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Manifold pressure issue

N8RV

Well Known Member
While my question relates to the MAP reading on a Dynon D-120 EMS, I put it in the Engines forum because the crux of the question revolves around the manifold pressure line.

A common issue with the MAP reading when drawing manifold pressure from a cylinder is that the digital readout fluctuates wildly because it's so sensitive to the changes in pressure as the intake valve opens and closes rapidly -- or so goes the explanation from Dynon.

The solution is to restrict the airflow in the manifold pressure line by inserting something in the line -- some have used a cigarette filter or dense foam with good results. I chose the cigarette filter route and it worked dandy. Rock solid MAP readout.

Then, it suddenly started fluctuating again. It was time for an oil change anyway, so I pulled the cowling and saw that the cigarette filter was starting to pull apart! The center fibers were being sucked toward the cylinder.

Anybody have a guess as to the consequences of a cigarette filter being sucked into a cylinder?? Will it just be burned in combustion or might something more disastrous be the outcome?

Thoughts?
 
Maybe you need to change brands? ;)

I would do a compression check on that cylinder and see what you get based on readings from your last condition inspection.

Call Dynon and see what they recommend. My guess is it is not a cig filter.
 
Call Dynon and see what they recommend. My guess is it is not a cig filter.

Actually, that's what was recommended ... sort of ...

Dynon Technical
Support

Posts: 4286
Re: Fluctuating Manifold Pressure
Reply #1 - Jun 2nd, 2009, 1:56pm Quote This is somewhat normal, and reflects real fluctuations occurring in the engine. A restrictor between the engine and the MAP sensor will damp it out and not mess up the reading. It needs to be very, very restrictive. Some people have had luck with cigarette filters or tightly packed foam. You can search the forum for more ideas and there is also some info in the install guides.
 
Yeah, Larry, that's what I thought. I checked the installation instructions and they were no real help. It just says, "If you notice fluctuations on the manifold pressure reading on the EMS-D120, you may need to install a restrictor with a small hole inline between the sensor and the head where the manifold pressure line is split off."

I would think that they'd include some kind of restrictor fitting, or make it available, since this is apparently a common issue.

I think I'll try something solid in the line -- like a piece of plastic rod with a teeny-tiny hole drilled in it.

I was just curious if anyone who knows engines would have a guess at the likely outcome of a cigarette filter being sucked into a cylinder.
 
Manifold Pressure Restrictor

I remember on Dan Checkoway's site (now out of print), showed a plastic restrictor that he placed inline. It narrowed the orifice and dampened the oscillations. Man I miss that site.

S S Anderson
RV 7A Flying
 
soldered end of steel fitting, drilled #60

I had the same issue with my GRT readings. I finally ended up getting a steel 45 degree fitting, drilled out the firewall side slightly, filling it with solder then drilled with a #60 drill bit. I drilled the fitting out slightly so that the solder plug could never be sucked in to the engine. Now my manifold pressure is rock solid.
 
I had the same issue with my GRT readings. I finally ended up getting a steel 45 degree fitting, drilled out the firewall side slightly, filling it with solder then drilled with a #60 drill bit.

I did the same thing, except I used a straight fitting (angled fitting won't fit on my cylinders) and JB weld instead of solder:

20080518_restrictor1.jpg
20080518_restrictor2.jpg


Probably a hundred different ways to do it!

mcb
 
Ah, another reason to hit the search button.

I drilled a small hole in a AN470 AD4-?? rivet, length does not matter, and put it in one of the MP line fittings. The shank of the rivet will be lose but the head will keep the rivet from moving.

That restriction will smooth out the MP gauge readings.
 
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Ah, another reason to hit the search button.

Not sure exactly what you would have wanted me to search for, Bill ... cigarette butts being sucked into engine? :D

All great suggestions, guys, and I'm sure I'll use one of them -- or a variation on a theme. However, nobody has approached my original question: What's likely to happen if the filter is sucked into the cylinder?

I'm just curious. I'll probably pull the cowling again tonight and come up with *something* more reliable than the cigarette filter. And, if I find that it's no longer there, I guess that will answer my question.
 
Not sure exactly what you would have wanted me to search for, Bill ... cigarette butts being sucked into engine? :D

All great suggestions, guys, and I'm sure I'll use one of them -- or a variation on a theme. However, nobody has approached my original question: What's likely to happen if the filter is sucked into the cylinder?

I'm just curious. I'll probably pull the cowling again tonight and come up with *something* more reliable than the cigarette filter. And, if I find that it's no longer there, I guess that will answer my question.
No, but cigarette filter brings up a number of posts.

The filter material is nothing but acetate (I used to work for the company that makes that stuff.), in other words it is highly processed wood pulp with some vinegar (the acid part of acetate) thrown in. Chances are it wouldn't hurt your engine but your issue is the exact reason why I went with the rivet method of smoothing the MP. That is assuming it gets sucked into the cylinder and doesn't fall down into the carb, that might cause you some issues, as you probably surmised.

Also, because of the vacuum advance feature of my dual electronic ignition, I was afraid the bouncing MP might play havoc with my timing.
 
