Bill.Peyton
Well Known Member
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
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