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  #1  
Old 02-06-2013, 09:29 PM
Bill.Peyton's Avatar
Bill.Peyton Bill.Peyton is offline
 
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Location: St. Louis, MO
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Default Manifold Pressure Reading Fluctuations

As a background. I am using a three screen G3X system with a factory IO 540.

Throughout the first phase and continuing into phase II I noticed a tendency for the displayed MP to vary as much as 2". The MP was not truly changing just the displayed value. After downloading the data and plotting it next to the RPM, it became evident that the worst case occurred at 2380-2400 RPM. A search found another thread regarding this issue on a O-360, with the solution being an .030 restrictor inserted in the MP line. I tried this solution with a .010 restrictor. This helped about 50%, but did not totally solve the issue. McMaster Carr carries "snubber/restrictor" devices as small as .004", designed to reduce pressure fluctuations in pneumatic gauges. After purchasing and installing a .004 restrictor at the firewall manifold the problem is essentially cured. I also purchased a .008" restrictor, which I am planning to try. My concern is, it would not take much debris to block a .004" dia. restrictor. Here is a link to the catalog http://www.mcmaster.com/#catalog/119/577/=ld9aqc

I suspect that anyone using a piezo element based transducer could have this same issue if you are using the Lycoming provided tap at the top of the cylinder. The tap is so close to the intake valve, that this extremely fast acting sensor is able track the back pressure pulses occurring in the valve cavity.

Below are the plots of MP over RPM
Top is no restrictor
Middle is .010" restrictor
Bottom is .004" restrictor










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  #2  
Old 02-07-2013, 08:32 AM
DennisRhodes DennisRhodes is offline
 
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Default eradic signal

You can dampen the signal and eliminate the spikes by installing an orifice in the line. Usually done in the fitting that screws into the cylinder. These type fitting are available and used on oil pressure indication to an instrument. Or you can make your own with anything to plug the fitting and then drill out a very small hole. Be careful that anything you might use as an orifice can not get lose in the line. A steel fitting which has been soldered closed and drilled (small hole) out works well in my RV9
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  #3  
Old 02-07-2013, 02:17 PM
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lostpilot28 lostpilot28 is offline
 
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Default

Quote:
Originally Posted by DennisRhodes View Post
You can dampen the signal and eliminate the spikes by installing an orifice in the line. Usually done in the fitting that screws into the cylinder. These type fitting are available and used on oil pressure indication to an instrument. Or you can make your own with anything to plug the fitting and then drill out a very small hole. Be careful that anything you might use as an orifice can not get lose in the line. A steel fitting which has been soldered closed and drilled (small hole) out works well in my RV9
Did you even read the OP's post?

To the OP, I'm wondering if sandwiching something breathable between 2 plugs with a .010" orifice would help your problem. It sounds like it's overly sensitive and needs a buffer. Heck, maybe having 2 orifices of .010" at each end of the manifold hose would work (so you would have a volume buffer the length of the hose)?

I'm using MGL Voyager EFIS and had the same issue with no restrictor. I made a small restrictor and that fixed it for me.
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  #4  
Old 02-07-2013, 04:01 PM
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The problem has been fixed with the .004 orifice.
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  #5  
Old 02-07-2013, 05:28 PM
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Auburntsts Auburntsts is offline
 
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Bill,
Which part number did you go with? I copy the .004 orifice, but there's multiple snubbers to choose from in that size.
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  #6  
Old 02-07-2013, 05:42 PM
myrv6180 myrv6180 is offline
 
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Talking manifold pressure fluctuations

I had the same problem on an O-540 and installed a small reservoir in the line and it dampned it out as smooth as a baby's B--t. I just soldered a couple of brass tubes in the lid of a small tin can and then soldered the lid tight. Worked like a charm.
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  #7  
Old 02-07-2013, 06:01 PM
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Bill.Peyton Bill.Peyton is offline
 
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Default

This is a much more elegant solution. The McMaster Carr part number is 3820K26. 1/8 NPT, .004 or .008
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  #8  
Old 02-07-2013, 06:30 PM
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chepburn chepburn is offline
 
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Quote:
Originally Posted by Bill.Peyton View Post
This is a much more elegant solution. The McMaster Carr part number is 3820K26. 1/8 NPT, .004 or .008
Bill,
I agree....I have the same issue as you. I tried making my own restrictor with a #80 drill hole, but it is not enough....soooo I hopped on the link you provided and promptly ordered a 004 and a 008. Two hour later, I received this polite email from McMaster....

Chris,

Due to the complexity of U.S. export regulations, McMaster-Carr accepts international orders only from our established customers. This decision also applies to orders shipping within the United States, because it is based on the final destination of the items. We cannot accept this order or future orders.


Sigh....I don't know what nefarious deeds they might think I can undertake with a .004 restrictor fitting....
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  #9  
Old 02-07-2013, 06:35 PM
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Bill.Peyton Bill.Peyton is offline
 
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Order it and have it sent to my address and I will ship it to you. Pm me for my address
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  #10  
Old 02-08-2013, 10:58 AM
brad walton brad walton is offline
 
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Default Damping fluctuation

I had the same problem on my RV-8, Superior IO360 and MGL Odyssey G2 ccombination. I tried the orifice, but the smallest drill bit I had was 1/16. I bought an in-line gasoline filter and installed it in the sensor tubing behind the firewall. It damps out the fluctuation nicely by acting as an accumulator.
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