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snubber

Hey Fellas:

I use an AVEX rivet in a straight fitting: pull the rivet as usual, then knock the stem out with a punch. File the rivet head as required to allow the flare to fit tightly.

Piece o cake!

Mark
 
Answer

Well, it looks like I answered my own question.

I pulled the cowling tonight and looked for the disintegrating cigarette filter ... GONE. I took off the tube to the cylinder fitting and checked to see if it was clogged in there. Nope.

So, since I first noticed the MAP fluctuating shortly after climbout on my last flight and it ran just dandy for an hour, I guess the answer is that a cigarette filter, sucked into a cylinder, is burned. POOF.

I cleaned out the line of any residue, drilled a small hole in a rivet and made a plastic fitting that fit snugly into the MAP plastic tubing and put the rivet in the plastic holder. Sounds more complex than it was. No way it can move toward the cylinder. I'll find out if the hole is small enough next time I crank it up.

Lesson learned: DO NOT USE A CIGARETTE FILTER AS A MANIFOLD PRESSURE DAMPER!
 
I have a dental lab and use a micro sandblaster. There is a filter screen that works perfectly for the MP line. Zahn Dental has them for around $17 for 3. Item # 564-8908 Itegral Systems filter screen. Mine has over 150 hrs on it with no problems. Don
 
Just to be fair to the folks at Dynon, they were not exactly recommending a cigarrette filter:

Some people have had luck with cigarette filters or tightly packed foam

You need to always keep in mind that most of the experimental EFIS / electronics makers are concerned about liability, and may additionally not be A&P qualified. In trying to be helpful they may pass along "things they have heard."

Always take it with a grain of salt...

:D
 
As someone who early on suggested the cigarette filter I should have mentioned that I also have a metal screen in the line as well. The screen is actually a model aircraft fuel filter which was left from an earlier experiment to dampen the flucations. So my cig filter cannot escape.
2 years and 130 hours with rock steady readings.

Peter
 
Pressure Snubber

I just installed one of these pressure snubbers for the oil pressure on our research engine. The snubber came with several "filter elements" for various viscosities from air to heavy oil. The snubber has an internal cavity where the filter goes and is retained by an inline set screw that can not get out. This cost me the grand total of $6 Cnd. Hard to beat and should be much more effective than a cigarette filter.

http://www.arrowpneumatics.com/accessories.html#Psnubbers

Bob Parry
 
thanks to Mark for the pop-rivet suggestion

I have a bulkhead fitting that is AN flare on the firewall side, with a braided line to the engine, and a hose-barb tube on the cockpit side. I put a 5/32 pop rivet in the end of the barb side, knocked the nail out, and filed the head to match the diameter of the hose barb. Rivet can't go toward engine.

We will test this weekend to see if it is sufficient restriction. But it was a very fast, easy thing to do.

Thanks Mark.
 
Made a fitting for my MP line too.

I made a new fitting when I was hooking my manifold pressure line into my PMag and AF3500. I stole Scott Card's idea and threaded the inside of a 45 degree fitting, then put an AN-3 bolt into the end of the fitting with some JB Weld. I cut the bolt off flush and got the end smooth. I then drilled a very small hole through the center of the bolt. If the bolt were to get pulled loose toward the cylinder it wouldn't get past the bend in the fitting. Take a look at the URL below.

http://rv9a.card-net.org/archives/2006/12/18/manifold-pressure-fittings/
 
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Sorry to resurrect an old thread but...

Since I first flew back in 2012, my MAP on my Dynon danced around like crazy. I always wondered why it did that since I had a restricter in the line.

Fast forward to this past Saturday. I finally decided I'd take care of a few nuisance items and this was one of them. You can probably guess that when I took it apart I found that I had installed NO restricter! Brain fart!

I searched VAF and decided that I'd just buy an off the shelf part rather than go the home grown route. Well, Van's doesn't sell a straight one, Wick's doesn't have a picture of one, and I can't remember the issue with Aircraft Spruce.

Since I do have a bunch of rivets at the hangar I decided to take a trip over to Harbor Freight. I found a pack of teeny tiny itty bitty drill bits for $4. I also had my fake Dremel at the hangar with a small drill chuck I had bought for it years ago. After breaking one drill bit I did manage to put a hole in an AN470 rivet. I put it in line as described earlier in this thread, started up the engine, and SHAZAM, my freaked out MAP is now calmer than ever!

It feels good to put to bed a nagging annoyance like that. I actually put 3 annoyances to bed on Saturday in short order. Now if the nagging overcast would just go away I'll go flying!

Thanks VAF!
 
Crimped primer line

I turned my old broken primer line coming off #4 into my MAP. Using the copper tubing as a high temp heat material and connecting that to silicon MAP tubing from my Dynon. Teed those off to my two P-Mags. In the copper tube I added a crimp which seemed to do the MAP trick. MAPs on the Dynon readout are smooth.
 
Just curious is this would work for a standard Van's electric manifold pressure
gauge reading low?
 
I used the gauge fitting on the fuel spider on top of the engine. It is a straight restricted fitting and is only needed if you are using an analogue pressure gauge. Swap it with the manifold plug on a cylinder of your choice and it should help dampen the manifold pressure reading fluctuations
 
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