this thread. It is one of many with good ideas on how to solve this problem,

I had a vacuum gauge on my little 4-cylinder VW Rabbit, and it swung wildly until I made a clamp that went over the rubber tubing to clamp it down to where the opening must have been 1/32" or less. Just releasing the screws on the clamp ever so slightly would make it buzz and vibrate. As long as there is just a very tiny opening the gauge will read the pressure. The only drawback is that the response time gets longer. So what! You're not jockeying the throttle as you would in a car, and when you do, you're looking at other things than MAP. How about a #60 drill.

lostpilot28 said:

Thanks, Dave...what was it doing before the filter that it required fixing? Was it the same problem I'm having?

Click to expand...

Yep .... displayed about +/- 1